requirements.c

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/***********************************************************************
 Freeciv - Copyright (C) 1996-2004 - The Freeciv Project
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.
 
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.
***********************************************************************/
#ifdef HAVE_CONFIG_H
#include <fc_config.h>
#endif
 
#include <stdarg.h>
 
/* utility */
#include "astring.h"
#include "fcintl.h"
#include "log.h"
#include "support.h"
 
/* common */
#include "achievements.h"
#include "calendar.h"
#include "citizens.h"
#include "counters.h"
#include "culture.h"
#include "game.h"
#include "government.h"
#include "improvement.h"
#include "movement.h"
#include "nation.h"
#include "player.h"
#include "map.h"
#include "research.h"
#include "road.h"
#include "server_settings.h"
#include "specialist.h"
#include "style.h"
#include "tiledef.h"
#include "victory.h" /* victory_enabled() */
 
#include "requirements.h"
 
struct thr_req_data
{
  fc_thread_id thr_id;
};
 
/* get 'struct thr_req_data_list' and related functions: */
#define SPECLIST_TAG thr_req_data
#define SPECLIST_TYPE struct thr_req_data
#include "speclist.h"
 
#define thr_req_data_list_iterate(trlist, ptrdata) \
    TYPED_LIST_ITERATE(struct thr_req_data, trlist, ptrdata)
#define thr_req_data_list_iterate_end  LIST_ITERATE_END
 
static struct thr_req_data_list *trdatas;
static fc_mutex trmutex;
 
/************************************************************************
  Container for req_item_found functions
************************************************************************/
typedef enum req_item_found (*universal_found)(const struct requirement *,
                                               const struct universal *);
static universal_found universal_found_function[VUT_COUNT] = {nullptr};
 
static
enum fc_tristate tri_req_present(const struct civ_map *nmap,
                                 const struct req_context *context,
                                 const struct req_context *other_context,
                                 const struct requirement *req);
 
/* Function pointer for requirement-type-specific is_req_active handlers */
typedef enum fc_tristate
(*is_req_active_cb)(const struct civ_map *nmap,
                    const struct req_context *context,
                    const struct req_context *other_context,
                    const struct requirement *req);
 
static inline bool are_tiles_in_range(const struct tile *tile1,
                                      const struct tile *tile2,
                                      enum req_range range);
 
/**********************************************************************/
static enum req_unchanging_status
  unchanging_local(const struct civ_map *nmap,
                   enum req_unchanging_status def,
                   const struct req_context *context,
                   const struct requirement *req)
{
  return req->range == REQ_RANGE_LOCAL ? REQUCH_YES : def;
}
#define REQUC_LOCAL unchanging_local
 
/**********************************************************************/
static enum req_unchanging_status
  unchanging_present(const struct civ_map *nmap,
                     enum req_unchanging_status def,
                     const struct req_context *context,
                     const struct requirement *req)
{
  if (TRI_YES != tri_req_present(nmap, context, nullptr, req)) {
    return REQUCH_NO;
  }
  return def;
}
#define REQUC_PRESENT unchanging_present
 
/**********************************************************************/
static enum req_unchanging_status
  unchanging_world(const struct civ_map *nmap,
                   enum req_unchanging_status def,
                   const struct req_context *context,
                   const struct requirement *req)
{
  return
    unchanging_present(nmap, req->range == REQ_RANGE_WORLD ? REQUCH_YES : def,
                       context, req);
}
#define REQUC_WORLD unchanging_world
 
/**********************************************************************/
static enum req_unchanging_status
  unchanging_noally(const struct civ_map *nmap,
                    enum req_unchanging_status def,
                    const struct req_context *context,
                    const struct requirement *req)
{
  if (REQ_RANGE_ALLIANCE == req->range
      || REQ_RANGE_TEAM == req->range) {
    struct requirement preq;
 
    req_copy(&preq, req);
    preq.range = REQ_RANGE_PLAYER;
    if (TRI_YES != tri_req_present(nmap, context, nullptr, &preq)) {
      return REQ_RANGE_TEAM == req->range ? REQUCH_ACT : REQUCH_NO;
    }
  }
  return def;
}
#define REQUC_NALLY unchanging_noally
 
/**********************************************************************/
static enum req_unchanging_status
  unchanging_citytile(const struct civ_map *nmap,
                      enum req_unchanging_status def,
                      const struct req_context *context,
                      const struct requirement *req)
{
  fc_assert_ret_val(VUT_CITYTILE == req->source.kind, REQUCH_NO);
  if (CITYT_CENTER == req->source.value.citytile
      || (CITYT_BORDERING_TCLASS_REGION != req->source.value.citytile
          && context->city != nullptr && context->tile != nullptr
          && city_tile(context->city) != nullptr
          && are_tiles_in_range(city_tile(context->city), context->tile,
                                req->range))){
    /* Cities don't move, and most reqs are present on city center */
    return REQUCH_YES;
  }
  return def;
}
#define REQUC_CITYTILE unchanging_citytile
 
/**********************************************************************/
static enum req_unchanging_status
  unchanging_citystatus(const struct civ_map *nmap,
                        enum req_unchanging_status def,
                        const struct req_context *context,
                        const struct requirement *req)
{
  fc_assert_ret_val(VUT_CITYSTATUS == req->source.kind, REQUCH_NO);
 
  if (REQ_RANGE_CITY == req->range
      && (CITYS_OWNED_BY_ORIGINAL == req->source.value.citystatus
          || CITYS_TRANSFERRED == req->source.value.citystatus)) {
    return REQUCH_CTRL;
  }
 
  return def;
}
#define REQUC_CITYSTATUS unchanging_citystatus
 
/**********************************************************************/
static enum req_unchanging_status
  unchanging_building(const struct civ_map *nmap,
                      enum req_unchanging_status def,
                      const struct req_context *context,
                      const struct requirement *req)
{
  const struct impr_type *b = req->source.value.building;
 
  fc_assert_ret_val(VUT_IMPROVEMENT == req->source.kind
                    || VUT_SITE == req->source.kind, REQUCH_NO);
  if (REQ_RANGE_LOCAL == req->range) {
    /* Likely, won't be questioned for an obsolete building */
    return REQUCH_YES;
  }
 
  if (req->source.kind == VUT_IMPROVEMENT
      && improvement_obsolete(context->player, b, context->city)) {
    /* FIXME: Sometimes can unobsolete, but considering it
     * may sometimes put the function on endless recursion */
    return REQUCH_ACT; /* Mostly about techs */
  }
  if (is_great_wonder(b)) {
    if (great_wonder_is_destroyed(b)
        || (!great_wonder_is_available(b)
            && (req->range <= REQ_RANGE_CITY && TRI_YES
                == tri_req_present(nmap, context, nullptr, req)))) {
      /* If the wonder stays somewhere, it may either remain there
       * or be destroyed. If it is destroyed, it is nowhere. */
      return REQUCH_SCRIPTS;
    }
  }
  return def;
}
#define REQUC_IMPR unchanging_building
 
struct req_def {
  const is_req_active_cb cb;
  enum req_unchanging_status unchanging;
  req_unchanging_cond_cb unchanging_cond;
};
 
/**********************************************************************/
static void thr_exit_cb(void)
{
  fc_thread_id self = fc_thread_self();
 
  fc_mutex_allocate(&trmutex);
  thr_req_data_list_iterate(trdatas, data) {
    if (fc_threads_equal(self, data->thr_id)) {
      thr_req_data_list_remove(trdatas, data);
      free(data);
      break;
    }
  } thr_req_data_list_iterate_end;
  fc_mutex_release(&trmutex);
}
 
/**********************************************************************/
void requirements_init(void)
{
  trdatas = thr_req_data_list_new();
  fc_mutex_init(&trmutex);
  register_at_thread_exit_callback(thr_exit_cb);
}
 
/**********************************************************************/
void requirements_free(void)
{
  fc_mutex_destroy(&trmutex);
  thr_req_data_list_destroy(trdatas);
}
 
/**********************************************************************/
struct universal universal_by_rule_name(const char *kind,
                                        const char *value)
{
  struct universal source;
 
  source.kind = universals_n_by_name(kind, fc_strcasecmp);
  if (!universals_n_is_valid(source.kind)) {
    return source;
  }
 
  universal_value_from_str(&source, value);
 
  return source;
}
 
/**********************************************************************/
static bool activity_is_valid_in_requirement(enum unit_activity act)
{
  return unit_activity_is_valid(act)
    && act != ACTIVITY_SENTRY
    && act != ACTIVITY_GOTO
    && act != ACTIVITY_EXPLORE;
}
 
/**********************************************************************/
bool universal_is_legal_in_requirement(const struct universal *univ)
{
  if (univ->kind == VUT_ACTIVITY) {
    return activity_is_valid_in_requirement(univ->value.activity);
  }
 
  return TRUE;
}
 
/**********************************************************************/
void universal_value_from_str(struct universal *source, const char *value)
{
  /* Finally scan the value string based on the type of the source. */
  switch (source->kind) {
  case VUT_NONE:
    return;
  case VUT_ADVANCE:
    source->value.advance = advance_by_rule_name(value);
    if (source->value.advance != nullptr) {
      return;
    }
    break;
  case VUT_TECHFLAG:
    source->value.techflag
      = tech_flag_id_by_name(value, fc_strcasecmp);
    if (tech_flag_id_is_valid(source->value.techflag)) {
      return;
    }
    break;
  case VUT_GOVERNMENT:
    source->value.govern = government_by_rule_name(value);
    if (source->value.govern != nullptr) {
      return;
    }
    break;
  case VUT_GOVFLAG:
    source->value.govflag = gov_flag_id_by_name(value, fc_strcasecmp);
    if (gov_flag_id_is_valid(source->value.govflag)) {
      return;
    }
    break;
  case VUT_ACHIEVEMENT:
    source->value.achievement = achievement_by_rule_name(value);
    if (source->value.achievement != nullptr) {
      return;
    }
    break;
  case VUT_STYLE:
    source->value.style = style_by_rule_name(value);
    if (source->value.style != nullptr) {
      return;
    }
    break;
  case VUT_IMPROVEMENT:
  case VUT_SITE:
    source->value.building = improvement_by_rule_name(value);
    if (source->value.building != nullptr) {
      return;
    }
    break;
  case VUT_IMPR_GENUS:
    source->value.impr_genus = impr_genus_id_by_name(value, fc_strcasecmp);
    if (impr_genus_id_is_valid(source->value.impr_genus)) {
      return;
    }
    break;
  case VUT_IMPR_FLAG:
    source->value.impr_flag = impr_flag_id_by_name(value, fc_strcasecmp);
    if (impr_flag_id_is_valid(source->value.impr_flag)) {
      return;
    }
    break;
  case VUT_PLAYER_FLAG:
    source->value.plr_flag = plr_flag_id_by_name(value, fc_strcasecmp);
    if (plr_flag_id_is_valid(source->value.plr_flag)) {
      return;
    }
    break;
  case VUT_EXTRA:
    source->value.extra = extra_type_by_rule_name(value);
    if (source->value.extra != nullptr) {
      return;
    }
    break;
  case VUT_TILEDEF:
    source->value.tiledef = tiledef_by_rule_name(value);
    if (source->value.tiledef != nullptr) {
      return;
    }
    break;
  case VUT_GOOD:
    source->value.good = goods_by_rule_name(value);
    if (source->value.good != nullptr) {
      return;
    }
    break;
  case VUT_TERRAIN:
    source->value.terrain = terrain_by_rule_name(value);
    if (source->value.terrain != T_UNKNOWN) {
      return;
    }
    break;
  case VUT_TERRFLAG:
    source->value.terrainflag
      = terrain_flag_id_by_name(value, fc_strcasecmp);
    if (terrain_flag_id_is_valid(source->value.terrainflag)) {
      return;
    }
    break;
  case VUT_NATION:
    source->value.nation = nation_by_rule_name(value);
    if (source->value.nation != NO_NATION_SELECTED) {
      return;
    }
    break;
  case VUT_NATIONGROUP:
    source->value.nationgroup = nation_group_by_rule_name(value);
    if (source->value.nationgroup != nullptr) {
      return;
    }
    break;
  case VUT_NATIONALITY:
    source->value.nationality = nation_by_rule_name(value);
    if (source->value.nationality != NO_NATION_SELECTED) {
      return;
    }
    break;
   case VUT_ORIGINAL_OWNER:
    source->value.origowner = nation_by_rule_name(value);
    if (source->value.origowner != NO_NATION_SELECTED) {
      return;
    }
    break;
  case VUT_DIPLREL:
  case VUT_DIPLREL_TILE:
  case VUT_DIPLREL_TILE_O:
  case VUT_DIPLREL_UNITANY:
  case VUT_DIPLREL_UNITANY_O:
    source->value.diplrel = diplrel_by_rule_name(value);
    if (source->value.diplrel != diplrel_other_invalid()) {
      return;
    }
    break;
  case VUT_UTYPE:
    source->value.utype = unit_type_by_rule_name(value);
    if (source->value.utype) {
      return;
    }
    break;
  case VUT_UTFLAG:
    source->value.unitflag = unit_type_flag_id_by_name(value, fc_strcasecmp);
    if (unit_type_flag_id_is_valid(source->value.unitflag)) {
      return;
    }
    break;
  case VUT_UCLASS:
    source->value.uclass = unit_class_by_rule_name(value);
    if (source->value.uclass) {
      return;
    }
    break;
  case VUT_UCFLAG:
    source->value.unitclassflag
      = unit_class_flag_id_by_name(value, fc_strcasecmp);
    if (unit_class_flag_id_is_valid(source->value.unitclassflag)) {
      return;
    }
    break;
  case VUT_MINVETERAN:
    source->value.minveteran = atoi(value);
    if (source->value.minveteran > 0) {
      return;
    }
    break;
  case VUT_UNITSTATE:
    source->value.unit_state = ustate_prop_by_name(value, fc_strcasecmp);
    if (ustate_prop_is_valid(source->value.unit_state)) {
      return;
    }
    break;
  case VUT_ACTIVITY:
    source->value.activity = unit_activity_by_name(value, fc_strcasecmp);
    if (activity_is_valid_in_requirement(source->value.activity)) {
      return;
    }
    break;
  case VUT_MINMOVES:
    source->value.minmoves = atoi(value);
    if (source->value.minmoves > 0) {
      return;
    }
    break;
  case VUT_MINHP:
    source->value.min_hit_points = atoi(value);
    if (source->value.min_hit_points > 0) {
      return;
    }
    break;
  case VUT_AGE:
    source->value.age = atoi(value);
    if (source->value.age > 0) {
      return;
    }
    break;
  case VUT_FORM_AGE:
    source->value.form_age = atoi(value);
    if (source->value.form_age > 0) {
      return;
    }
    break;
  case VUT_MINTECHS:
    source->value.min_techs = atoi(value);
    if (source->value.min_techs > 0) {
      return;
    }
    break;
  case VUT_FUTURETECHS:
    source->value.future_techs = atoi(value);
    if (source->value.future_techs > 0) {
      return;
    }
    break;
  case VUT_MINCITIES:
    source->value.min_cities = atoi(value);
    if (source->value.min_cities > 0) {
      return;
    }
    break;
  case VUT_ACTION:
    source->value.action = action_by_rule_name(value);
    if (source->value.action != nullptr) {
      return;
    }
    break;
  case VUT_OTYPE:
    source->value.outputtype = output_type_by_identifier(value);
    if (source->value.outputtype != O_LAST) {
      return;
    }
    break;
  case VUT_SPECIALIST:
    source->value.specialist = specialist_by_rule_name(value);
    if (source->value.specialist) {
      return;
    }
    break;
  case VUT_MINSIZE:
    source->value.minsize = atoi(value);
    if (source->value.minsize > 0) {
      return;
    }
    break;
  case VUT_MINCULTURE:
    source->value.minculture = atoi(value);
    if (source->value.minculture > 0) {
      return;
    }
    break;
  case VUT_MINFOREIGNPCT:
    source->value.minforeignpct = atoi(value);
    if (source->value.minforeignpct > 0) {
      return;
    }
    break;
  case VUT_AI_LEVEL:
    source->value.ai_level = ai_level_by_name(value, fc_strcasecmp);
    if (ai_level_is_valid(source->value.ai_level)) {
      return;
    }
    break;
  case VUT_MAXTILETOTALUNITS:
    source->value.max_tile_total_units = atoi(value);
    if (0 <= source->value.max_tile_total_units) {
      return;
    }
    break;
  case VUT_MAXTILETOPUNITS:
    source->value.max_tile_top_units = atoi(value);
    if (0 <= source->value.max_tile_top_units) {
      return;
    }
    break;
  case VUT_TERRAINCLASS:
    source->value.terrainclass
      = terrain_class_by_name(value, fc_strcasecmp);
    if (terrain_class_is_valid(source->value.terrainclass)) {
      return;
    }
    break;
  case VUT_ROADFLAG:
    source->value.roadflag = road_flag_id_by_name(value, fc_strcasecmp);
    if (road_flag_id_is_valid(source->value.roadflag)) {
      return;
    }
    break;
  case VUT_EXTRAFLAG:
    source->value.extraflag = extra_flag_id_by_name(value, fc_strcasecmp);
    if (extra_flag_id_is_valid(source->value.extraflag)) {
      return;
    }
    break;
  case VUT_MINYEAR:
    source->value.minyear = atoi(value);
    return;
  case VUT_MINCALFRAG:
    /* Rule names are 0-based numbers, not pretty names from ruleset */
    source->value.mincalfrag = atoi(value);
    if (source->value.mincalfrag >= 0) {
      /* More range checking done later, in sanity_check_req_individual() */
      return;
    }
    break;
  case VUT_TOPO:
    source->value.topo_property = topo_flag_by_name(value, fc_strcasecmp);
    if (topo_flag_is_valid(source->value.topo_property)) {
      return;
    }
    break;
  case VUT_WRAP:
    source->value.wrap_property = wrap_flag_by_name(value, fc_strcasecmp);
    if (wrap_flag_is_valid(source->value.wrap_property)) {
      return;
    }
    break;
  case VUT_SERVERSETTING:
    source->value.ssetval = ssetv_by_rule_name(value);
    if (source->value.ssetval != SSETV_NONE) {
      return;
    }
    break;
  case VUT_TERRAINALTER:
    source->value.terrainalter
      = terrain_alteration_by_name(value, fc_strcasecmp);
    if (terrain_alteration_is_valid(source->value.terrainalter)) {
      return;
    }
    break;
  case VUT_CITYTILE:
    source->value.citytile = citytile_type_by_name(value, fc_strcasecmp);
    if (source->value.citytile != CITYT_LAST) {
      return;
    }
    break;
  case VUT_CITYSTATUS:
    source->value.citystatus = citystatus_type_by_name(value, fc_strcasecmp);
    if (source->value.citystatus != CITYS_LAST) {
      return;
    }
    break;
  case VUT_PLAYER_STATE:
    source->value.plrstate = plrstate_type_by_name(value, fc_strcasecmp);
    if (source->value.plrstate != PLRS_LAST) {
      return;
    }
    break;
  case VUT_MINLATITUDE:
  case VUT_MAXLATITUDE:
    source->value.latitude = atoi(value);
    if (source->value.latitude >= -MAP_MAX_LATITUDE
        && source->value.latitude <= MAP_MAX_LATITUDE) {
      return;
    }
    break;
  case VUT_COUNTER:
   source->value.counter = counter_by_rule_name(value);
   if (source->value.counter != nullptr) {
      return;
   }
   break;
  case VUT_MAX_DISTANCE_SQ:
    source->value.distance_sq = atoi(value);
    if (0 <= source->value.distance_sq) {
      return;
    }
    break;
  case VUT_MAX_REGION_TILES:
    source->value.region_tiles = atoi(value);
    if (0 < source->value.region_tiles) {
      return;
    }
    break;
  case VUT_TILE_REL:
    source->value.tilerel = tilerel_type_by_name(value, fc_strcasecmp);
    if (source->value.tilerel != TREL_COUNT) {
      return;
    }
    break;
  case VUT_COUNT:
    break;
  }
 
  /* If we reach here there's been an error. */
  source->kind = universals_n_invalid();
}
 
/**********************************************************************/
struct universal universal_by_number(const enum universals_n kind,
                                     const int value)
{
  struct universal source;
 
  source.kind = kind;
 
  switch (source.kind) {
  case VUT_NONE:
    /* Avoid compiler warning about uninitialized source.value */
    source.value.advance = nullptr;
 
    return source;
  case VUT_ADVANCE:
    source.value.advance = advance_by_number(value);
    if (source.value.advance != nullptr) {
      return source;
    }
    break;
  case VUT_TECHFLAG:
    source.value.techflag = value;
    return source;
  case VUT_GOVERNMENT:
    source.value.govern = government_by_number(value);
    if (source.value.govern != nullptr) {
      return source;
    }
    break;
  case VUT_GOVFLAG:
    source.value.govflag = value;
    return source;
  case VUT_ACHIEVEMENT:
    source.value.achievement = achievement_by_number(value);
    if (source.value.achievement != nullptr) {
      return source;
    }
    break;
  case VUT_STYLE:
    source.value.style = style_by_number(value);
    if (source.value.style != nullptr) {
      return source;
    }
    break;
  case VUT_IMPROVEMENT:
  case VUT_SITE:
    source.value.building = improvement_by_number(value);
    if (source.value.building != nullptr) {
      return source;
    }
    break;
  case VUT_IMPR_GENUS:
    source.value.impr_genus = value;
    return source;
  case VUT_IMPR_FLAG:
    source.value.impr_flag = value;
    return source;
  case VUT_PLAYER_FLAG:
    source.value.plr_flag = value;
    return source;
  case VUT_EXTRA:
    source.value.extra = extra_by_number(value);
    return source;
  case VUT_TILEDEF:
    source.value.tiledef = tiledef_by_number(value);
    return source;
  case VUT_GOOD:
    source.value.good = goods_by_number(value);
    return source;
  case VUT_TERRAIN:
    source.value.terrain = terrain_by_number(value);
    if (source.value.terrain != nullptr) {
      return source;
    }
    break;
  case VUT_TERRFLAG:
    source.value.terrainflag = value;
    return source;
  case VUT_NATION:
    source.value.nation = nation_by_number(value);
    if (source.value.nation != nullptr) {
      return source;
    }
    break;
  case VUT_NATIONGROUP:
    source.value.nationgroup = nation_group_by_number(value);
    if (source.value.nationgroup != nullptr) {
      return source;
    }
    break;
  case VUT_DIPLREL:
  case VUT_DIPLREL_TILE:
  case VUT_DIPLREL_TILE_O:
  case VUT_DIPLREL_UNITANY:
  case VUT_DIPLREL_UNITANY_O:
    source.value.diplrel = value;
    if (source.value.diplrel != diplrel_other_invalid()) {
      return source;
    }
    break;
  case VUT_NATIONALITY:
    source.value.nationality = nation_by_number(value);
    if (source.value.nationality != nullptr) {
      return source;
    }
    break;
  case VUT_ORIGINAL_OWNER:
    source.value.origowner = nation_by_number(value);
    if (source.value.origowner != nullptr) {
      return source;
    }
    break;
  case VUT_UTYPE:
    source.value.utype = utype_by_number(value);
    if (source.value.utype != nullptr) {
      return source;
    }
    break;
  case VUT_UTFLAG:
    source.value.unitflag = value;
    return source;
  case VUT_UCLASS:
    source.value.uclass = uclass_by_number(value);
    if (source.value.uclass != nullptr) {
      return source;
    }
    break;
  case VUT_UCFLAG:
    source.value.unitclassflag = value;
    return source;
  case VUT_MINVETERAN:
    source.value.minveteran = value;
    return source;
  case VUT_UNITSTATE:
    source.value.unit_state = value;
    return source;
  case VUT_ACTIVITY:
    source.value.activity = value;
    return source;
  case VUT_MINMOVES:
    source.value.minmoves = value;
    return source;
  case VUT_MINHP:
    source.value.min_hit_points = value;
    return source;
  case VUT_AGE:
    source.value.age = value;
    return source;
  case VUT_FORM_AGE:
    source.value.form_age = value;
    return source;
  case VUT_MINTECHS:
    source.value.min_techs = value;
    return source;
  case VUT_FUTURETECHS:
    source.value.future_techs = value;
    return source;
  case VUT_MINCITIES:
    source.value.min_cities = value;
    return source;
  case VUT_ACTION:
    source.value.action = action_by_number(value);
    if (source.value.action != nullptr) {
      return source;
    }
    break;
  case VUT_OTYPE:
    source.value.outputtype = value;
    return source;
  case VUT_SPECIALIST:
    source.value.specialist = specialist_by_number(value);
    return source;
  case VUT_MINSIZE:
    source.value.minsize = value;
    return source;
  case VUT_MINCULTURE:
    source.value.minculture = value;
    return source;
  case VUT_MINFOREIGNPCT:
    source.value.minforeignpct = value;
    return source;
  case VUT_AI_LEVEL:
    source.value.ai_level = value;
    return source;
  case VUT_MAXTILETOTALUNITS:
    source.value.max_tile_total_units = value;
    return source;
  case VUT_MAXTILETOPUNITS:
    source.value.max_tile_top_units = value;
    return source;
  case VUT_TERRAINCLASS:
    source.value.terrainclass = value;
    return source;
  case VUT_ROADFLAG:
    source.value.roadflag = value;
    return source;
  case VUT_EXTRAFLAG:
    source.value.extraflag = value;
    return source;
  case VUT_MINYEAR:
    source.value.minyear = value;
    return source;
  case VUT_MINCALFRAG:
    source.value.mincalfrag = value;
    return source;
  case VUT_TOPO:
    source.value.topo_property = value;
    return source;
  case VUT_WRAP:
    source.value.wrap_property = value;
    return source;
  case VUT_SERVERSETTING:
    source.value.ssetval = value;
    return source;
  case VUT_TERRAINALTER:
    source.value.terrainalter = value;
    return source;
  case VUT_CITYTILE:
    source.value.citytile = value;
    return source;
  case VUT_CITYSTATUS:
    source.value.citystatus = value;
    return source;
  case VUT_PLAYER_STATE:
    source.value.plrstate = value;
    return source;
  case VUT_COUNTER:
    source.value.counter = counter_by_id(value);
    return source;
  case VUT_MINLATITUDE:
  case VUT_MAXLATITUDE:
    source.value.latitude = value;
    return source;
  case VUT_MAX_DISTANCE_SQ:
    source.value.distance_sq = value;
    return source;
  case VUT_MAX_REGION_TILES:
    source.value.region_tiles = value;
    return source;
  case VUT_TILE_REL:
    source.value.tilerel = value;
    return source;
  case VUT_COUNT:
    break;
  }
 
  /* If we reach here there's been an error. */
  source.kind = universals_n_invalid();
  /* Avoid compiler warning about uninitialized source.value */
  source.value.advance = nullptr;
 
  return source;
}
 
/**********************************************************************/
void universal_copy(struct universal *dst, const struct universal *src)
{
  dst->value = src->value;
  dst->kind = src->kind;
}
 
/**********************************************************************/
void universal_extraction(const struct universal *source,
                          int *kind, int *value)
{
  *kind = source->kind;
  *value = universal_number(source);
}
 
/**********************************************************************/
int universal_number(const struct universal *source)
{
  switch (source->kind) {
  case VUT_NONE:
    return 0;
  case VUT_ADVANCE:
    return advance_number(source->value.advance);
  case VUT_TECHFLAG:
    return source->value.techflag;
  case VUT_GOVERNMENT:
    return government_number(source->value.govern);
  case VUT_GOVFLAG:
    return source->value.govflag;
  case VUT_ACHIEVEMENT:
    return achievement_number(source->value.achievement);
  case VUT_STYLE:
    return style_number(source->value.style);
  case VUT_IMPROVEMENT:
  case VUT_SITE:
    return improvement_number(source->value.building);
  case VUT_IMPR_GENUS:
    return source->value.impr_genus;
  case VUT_IMPR_FLAG:
    return source->value.impr_flag;
  case VUT_PLAYER_FLAG:
    return source->value.plr_flag;
  case VUT_EXTRA:
    return extra_number(source->value.extra);
  case VUT_TILEDEF:
    return tiledef_number(source->value.tiledef);
  case VUT_GOOD:
    return goods_number(source->value.good);
  case VUT_TERRAIN:
    return terrain_number(source->value.terrain);
  case VUT_TERRFLAG:
    return source->value.terrainflag;
  case VUT_NATION:
    return nation_number(source->value.nation);
  case VUT_NATIONGROUP:
    return nation_group_number(source->value.nationgroup);
  case VUT_NATIONALITY:
    return nation_number(source->value.nationality);
  case VUT_ORIGINAL_OWNER:
    return nation_number(source->value.origowner);
  case VUT_DIPLREL:
  case VUT_DIPLREL_TILE:
  case VUT_DIPLREL_TILE_O:
  case VUT_DIPLREL_UNITANY:
  case VUT_DIPLREL_UNITANY_O:
    return source->value.diplrel;
  case VUT_UTYPE:
    return utype_number(source->value.utype);
  case VUT_UTFLAG:
    return source->value.unitflag;
  case VUT_UCLASS:
    return uclass_number(source->value.uclass);
  case VUT_UCFLAG:
    return source->value.unitclassflag;
  case VUT_MINVETERAN:
    return source->value.minveteran;
  case VUT_UNITSTATE:
    return source->value.unit_state;
  case VUT_ACTIVITY:
    return source->value.activity;
  case VUT_MINMOVES:
    return source->value.minmoves;
  case VUT_MINHP:
    return source->value.min_hit_points;
  case VUT_AGE:
    return source->value.age;
  case VUT_FORM_AGE:
    return source->value.form_age;
  case VUT_MINTECHS:
    return source->value.min_techs;
  case VUT_FUTURETECHS:
    return source->value.future_techs;
  case VUT_MINCITIES:
    return source->value.min_cities;
  case VUT_ACTION:
    return action_number(source->value.action);
  case VUT_OTYPE:
    return source->value.outputtype;
  case VUT_SPECIALIST:
    return specialist_number(source->value.specialist);
  case VUT_MINSIZE:
    return source->value.minsize;
  case VUT_MINCULTURE:
    return source->value.minculture;
  case VUT_MINFOREIGNPCT:
    return source->value.minforeignpct;
  case VUT_AI_LEVEL:
    return source->value.ai_level;
  case VUT_MAXTILETOTALUNITS:
    return source->value.max_tile_total_units;
  case VUT_MAXTILETOPUNITS:
    return source->value.max_tile_top_units;
  case VUT_TERRAINCLASS:
    return source->value.terrainclass;
   case VUT_ROADFLAG:
    return source->value.roadflag;
  case VUT_EXTRAFLAG:
    return source->value.extraflag;
  case VUT_MINYEAR:
    return source->value.minyear;
  case VUT_MINCALFRAG:
    return source->value.mincalfrag;
  case VUT_TOPO:
    return source->value.topo_property;
  case VUT_WRAP:
    return source->value.wrap_property;
  case VUT_SERVERSETTING:
    return source->value.ssetval;
  case VUT_TERRAINALTER:
    return source->value.terrainalter;
  case VUT_CITYTILE:
    return source->value.citytile;
  case VUT_CITYSTATUS:
    return source->value.citystatus;
  case VUT_PLAYER_STATE:
    return source->value.plrstate;
  case VUT_COUNTER:
    return counter_id(source->value.counter);
  case VUT_MINLATITUDE:
  case VUT_MAXLATITUDE:
    return source->value.latitude;
  case VUT_MAX_DISTANCE_SQ:
    return source->value.distance_sq;
  case VUT_MAX_REGION_TILES:
    return source->value.region_tiles;
  case VUT_TILE_REL:
    return source->value.tilerel;
  case VUT_COUNT:
    break;
  }
 
  /* If we reach here there's been an error. */
  fc_assert_msg(FALSE, "universal_number(): invalid source kind %d.",
                source->kind);
  return 0;
}
 
 
/**********************************************************************/
const struct req_context *req_context_empty(void)
{
  static const struct req_context empty = {};
  return &empty;
}
 
 
/**********************************************************************/
const char *req_to_fstring(const struct requirement *req,
                           struct astring *astr)
{
  astr_init(astr);
 
  astr_set(astr, "%s%s %s %s%s",
           req->survives ? "surviving " : "",
           req_range_name(req->range),
           universal_type_rule_name(&req->source),
           req->present ? "" : "!",
           universal_rule_name(&req->source));
 
  return astr_str(astr);
}
 
/**********************************************************************/
struct requirement req_from_str(const char *type, const char *range,
                                bool survives, bool present, bool quiet,
                                const char *value)
{
  struct requirement req;
  bool invalid;
  const char *error = nullptr;
 
  req.source = universal_by_rule_name(type, value);
 
  invalid = !universals_n_is_valid(req.source.kind);
  if (invalid) {
    error = "bad type or name";
  } else {
    /* Scan the range string to find the range. If no range is given a
     * default fallback is used rather than giving an error. */
    if (range != nullptr) {
      req.range = req_range_by_name(range, fc_strcasecmp);
      if (!req_range_is_valid(req.range)) {
        invalid = TRUE;
      }
    } else {
      switch (req.source.kind) {
      case VUT_NONE:
      case VUT_COUNT:
        break;
      case VUT_IMPROVEMENT:
      case VUT_SITE:
      case VUT_IMPR_GENUS:
      case VUT_IMPR_FLAG:
      case VUT_UTYPE:
      case VUT_UTFLAG:
      case VUT_UCLASS:
      case VUT_UCFLAG:
      case VUT_MINVETERAN:
      case VUT_UNITSTATE:
      case VUT_ACTIVITY:
      case VUT_MINMOVES:
      case VUT_MINHP:
      case VUT_AGE:
      case VUT_FORM_AGE:
      case VUT_ACTION:
      case VUT_OTYPE:
      case VUT_SPECIALIST:
      case VUT_DIPLREL_TILE_O:
      case VUT_DIPLREL_UNITANY_O:
        req.range = REQ_RANGE_LOCAL;
        break;
      case VUT_EXTRA:
      case VUT_TILEDEF:
      case VUT_ROADFLAG:
      case VUT_EXTRAFLAG:
        /* Keep old behavior */
        req.range = REQ_RANGE_TILE;
        break;
      case VUT_TERRAIN:
      case VUT_TERRFLAG:
      case VUT_TERRAINCLASS:
      case VUT_TERRAINALTER:
      case VUT_CITYTILE:
      case VUT_MAXTILETOTALUNITS:
      case VUT_MAXTILETOPUNITS:
      case VUT_MINLATITUDE:
      case VUT_MAXLATITUDE:
      case VUT_MAX_DISTANCE_SQ:
        req.range = REQ_RANGE_TILE;
        break;
      case VUT_COUNTER:
      case VUT_MINSIZE:
      case VUT_MINCULTURE:
      case VUT_MINFOREIGNPCT:
      case VUT_NATIONALITY:
      case VUT_ORIGINAL_OWNER:
      case VUT_CITYSTATUS:
      case VUT_GOOD:
        req.range = REQ_RANGE_CITY;
        break;
      case VUT_GOVERNMENT:
      case VUT_GOVFLAG:
      case VUT_ACHIEVEMENT:
      case VUT_STYLE:
      case VUT_ADVANCE:
      case VUT_TECHFLAG:
      case VUT_NATION:
      case VUT_NATIONGROUP:
      case VUT_DIPLREL:
      case VUT_DIPLREL_TILE:
      case VUT_DIPLREL_UNITANY:
      case VUT_AI_LEVEL:
      case VUT_PLAYER_FLAG:
      case VUT_PLAYER_STATE:
      case VUT_MINCITIES:
      case VUT_FUTURETECHS:
        req.range = REQ_RANGE_PLAYER;
        break;
      case VUT_MINYEAR:
      case VUT_MINCALFRAG:
      case VUT_TOPO:
      case VUT_WRAP:
      case VUT_MINTECHS:
      case VUT_SERVERSETTING:
        req.range = REQ_RANGE_WORLD;
        break;
      case VUT_MAX_REGION_TILES:
        req.range = REQ_RANGE_CONTINENT;
        break;
      case VUT_TILE_REL:
        req.range = REQ_RANGE_TILE;
        if (req.source.value.tilerel == TREL_ONLY_OTHER_REGION) {
          /* Not available at Tile range */
          req.range = REQ_RANGE_ADJACENT;
        }
        break;
      }
    }
 
    req.survives = survives;
    req.present = present;
    req.quiet = quiet;
 
    /* These checks match what combinations are supported inside
     * is_req_active(). However, it's only possible to do basic checks,
     * not anything that might depend on the rest of the ruleset which
     * might not have been loaded yet. */
    switch (req.source.kind) {
    case VUT_TERRAIN:
    case VUT_TERRAINCLASS:
    case VUT_TERRFLAG:
      invalid = (req.range != REQ_RANGE_TILE
                 && req.range != REQ_RANGE_CADJACENT
                 && req.range != REQ_RANGE_ADJACENT
                 && req.range != REQ_RANGE_CITY
                 && req.range != REQ_RANGE_TRADE_ROUTE);
      break;
    case VUT_EXTRA:
    case VUT_ROADFLAG:
    case VUT_EXTRAFLAG:
      invalid = (req.range > REQ_RANGE_TRADE_ROUTE);
      break;
    case VUT_TILEDEF:
      invalid = (req.range > REQ_RANGE_TRADE_ROUTE
                 || req.range == REQ_RANGE_LOCAL);
      break;
    case VUT_ACHIEVEMENT:
    case VUT_MINTECHS:
    case VUT_FUTURETECHS:
      invalid = (req.range < REQ_RANGE_PLAYER);
      break;
    case VUT_ADVANCE:
    case VUT_TECHFLAG:
      invalid = (req.range < REQ_RANGE_PLAYER
                 && req.range != REQ_RANGE_LOCAL);
      break;
    case VUT_GOVERNMENT:
    case VUT_GOVFLAG:
    case VUT_AI_LEVEL:
    case VUT_STYLE:
    case VUT_MINCITIES:
      invalid = (req.range != REQ_RANGE_PLAYER);
      break;
    case VUT_MINSIZE:
    case VUT_MINFOREIGNPCT:
    case VUT_NATIONALITY:
    case VUT_CITYSTATUS:
      invalid = (req.range != REQ_RANGE_CITY
                 && req.range != REQ_RANGE_TRADE_ROUTE);
      break;
    case VUT_GOOD:
    case VUT_ORIGINAL_OWNER:
      invalid = (req.range != REQ_RANGE_CITY);
      break;
    case VUT_MINCULTURE:
      invalid = (req.range != REQ_RANGE_CITY
                 && req.range != REQ_RANGE_TRADE_ROUTE
                 && req.range != REQ_RANGE_PLAYER
                 && req.range != REQ_RANGE_TEAM
                 && req.range != REQ_RANGE_ALLIANCE
                 && req.range != REQ_RANGE_WORLD);
      break;
    case VUT_DIPLREL:
      invalid = (req.range != REQ_RANGE_LOCAL
                 && req.range != REQ_RANGE_PLAYER
                 && req.range != REQ_RANGE_TEAM
                 && req.range != REQ_RANGE_ALLIANCE
                 && req.range != REQ_RANGE_WORLD)
                /* Non local foreign makes no sense. */
                || (req.source.value.diplrel == DRO_FOREIGN
                    && req.range != REQ_RANGE_LOCAL);
      break;
    case VUT_DIPLREL_TILE:
      invalid = (req.range != REQ_RANGE_LOCAL
                 && req.range != REQ_RANGE_PLAYER
                 && req.range != REQ_RANGE_TEAM
                 && req.range != REQ_RANGE_ALLIANCE)
                /* Non local foreign makes no sense. */
                || (req.source.value.diplrel == DRO_FOREIGN
                    && req.range != REQ_RANGE_LOCAL);
      break;
    case VUT_DIPLREL_TILE_O:
      invalid = (req.range != REQ_RANGE_LOCAL);
      break;
    case VUT_DIPLREL_UNITANY:
      invalid = (req.range != REQ_RANGE_LOCAL
                 && req.range != REQ_RANGE_PLAYER
                 && req.range != REQ_RANGE_TEAM
                 && req.range != REQ_RANGE_ALLIANCE)
                /* Non local foreign makes no sense. */
                || (req.source.value.diplrel == DRO_FOREIGN
                    && req.range != REQ_RANGE_LOCAL);
      break;
    case VUT_DIPLREL_UNITANY_O:
      invalid = (req.range != REQ_RANGE_LOCAL);
      break;
    case VUT_NATION:
    case VUT_NATIONGROUP:
      invalid = (req.range != REQ_RANGE_PLAYER
                 && req.range != REQ_RANGE_TEAM
                 && req.range != REQ_RANGE_ALLIANCE
                 && req.range != REQ_RANGE_WORLD);
      break;
    case VUT_MINVETERAN:
    case VUT_UNITSTATE:
    case VUT_ACTIVITY:
    case VUT_MINMOVES:
    case VUT_MINHP:
    case VUT_ACTION:
    case VUT_OTYPE:
    case VUT_SPECIALIST:
      invalid = (req.range != REQ_RANGE_LOCAL);
      break;
    case VUT_UTYPE:
    case VUT_UTFLAG:
    case VUT_UCLASS:
    case VUT_UCFLAG:
      invalid = (req.range != REQ_RANGE_LOCAL
                 && req.range != REQ_RANGE_TILE
                 && req.range != REQ_RANGE_CADJACENT
                 && req.range != REQ_RANGE_ADJACENT);
      break;
    case VUT_TERRAINALTER: /* XXX could in principle support C/ADJACENT */
    case VUT_MAX_DISTANCE_SQ:
      invalid = (req.range != REQ_RANGE_TILE);
      break;
    case VUT_CITYTILE:
    case VUT_MAXTILETOTALUNITS:
    case VUT_MAXTILETOPUNITS:
      invalid = (req.range != REQ_RANGE_TILE
                 && req.range != REQ_RANGE_CADJACENT
                 && req.range != REQ_RANGE_ADJACENT);
      break;
    case VUT_MINLATITUDE:
    case VUT_MAXLATITUDE:
      invalid = (req.range != REQ_RANGE_TILE
                 && req.range != REQ_RANGE_CADJACENT
                 && req.range != REQ_RANGE_ADJACENT
                 && req.range != REQ_RANGE_WORLD)
                /* Avoid redundancy at tile range: no negated requirements
                 * that could be emulated by a present requirement of the
                 * other type */
                || (req.range == REQ_RANGE_TILE && !req.present);
      break;
    case VUT_MINYEAR:
    case VUT_MINCALFRAG:
    case VUT_TOPO:
    case VUT_WRAP:
    case VUT_SERVERSETTING:
      invalid = (req.range != REQ_RANGE_WORLD);
      break;
    case VUT_AGE:
      /* FIXME: Could support TRADE_ROUTE, TEAM, etc */
      invalid = (req.range != REQ_RANGE_LOCAL
                 && req.range != REQ_RANGE_CITY
                 && req.range != REQ_RANGE_PLAYER);
      break;
    case VUT_FORM_AGE:
      invalid = (req.range != REQ_RANGE_LOCAL);
      break;
    case VUT_IMPR_GENUS:
      /* TODO: Support other ranges too. */
      invalid = (req.range != REQ_RANGE_LOCAL);
      break;
    case VUT_IMPR_FLAG:
      invalid = (req.range != REQ_RANGE_LOCAL
                 && req.range != REQ_RANGE_TILE
                 && req.range != REQ_RANGE_CITY);
      break;
    case VUT_COUNTER:
      invalid = req.range != REQ_RANGE_CITY;
      break;
    case VUT_PLAYER_FLAG:
    case VUT_PLAYER_STATE:
      invalid = (req.range != REQ_RANGE_PLAYER);
      break;
    case VUT_MAX_REGION_TILES:
      invalid = (req.range != REQ_RANGE_CONTINENT
                 && req.range != REQ_RANGE_CADJACENT
                 && req.range != REQ_RANGE_ADJACENT);
      break;
    case VUT_TILE_REL:
      invalid = (req.range != REQ_RANGE_ADJACENT
                 && req.range != REQ_RANGE_CADJACENT
                 && req.range != REQ_RANGE_TILE)
                /* TREL_ONLY_OTHER_REGION not supported at Tile range */
                || (req.source.value.tilerel == TREL_ONLY_OTHER_REGION
                    && req.range == REQ_RANGE_TILE);
      break;
    case VUT_IMPROVEMENT:
    case VUT_SITE:
      /* Valid ranges depend on the building genus (wonder/improvement),
       * which might not have been loaded from the ruleset yet.
       * So we allow anything here, and do a proper check once ruleset
       * loading is complete, in sanity_check_req_individual(). */
    case VUT_NONE:
      invalid = FALSE;
      break;
    case VUT_COUNT:
      break;
    }
    if (invalid) {
      error = "bad range";
    }
  }
 
  if (!invalid) {
    /* Check 'survives'. */
    switch (req.source.kind) {
    case VUT_IMPROVEMENT:
    case VUT_SITE:
      /* See buildings_in_range(). */
      invalid = survives && req.range <= REQ_RANGE_CONTINENT;
      break;
    case VUT_NATION:
    case VUT_ADVANCE:
      invalid = survives && req.range != REQ_RANGE_WORLD;
      break;
    case VUT_COUNTER:
    case VUT_IMPR_GENUS:
    case VUT_IMPR_FLAG:
    case VUT_PLAYER_FLAG:
    case VUT_PLAYER_STATE:
    case VUT_GOVERNMENT:
    case VUT_GOVFLAG:
    case VUT_TERRAIN:
    case VUT_UTYPE:
    case VUT_UTFLAG:
    case VUT_UCLASS:
    case VUT_UCFLAG:
    case VUT_MINVETERAN:
    case VUT_UNITSTATE:
    case VUT_ACTIVITY:
    case VUT_MINMOVES:
    case VUT_MINHP:
    case VUT_AGE:
    case VUT_FORM_AGE:
    case VUT_ACTION:
    case VUT_OTYPE:
    case VUT_SPECIALIST:
    case VUT_MINSIZE:
    case VUT_MINCULTURE:
    case VUT_MINFOREIGNPCT:
    case VUT_AI_LEVEL:
    case VUT_TERRAINCLASS:
    case VUT_MINYEAR:
    case VUT_MINCALFRAG:
    case VUT_TOPO:
    case VUT_WRAP:
    case VUT_SERVERSETTING:
    case VUT_TERRAINALTER:
    case VUT_CITYTILE:
    case VUT_CITYSTATUS:
    case VUT_TERRFLAG:
    case VUT_NATIONALITY:
    case VUT_ORIGINAL_OWNER:
    case VUT_ROADFLAG:
    case VUT_EXTRAFLAG:
    case VUT_EXTRA:
    case VUT_TILEDEF:
    case VUT_GOOD:
    case VUT_TECHFLAG:
    case VUT_ACHIEVEMENT:
    case VUT_NATIONGROUP:
    case VUT_STYLE:
    case VUT_DIPLREL:
    case VUT_DIPLREL_TILE:
    case VUT_DIPLREL_TILE_O:
    case VUT_DIPLREL_UNITANY:
    case VUT_DIPLREL_UNITANY_O:
    case VUT_MAXTILETOTALUNITS:
    case VUT_MAXTILETOPUNITS:
    case VUT_MINTECHS:
    case VUT_FUTURETECHS:
    case VUT_MINCITIES:
    case VUT_MINLATITUDE:
    case VUT_MAXLATITUDE:
    case VUT_MAX_DISTANCE_SQ:
    case VUT_MAX_REGION_TILES:
    case VUT_TILE_REL:
      /* Most requirements don't support 'survives'. */
      invalid = survives;
      break;
    case VUT_NONE:
    case VUT_COUNT:
      break;
    }
    if (invalid) {
      error = "bad 'survives'";
    }
  }
 
  if (invalid) {
    log_error("Invalid requirement %s | %s | %s | %s | %s: %s",
              type, range, survives ? "survives" : "",
              present ? "present" : "", value, error);
    req.source.kind = universals_n_invalid();
  }
 
  return req;
}
 
/**********************************************************************/
struct requirement req_from_values(int type, int range,
                                   bool survives, bool present, bool quiet,
                                   int value)
{
  struct requirement req;
 
  req.source = universal_by_number(type, value);
  req.range = range;
  req.survives = survives;
  req.present = present;
  req.quiet = quiet;
 
  return req;
}
 
/**********************************************************************/
void req_get_values(const struct requirement *req,
                    int *type, int *range,
                    bool *survives, bool *present, bool *quiet,
                    int *value)
{
  universal_extraction(&req->source, type, value);
  *range = req->range;
  *survives = req->survives;
  *present = req->present;
  *quiet = req->quiet;
}
 
/**********************************************************************/
void req_copy(struct requirement *dst, const struct requirement *src)
{
  universal_copy(&(dst->source), &(src->source));
  dst->range = src->range;
  dst->survives = src->survives;
  dst->present = src->present;
  dst->quiet = src->quiet;
}
 
/**********************************************************************/
bool are_requirements_equal(const struct requirement *req1,
                            const struct requirement *req2)
{
  return (are_universals_equal(&req1->source, &req2->source)
          && req1->range == req2->range
          && req1->survives == req2->survives
          && req1->present == req2->present);
}
 
/**********************************************************************/
static bool are_requirements_opposites(const struct requirement *req1,
                                       const struct requirement *req2)
{
  return (are_universals_equal(&req1->source, &req2->source)
          && req1->range == req2->range
          && req1->survives == req2->survives
          && req1->present != req2->present);
}
 
/**********************************************************************/
static bool impr_contra_genus(const struct requirement *impr_req,
                              const struct requirement *genus_req)
{
  /* The input is sane. */
  fc_assert_ret_val(impr_req->source.kind == VUT_IMPROVEMENT
                    || impr_req->source.kind == VUT_SITE, FALSE);
  fc_assert_ret_val(genus_req->source.kind == VUT_IMPR_GENUS, FALSE);
 
  if (impr_req->range == REQ_RANGE_LOCAL
      && genus_req->range == REQ_RANGE_LOCAL) {
    /* Applies to the same target building. */
 
    if (impr_req->present && !genus_req->present) {
      /* The target building can't not have the genus it has. */
      return (impr_req->source.value.building->genus
              == genus_req->source.value.impr_genus);
    }
 
    if (impr_req->present && genus_req->present) {
      /* The target building can't have another genus than it has. */
      return (impr_req->source.value.building->genus
              != genus_req->source.value.impr_genus);
    }
  }
 
  /* No special knowledge. */
  return FALSE;
}
 
/**********************************************************************/
static bool impr_contra_flag(const struct requirement *impr_req,
                             const struct requirement *flag_req)
{
  /* The input is sane. */
  fc_assert_ret_val(impr_req->source.kind == VUT_IMPROVEMENT
                    || impr_req->source.kind == VUT_SITE, FALSE);
  fc_assert_ret_val(flag_req->source.kind == VUT_IMPR_FLAG, FALSE);
 
  if (impr_req->range == REQ_RANGE_LOCAL
      && flag_req->range == REQ_RANGE_LOCAL) {
    /* Applies to the same target building. */
 
    if (impr_req->present && !flag_req->present) {
      /* The target building can't not have the flag it has. */
      return improvement_has_flag(impr_req->source.value.building,
                                  flag_req->source.value.impr_flag);
    }
 
    if (impr_req->present && flag_req->present) {
      /* The target building can't have another flag than it has. */
      return !improvement_has_flag(impr_req->source.value.building,
                                   flag_req->source.value.impr_flag);
    }
  }
 
  /* No special knowledge. */
  return FALSE;
}
 
/**********************************************************************/
static bool nation_contra_group(const struct requirement *nation_req,
                                const struct requirement *group_req)
{
  /* The input is sane. */
  fc_assert_ret_val(nation_req->source.kind == VUT_NATION, FALSE);
  fc_assert_ret_val(group_req->source.kind == VUT_NATIONGROUP, FALSE);
 
  if (nation_req->range == REQ_RANGE_PLAYER
      && group_req->range == REQ_RANGE_PLAYER) {
    /* Applies to the same target building. */
 
    if (nation_req->present && !group_req->present) {
      /* The target nation can't be in the group. */
      return nation_is_in_group(nation_req->source.value.nation,
                                group_req->source.value.nationgroup);
    }
  }
 
  /* No special knowledge. */
  return FALSE;
}
 
/**********************************************************************/
static bool city_center_contra(const struct requirement *cc_req,
                               const struct requirement *ct_req)
{
  /* The input is sane. */
  fc_assert_ret_val(cc_req->source.kind == VUT_CITYTILE
                    && ct_req->source.kind == VUT_CITYTILE, FALSE);
 
  if (cc_req->source.value.citytile == CITYT_CENTER
      && cc_req->present && cc_req->range <= ct_req->range) {
    switch (ct_req->source.value.citytile) {
    case CITYT_CENTER:
    case CITYT_CLAIMED:
    case CITYT_EXTRAS_OWNED:
    case CITYT_WORKED:
    case CITYT_SAME_CONTINENT:
      /* Should be always on city center */
      return !ct_req->present;
    case CITYT_BORDERING_TCLASS_REGION:
      /* Handled later */
      break;
    case CITYT_LAST:
      /* Error */
      fc_assert_ret_val(ct_req->source.value.citytile != CITYT_LAST, FALSE);
    }
  }
  if ((cc_req->source.value.citytile == CITYT_SAME_CONTINENT
       || cc_req->source.value.citytile == CITYT_CENTER)
      && ct_req->source.value.citytile
         == CITYT_BORDERING_TCLASS_REGION
      && REQ_RANGE_TILE == cc_req->range
      && REQ_RANGE_TILE == ct_req->range) {
    /* Can't coexist */
    return cc_req->present ? ct_req->present : !ct_req->present;
  }
 
  return FALSE;
}
 
/**********************************************************************/
static bool present_implies_not_present(const struct requirement *req1,
                                        const struct requirement *req2)
{
  const struct requirement *absent, *present;
 
  if (req1->present == req2->present) {
    /* Can't use the knowledge in universal_found_function when both are
     * required to be absent or when both are required to be present.
     * It is no contradiction to require !Spy unit and !Missile unit class.
     * It is no contradiction to require River and Irrigation at the same
     * tile. */
    return FALSE;
  }
 
  if (req1->present) {
    absent = req2;
    present = req1;
  } else {
    absent = req1;
    present = req2;
  }
 
  if (!universal_found_function[present->source.kind]) {
    /* No knowledge to exploit. */
    return FALSE;
  }
 
  if (present->range != absent->range) {
    /* Larger ranges are not always strict supersets of smaller ranges.
     * Example: Trade Route > CAdjacent but something may be in CAdjacent
     * but not in Trade Route. */
    return FALSE;
  }
 
  return ITF_YES == universal_fulfills_requirement(absent,
                                                   &present->source);
}
 
/**********************************************************************/
bool req_implies_req(const struct requirement *req1,
                     const struct requirement *req2)
{
  struct requirement nreq2;
 
  req_copy(&nreq2, req2);
  nreq2.present = !nreq2.present;
  return are_requirements_contradictions(req1, &nreq2);
}
 
/**********************************************************************/
static inline bool are_bounds_contradictions(int bound1, bool is_upper1,
                                             int bound2, bool is_upper2)
{
  /* If the bounds are on opposite sides, and one is inclusive, the other
   * exclusive, the number of values that satisfy both bounds is exactly
   * their difference, (upper bound) - (lower bound).
   * The bounds contradict each other iff this difference is 0 or less,
   * i.e. iff (upper bound) <= (lower bound) */
  if (is_upper1 && !is_upper2) {
    return bound1 <= bound2;
  } else if (!is_upper1 && is_upper2) {
    return bound1 >= bound2;
  }
  /* Both are upper or both are lower ~> no contradiction possible */
  return FALSE;
}
 
/**********************************************************************/
bool are_requirements_contradictions(const struct requirement *req1,
                                     const struct requirement *req2)
{
  if (are_requirements_opposites(req1, req2)) {
    /* The exact opposite. */
    return TRUE;
  }
 
  if (present_implies_not_present(req1, req2)) {
    return TRUE;
  }
 
  switch (req1->source.kind) {
  case VUT_IMPROVEMENT:
  case VUT_SITE:
    if (req2->source.kind == VUT_IMPR_GENUS) {
      return impr_contra_genus(req1, req2);
    } else if (req2->source.kind == VUT_IMPR_FLAG) {
      return impr_contra_flag(req1, req2);
    } else if (req2->source.kind == VUT_CITYTILE
               && req2->source.value.citytile == CITYT_CENTER
               && REQ_RANGE_TILE == req2->range
               && REQ_RANGE_TILE == req1->range
               && req1->present) {
      /* A building must be in a city */
      return !req2->present;
    }
 
    /* No special knowledge. */
    return FALSE;
  case VUT_IMPR_GENUS:
    if (req2->source.kind == VUT_IMPROVEMENT
        || req2->source.kind == VUT_SITE) {
      return impr_contra_genus(req2, req1);
    }
 
    /* No special knowledge. */
    return FALSE;
  case VUT_IMPR_FLAG:
    if (req2->source.kind == VUT_IMPROVEMENT
        || req2->source.kind == VUT_SITE) {
      return impr_contra_flag(req2, req1);
    }
 
    /* No special knowledge. */
    return FALSE;
  case VUT_DIPLREL:
  case VUT_DIPLREL_TILE:
  case VUT_DIPLREL_TILE_O:
  case VUT_DIPLREL_UNITANY:
  case VUT_DIPLREL_UNITANY_O:
    if (req2->source.kind != req1->source.kind) {
      /* Finding contradictions across requirement kinds aren't supported
       * for DiplRel requirements. */
      return FALSE;
    } else {
      /* Use the special knowledge about DiplRel requirements to find
       * contradictions. */
 
      bv_diplrel_all_reqs req1_contra;
      int req2_pos;
 
      req1_contra = diplrel_req_contradicts(req1);
      req2_pos = requirement_diplrel_ereq(req2->source.value.diplrel,
                                          req2->range,
                                          req2->present);
 
      return BV_ISSET(req1_contra, req2_pos);
    }
    break;
  case VUT_MINMOVES:
    if (req2->source.kind != VUT_MINMOVES) {
      /* Finding contradictions across requirement kinds aren't supported
       * for MinMoveFrags requirements. */
      return FALSE;
    }
    return are_bounds_contradictions(
        req1->source.value.minmoves, !req1->present,
        req2->source.value.minmoves, !req2->present);
  case VUT_MINLATITUDE:
  case VUT_MAXLATITUDE:
    if (req2->source.kind != VUT_MINLATITUDE
        && req2->source.kind != VUT_MAXLATITUDE) {
      /* Finding contradictions across requirement kinds other than each
       * other is not supported for MinLatitude and MaxLatitude. */
      return FALSE;
    } else {
      /* For a contradiction, we need
       * - a minimum (present MinLatitude or negated MaxLatitude)
       * - a maximum (negated MinLatitude or present MaxLatitude)
       * - the maximum to be less than the minimum
       * - a requirement at the larger range that applies to the entire
       *   range (i.e. a negated requirement, unless the range is Tile)
       *   Otherwise, the two requirements could still be fulfilled
       *   simultaneously by different tiles in the range */
 
      /* Initial values beyond the boundaries to avoid edge cases */
      int minimum = -MAP_MAX_LATITUDE - 1, maximum = MAP_MAX_LATITUDE + 1;
      enum req_range covered_range = REQ_RANGE_TILE;
 
#define EXTRACT_INFO(req)                                                 \
      if (req->present) {                                                 \
        if (req->source.kind == VUT_MINLATITUDE) {                        \
          /* present MinLatitude */                                       \
          minimum = MAX(minimum, req->source.value.latitude);             \
        } else {                                                          \
          /* present MaxLatitude */                                       \
          maximum = MIN(maximum, req->source.value.latitude);             \
        }                                                                 \
      } else {                                                            \
        covered_range = MAX(covered_range, req->range);                   \
        if (req->source.kind == VUT_MINLATITUDE) {                        \
          /* negated MinLatitude */                                       \
          maximum = MIN(maximum, req->source.value.latitude - 1);         \
        } else {                                                          \
          /* negated MaxLatitude */                                       \
          minimum = MAX(minimum, req->source.value.latitude + 1);         \
        }                                                                 \
      }
 
      EXTRACT_INFO(req1);
      EXTRACT_INFO(req2);
 
#undef EXTRACT_INFO
 
      return (maximum < minimum
              && covered_range >= req1->range
              && covered_range >= req2->range);
    }
    break;
  case VUT_NATION:
    if (req2->source.kind == VUT_NATIONGROUP) {
      return nation_contra_group(req1, req2);
    }
 
    /* No special knowledge. */
    return FALSE;
    break;
  case VUT_NATIONGROUP:
    if (req2->source.kind == VUT_NATION) {
      return nation_contra_group(req2, req1);
    }
 
    /* No special knowledge. */
    return FALSE;
    break;
  case VUT_CITYTILE:
    if (req2->source.kind == VUT_CITYTILE) {
      return city_center_contra(req1, req2)
        || city_center_contra(req2, req1);
    } else if (req1->source.value.citytile == CITYT_CENTER
               && (req2->source.kind == VUT_IMPROVEMENT
                   || req2->source.kind == VUT_SITE)
               && REQ_RANGE_TILE == req2->range
               && REQ_RANGE_TILE == req1->range
               && req2->present) {
      /* A building must be in a city */
      return !req1->present;
    }
 
    return FALSE;
  case VUT_MAX_DISTANCE_SQ:
    if (req2->source.kind != VUT_MAX_DISTANCE_SQ) {
      /* Finding contradictions across requirement kinds isn't supported
       * for MaxDistanceSq requirements. */
      return FALSE;
    }
    return are_bounds_contradictions(
        req1->source.value.distance_sq, req1->present,
        req2->source.value.distance_sq, req2->present);
  case VUT_MAX_REGION_TILES:
    if (req2->source.kind != VUT_MAX_REGION_TILES) {
      /* Finding contradictions across requirement kinds isn't supported
       * for MaxRegionTiles requirements. */
      return FALSE;
    } else if (req1->range != req2->range) {
      /* FIXME: Finding contradictions across ranges not yet supported.
       * In particular, a max at a small range and a min at a larger range
       * needs extra work to figure out. */
      return FALSE;
    }
    return are_bounds_contradictions(
        req1->source.value.region_tiles, req1->present,
        req2->source.value.region_tiles, req2->present);
  case VUT_TILE_REL:
    if (req2->source.kind != VUT_TILE_REL) {
      /* Finding contradictions across requirement kinds isn't supported
       * for TileRel requirements. */
      return FALSE;
    }
    if (req1->source.value.tilerel == req2->source.value.tilerel) {
      /* Same requirement at different ranges. Note that same range is
       * already covered by are_requirements_opposites() above. */
      switch (req1->source.value.tilerel) {
        case TREL_SAME_TCLASS:
        case TREL_SAME_REGION:
        case TREL_REGION_SURROUNDED:
          /* Negated req at larger range contradicts present req at
           * smaller range. */
          if (req1->range > req2->range) {
            return !req1->present && req2->present;
          } else {
            return req1->present && !req2->present;
          }
          break;
        case TREL_ONLY_OTHER_REGION:
          /* Present req at larger range contradicts negated req at
           * smaller range */
          if (req1->range > req2->range) {
            return req1->present && !req2->present;
          } else {
            return !req1->present && req2->present;
          }
          break;
        default:
          return FALSE;
      }
    }
    if (req1->source.value.tilerel == TREL_SAME_TCLASS
        && req2->source.value.tilerel == TREL_SAME_REGION) {
      /* Same region at any range implies same terrain class at that range
       * and any larger range ~> contradicts negated */
      return (!req1->present && req2->present
              && (req1->range >= req2->range));
    } else if (req2->source.value.tilerel == TREL_SAME_TCLASS
               && req1->source.value.tilerel == TREL_SAME_REGION) {
      /* Same as above */
      return (req1->present && !req2->present
              && (req1->range <= req2->range));
    } else if (req1->source.value.tilerel == TREL_REGION_SURROUNDED
               || req2->source.value.tilerel == TREL_REGION_SURROUNDED) {
      const struct requirement *surr, *other;
      if (req1->source.value.tilerel == TREL_REGION_SURROUNDED) {
        surr = req1;
        other = req2;
      } else {
        surr = req2;
        other = req1;
      }
      if (surr->present && surr->range == REQ_RANGE_TILE) {
        /* Target tile must be part of a surrounded region
         * ~> not the same terrain class
         * ~> not the same region
         * ~> not touched by a third region */
        switch (other->source.value.tilerel) {
        case TREL_SAME_TCLASS:
        case TREL_SAME_REGION:
          return (other->present && other->range == REQ_RANGE_TILE);
        case TREL_ONLY_OTHER_REGION:
          return (!other->present);
        default:
          break;
        }
      }
    }
    /* No further contradictions we can detect */
    return FALSE;
  default:
    /* No special knowledge exists. The requirements aren't the exact
     * opposite of each other per the initial check. */
    return FALSE;
  }
}
 
/**********************************************************************/
struct requirement *
req_vec_first_contradiction_in_vec(const struct requirement *req,
                                   const struct requirement_vector *vec)
{
  /* If the requirement is contradicted by any requirement in the vector it
   * contradicts the entire requirement vector. */
  requirement_vector_iterate(vec, preq) {
    if (are_requirements_contradictions(req, preq)) {
      return preq;
    }
  } requirement_vector_iterate_end;
 
  /* Not a single requirement in the requirement vector is contradicted to be
   * the specified requirement. */
  return nullptr;
}
 
/**********************************************************************/
bool does_req_contradicts_reqs(const struct requirement *req,
                               const struct requirement_vector *vec)
{
  return req_vec_first_contradiction_in_vec(req, vec) != nullptr;
}
 
/**********************************************************************/
static inline bool are_tiles_in_range(const struct tile *tile1,
                                      const struct tile *tile2,
                                      enum req_range range)
{
  switch (range) {
  case REQ_RANGE_ADJACENT:
    if (is_tiles_adjacent(tile1, tile2)) {
      return TRUE;
    }
    fc__fallthrough;
  case REQ_RANGE_TILE:
    return same_pos(tile1, tile2);
  case REQ_RANGE_CADJACENT:
    return map_distance(tile1, tile2) <= 1;
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    /* Invalid */
    fc_assert(FALSE);
  }
  return FALSE;
}
 
/**********************************************************************/
static inline bool players_in_same_range(const struct player *pplayer1,
                                         const struct player *pplayer2,
                                         enum req_range range)
{
  switch (range) {
  case REQ_RANGE_WORLD:
    return TRUE;
  case REQ_RANGE_ALLIANCE:
    return pplayers_allied(pplayer1, pplayer2);
  case REQ_RANGE_TEAM:
    return players_on_same_team(pplayer1, pplayer2);
  case REQ_RANGE_PLAYER:
    return pplayer1 == pplayer2;
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CITY:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_TILE:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", range);
  return FALSE;
}
 
#define IS_REQ_ACTIVE_VARIANT_ASSERT(_kind)                \
{                                                          \
  fc_assert_ret_val(req != nullptr, TRI_MAYBE);            \
  fc_assert_ret_val(req->source.kind == _kind, TRI_MAYBE); \
  fc_assert(context != nullptr);                           \
  fc_assert(other_context != nullptr);                     \
}
 
/**********************************************************************/
static enum fc_tristate
is_none_req_active(const struct civ_map *nmap,
                   const struct req_context *context,
                   const struct req_context *other_context,
                   const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_NONE);
 
  return TRI_YES;
}
 
/**********************************************************************/
static int num_world_buildings_total(const struct impr_type *building)
{
  if (is_great_wonder(building)) {
    return (great_wonder_is_built(building)
            || great_wonder_is_destroyed(building) ? 1 : 0);
  } else {
    log_error("World-ranged requirements are only supported for wonders.");
    return 0;
  }
}
 
/**********************************************************************/
static int num_world_buildings(const struct impr_type *building)
{
  if (is_great_wonder(building)) {
    return (great_wonder_is_built(building) ? 1 : 0);
  } else {
    log_error("World-ranged requirements are only supported for wonders.");
    return 0;
  }
}
 
/**********************************************************************/
static bool player_has_ever_built(const struct player *pplayer,
                                  const struct impr_type *building)
{
  if (is_wonder(building)) {
    return (wonder_is_built(pplayer, building)
            || wonder_is_lost(pplayer, building) ? TRUE : FALSE);
  } else {
    log_error("Player-ranged requirements are only supported for wonders.");
    return FALSE;
  }
}
 
/**********************************************************************/
static int num_player_buildings(const struct player *pplayer,
                                const struct impr_type *building)
{
  if (is_wonder(building)) {
    return (wonder_is_built(pplayer, building) ? 1 : 0);
  } else {
    log_error("Player-ranged requirements are only supported for wonders.");
    return 0;
  }
}
 
/**********************************************************************/
static int num_continent_buildings(const struct player *pplayer,
                                   int continent,
                                   const struct impr_type *building)
{
  if (is_wonder(building)) {
    const struct city *pcity;
 
    pcity = city_from_wonder(pplayer, building);
    if (pcity && pcity->tile && tile_continent(pcity->tile) == continent) {
      return 1;
    }
  } else {
    log_error("Island-ranged requirements are only supported for wonders.");
  }
  return 0;
}
 
/**********************************************************************/
static enum fc_tristate
is_building_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  const struct impr_type *building;
 
  /* Can't use this assertion, as both VUT_IMPROVEMENT and VUT_SITE
   * are handled here. */
  /* IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_IMPROVEMENT); */
 
  building = req->source.value.building;
 
  /* Check if it's certain that the building is obsolete given the
   * specification we have */
  if (req->source.kind == VUT_IMPROVEMENT
      && improvement_obsolete(context->player, building, context->city)) {
    return TRI_NO;
  }
 
  if (req->survives) {
 
    /* Check whether condition has ever held, using cached information. */
    switch (req->range) {
    case REQ_RANGE_WORLD:
      return BOOL_TO_TRISTATE(num_world_buildings_total(building) > 0);
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_TEAM:
      if (context->player == nullptr) {
        return TRI_MAYBE;
      }
      players_iterate_alive(plr2) {
        if (players_in_same_range(context->player, plr2, req->range)
            && player_has_ever_built(plr2, building)) {
          return TRI_YES;
        }
      } players_iterate_alive_end;
      return TRI_NO;
    case REQ_RANGE_PLAYER:
      if (context->player == nullptr) {
        return TRI_MAYBE;
      }
      return BOOL_TO_TRISTATE(player_has_ever_built(context->player,
                              building));
    case REQ_RANGE_CONTINENT:
    case REQ_RANGE_TRADE_ROUTE:
    case REQ_RANGE_CITY:
    case REQ_RANGE_LOCAL:
    case REQ_RANGE_TILE:
    case REQ_RANGE_CADJACENT:
    case REQ_RANGE_ADJACENT:
      /* There is no sources cache for this. */
      log_error("Surviving requirements are only supported at "
                "World/Alliance/Team/Player ranges.");
      return TRI_NO;
    case REQ_RANGE_COUNT:
      break;
    }
 
  } else {
 
    /* Non-surviving requirement. */
    switch (req->range) {
    case REQ_RANGE_WORLD:
      return BOOL_TO_TRISTATE(num_world_buildings(building) > 0);
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_TEAM:
      if (context->player == nullptr) {
        return TRI_MAYBE;
      }
      players_iterate_alive(plr2) {
        if (players_in_same_range(context->player, plr2, req->range)
            && num_player_buildings(plr2, building) > 0) {
          return TRI_YES;
        }
      } players_iterate_alive_end;
      return TRI_NO;
    case REQ_RANGE_PLAYER:
      if (context->player == nullptr) {
        return TRI_MAYBE;
      }
      return BOOL_TO_TRISTATE(num_player_buildings(context->player,
                                                   building)
                              > 0);
    case REQ_RANGE_CONTINENT:
      /* At present, "Continent" effects can affect only
       * cities and units in cities. */
      if (context->player && context->city) {
        int continent = tile_continent(context->city->tile);
        return BOOL_TO_TRISTATE(num_continent_buildings(context->player,
                                                        continent, building)
                                > 0);
      } else {
        return TRI_MAYBE;
      }
    case REQ_RANGE_TRADE_ROUTE:
      if (context->city) {
        if (city_has_building(context->city, building)) {
          return TRI_YES;
        } else {
          enum fc_tristate ret = TRI_NO;
 
          trade_partners_iterate(context->city, trade_partner) {
            if (trade_partner == nullptr) {
              ret = TRI_MAYBE;
            } else if (city_has_building(trade_partner, building)) {
              return TRI_YES;
            }
          } trade_partners_iterate_end;
 
          return ret;
        }
      } else {
        return TRI_MAYBE;
      }
    case REQ_RANGE_CITY:
      if (context->city) {
        return BOOL_TO_TRISTATE(city_has_building(context->city, building));
      } else {
        return TRI_MAYBE;
      }
    case REQ_RANGE_LOCAL:
      if (context->building) {
        if (context->building == building) {
          return TRI_YES;
        } else {
          return TRI_NO;
        }
      } else {
        /* TODO: Other local targets */
        return TRI_MAYBE;
      }
    case REQ_RANGE_TILE:
      if (context->tile) {
        const struct city *pcity = tile_city(context->tile);
 
        if (pcity) {
          return BOOL_TO_TRISTATE(city_has_building(pcity, building));
        } else {
          return TRI_NO;
        }
      } else {
        return TRI_MAYBE;
      }
    case REQ_RANGE_CADJACENT:
    case REQ_RANGE_ADJACENT:
    case REQ_RANGE_COUNT:
      break;
    }
 
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
  return TRI_NO;
}
 
/**********************************************************************/
static enum fc_tristate
is_buildinggenus_req_active(const struct civ_map *nmap,
                            const struct req_context *context,
                            const struct req_context *other_context,
                            const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_IMPR_GENUS);
 
  return (context->building ? BOOL_TO_TRISTATE(
                                context->building->genus
                                == req->source.value.impr_genus)
                            : TRI_MAYBE);
}
 
/**********************************************************************/
static enum fc_tristate is_buildingflag_in_city(const struct city *pcity,
                                                enum impr_flag_id flag)
{
  struct player *owner;
 
  if (pcity == nullptr) {
    return TRI_MAYBE;
  }
 
  owner = city_owner(pcity);
  city_built_iterate(pcity, impr) {
    if (improvement_has_flag(impr, flag)
        && !improvement_obsolete(owner, impr, pcity)) {
      return TRI_YES;
    }
  } city_built_iterate_end;
 
  return TRI_NO;
}
 
/**********************************************************************/
static enum fc_tristate
is_buildingflag_req_active(const struct civ_map *nmap,
                           const struct req_context *context,
                           const struct req_context *other_context,
                           const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_IMPR_FLAG);
 
  switch (req->range) {
  case REQ_RANGE_LOCAL:
    return (context->building
            ? BOOL_TO_TRISTATE(improvement_has_flag(context->building,
                                                    req->source.value.impr_flag))
            : TRI_MAYBE);
  case REQ_RANGE_CITY:
    return is_buildingflag_in_city(context->city, req->source.value.impr_flag);
  case REQ_RANGE_TILE:
    if (context->tile == nullptr) {
      return TRI_MAYBE;
    }
    return is_buildingflag_in_city(tile_city(context->tile),
                                   req->source.value.impr_flag);
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_plr_flag_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_PLAYER_FLAG);
 
  switch (req->range) {
  case REQ_RANGE_PLAYER:
    return (context->player != nullptr
            ? BOOL_TO_TRISTATE(player_has_flag(context->player,
                                               req->source.value.plr_flag))
            : TRI_MAYBE);
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_CITY:
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_plr_state_req_active(const struct civ_map *nmap,
                        const struct req_context *context,
                        const struct req_context *other_context,
                        const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_PLAYER_STATE);
 
  switch (req->range) {
  case REQ_RANGE_PLAYER:
    return (context->player != nullptr
            ? BOOL_TO_TRISTATE(player_has_state(context->player,
                                                req->source.value.plrstate))
            : TRI_MAYBE);
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_CITY:
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_tech_req_active(const struct civ_map *nmap,
                   const struct req_context *context,
                   const struct req_context *other_context,
                   const struct requirement *req)
{
  Tech_type_id tech;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_ADVANCE);
 
  tech = advance_number(req->source.value.advance);
 
  if (req->survives) {
    fc_assert(req->range == REQ_RANGE_WORLD);
    return BOOL_TO_TRISTATE(game.info.global_advances[tech]);
  }
 
  /* Not a 'surviving' requirement. */
  switch (req->range) {
  case REQ_RANGE_PLAYER:
    if (context->player != nullptr) {
      return BOOL_TO_TRISTATE(TECH_KNOWN == research_invention_state
                                (research_get(context->player), tech));
    } else {
      return TRI_MAYBE;
    }
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    }
    players_iterate_alive(plr2) {
      if (players_in_same_range(context->player, plr2, req->range)) {
        if (research_invention_state(research_get(plr2), tech)
            == TECH_KNOWN) {
          return TRI_YES;
        }
      }
    } players_iterate_alive_end;
 
    return TRI_NO;
  case REQ_RANGE_LOCAL:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    }
    if (research_get(context->player)->researching == tech) {
      return TRI_YES;
    }
    return TRI_NO;
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_techflag_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  enum tech_flag_id techflag;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_TECHFLAG);
 
  techflag = req->source.value.techflag;
 
  switch (req->range) {
  case REQ_RANGE_PLAYER:
    if (context->player != nullptr) {
      return BOOL_TO_TRISTATE(player_knows_techs_with_flag(context->player,
                                                           techflag));
    } else {
      return TRI_MAYBE;
    }
    break;
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    }
    players_iterate_alive(plr2) {
      if (players_in_same_range(context->player, plr2, req->range)
          && player_knows_techs_with_flag(plr2, techflag)) {
        return TRI_YES;
      }
    } players_iterate_alive_end;
    return TRI_NO;
  case REQ_RANGE_WORLD:
    players_iterate(pplayer) {
      if (player_knows_techs_with_flag(pplayer, techflag)) {
        return TRI_YES;
      }
    } players_iterate_end;
 
    return TRI_NO;
  case REQ_RANGE_LOCAL:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    }
    if (advance_has_flag(research_get(context->player)->researching, techflag)) {
      return TRI_YES;
    }
    return TRI_NO;
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_minculture_req_active(const struct civ_map *nmap,
                         const struct req_context *context,
                         const struct req_context *other_context,
                         const struct requirement *req)
{
  int minculture;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MINCULTURE);
 
  minculture = req->source.value.minculture;
 
  switch (req->range) {
  case REQ_RANGE_CITY:
    if (!context->city) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(city_culture(context->city) >= minculture);
  case REQ_RANGE_TRADE_ROUTE:
    if (!context->city) {
      return TRI_MAYBE;
    }
    if (city_culture(context->city) >= minculture) {
      return TRI_YES;
    } else {
      enum fc_tristate ret = TRI_NO;
 
      trade_partners_iterate(context->city, trade_partner) {
        if (trade_partner == nullptr) {
          ret = TRI_MAYBE;
        } else if (city_culture(trade_partner) >= minculture) {
          return TRI_YES;
        }
      } trade_partners_iterate_end;
 
      return ret;
    }
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    }
    players_iterate_alive(plr2) {
      if (players_in_same_range(context->player, plr2, req->range)) {
        if (player_culture(plr2) >= minculture) {
          return TRI_YES;
        }
      }
    } players_iterate_alive_end;
    return TRI_NO;
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_minforeignpct_req_active(const struct civ_map *nmap,
                            const struct req_context *context,
                            const struct req_context *other_context,
                            const struct requirement *req)
{
  int min_foreign_pct, foreign_pct;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MINFOREIGNPCT);
 
  min_foreign_pct = req->source.value.minforeignpct;
 
  switch (req->range) {
  case REQ_RANGE_CITY:
    if (!context->city) {
      return TRI_MAYBE;
    }
    foreign_pct = citizens_nation_foreign(context->city) * 100
      / city_size_get(context->city);
    return BOOL_TO_TRISTATE(foreign_pct >= min_foreign_pct);
  case REQ_RANGE_TRADE_ROUTE:
    if (!context->city) {
      return TRI_MAYBE;
    }
    foreign_pct = citizens_nation_foreign(context->city) * 100
      / city_size_get(context->city);
    if (foreign_pct >= min_foreign_pct) {
      return TRI_YES;
    } else {
      enum fc_tristate ret = TRI_NO;
 
      trade_partners_iterate(context->city, trade_partner) {
        if (trade_partner == nullptr) {
          ret = TRI_MAYBE;
        } else {
          foreign_pct = citizens_nation_foreign(trade_partner) * 100
            / city_size_get(trade_partner);
          if (foreign_pct >= min_foreign_pct) {
            return TRI_YES;
          }
        }
      } trade_partners_iterate_end;
 
      return ret;
    }
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_maxtotalunitsontile_req_active(const struct civ_map *nmap,
                                  const struct req_context *context,
                                  const struct req_context *other_context,
                                  const struct requirement *req)
{
  int max_units;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MAXTILETOTALUNITS);
 
  max_units = req->source.value.max_tile_total_units;
 
  /* TODO: If can't see V_INVIS -> TRI_MAYBE */
  switch (req->range) {
  case REQ_RANGE_TILE:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(unit_list_size(context->tile->units) <= max_units);
  case REQ_RANGE_CADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    if (unit_list_size(context->tile->units) <= max_units) {
      return TRI_YES;
    }
    cardinal_adjc_iterate(nmap, context->tile, adjc_tile) {
      if (unit_list_size(adjc_tile->units) <= max_units) {
        return TRI_YES;
      }
    } cardinal_adjc_iterate_end;
    return TRI_NO;
  case REQ_RANGE_ADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    if (unit_list_size(context->tile->units) <= max_units) {
      return TRI_YES;
    }
    adjc_iterate(nmap, context->tile, adjc_tile) {
      if (unit_list_size(adjc_tile->units) <= max_units) {
        return TRI_YES;
      }
    } adjc_iterate_end;
    return TRI_NO;
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
 
/**********************************************************************/
static enum fc_tristate
is_maxtopunitsontile_req_active(const struct civ_map *nmap,
                                const struct req_context *context,
                                const struct req_context *other_context,
                                const struct requirement *req)
{
  int max_units;
  int count;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MAXTILETOPUNITS);
 
  max_units = req->source.value.max_tile_top_units;
 
  /* TODO: If can't see V_INVIS -> TRI_MAYBE */
  switch (req->range) {
  case REQ_RANGE_TILE:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    count = 0;
    unit_list_iterate(context->tile->units, punit) {
      if (!unit_transported(punit)) {
        count++;
      }
    } unit_list_iterate_end;
    return BOOL_TO_TRISTATE(count <= max_units);
  case REQ_RANGE_CADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    count = 0;
    unit_list_iterate(context->tile->units, punit) {
      if (!unit_transported(punit)) {
        count++;
      }
    } unit_list_iterate_end;
    if (count <= max_units) {
      return TRI_YES;
    }
    cardinal_adjc_iterate(nmap, context->tile, adjc_tile) {
      count = 0;
      unit_list_iterate(adjc_tile->units, punit) {
        if (!unit_transported(punit)) {
          count++;
        }
      } unit_list_iterate_end;
      if (count <= max_units) {
        return TRI_YES;
      }
    } cardinal_adjc_iterate_end;
 
    return TRI_NO;
  case REQ_RANGE_ADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    count = 0;
    unit_list_iterate(context->tile->units, punit) {
      if (!unit_transported(punit)) {
        count++;
      }
    } unit_list_iterate_end;
    if (count <= max_units) {
      return TRI_YES;
    }
    adjc_iterate(nmap, context->tile, adjc_tile) {
      count = 0;
      unit_list_iterate(adjc_tile->units, punit) {
        if (!unit_transported(punit)) {
          count++;
        }
      } unit_list_iterate_end;
      if (count <= max_units) {
        return TRI_YES;
      }
    } adjc_iterate_end;
    return TRI_NO;
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_extra_req_active(const struct civ_map *nmap,
                    const struct req_context *context,
                    const struct req_context *other_context,
                    const struct requirement *req)
{
  const struct extra_type *pextra;
  enum fc_tristate ret;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_EXTRA);
 
  pextra = req->source.value.extra;
 
  switch (req->range) {
  case REQ_RANGE_LOCAL:
    if (!context->extra) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(context->extra == pextra);
  case REQ_RANGE_TILE:
    /* The requirement is filled if the tile has extra of requested type. */
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_has_extra(context->tile, pextra));
  case REQ_RANGE_CADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_has_extra(context->tile, pextra)
                            || is_extra_card_near(nmap, context->tile, pextra));
  case REQ_RANGE_ADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_has_extra(context->tile, pextra)
                            || is_extra_near_tile(nmap, context->tile, pextra));
  case REQ_RANGE_CITY:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      if (tile_has_extra(ptile, pextra)) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    return TRI_NO;
 
  case REQ_RANGE_TRADE_ROUTE:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      if (tile_has_extra(ptile, pextra)) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    ret = TRI_NO;
    trade_partners_iterate(context->city, trade_partner) {
      if (trade_partner == nullptr) {
        ret = TRI_MAYBE;
      } else {
        city_tile_iterate(nmap, city_map_radius_sq_get(trade_partner),
                          city_tile(trade_partner), ptile) {
          if (tile_has_extra(ptile, pextra)) {
            return TRI_YES;
          }
        } city_tile_iterate_end;
      }
    } trade_partners_iterate_end;
 
    return ret;
 
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_tiledef_req_active(const struct civ_map *nmap,
                      const struct req_context *context,
                      const struct req_context *other_context,
                      const struct requirement *req)
{
  const struct tiledef *ptdef;
  enum fc_tristate ret;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_TILEDEF);
 
  ptdef = req->source.value.tiledef;
 
  switch (req->range) {
  case REQ_RANGE_TILE:
    /* The requirement is filled if the tile has tiledef of requested type. */
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_matches_tiledef(ptdef, context->tile));
  case REQ_RANGE_CADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_matches_tiledef(ptdef, context->tile)
                            || is_tiledef_card_near(nmap, context->tile, ptdef));
  case REQ_RANGE_ADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_matches_tiledef(ptdef, context->tile)
                            || is_tiledef_near_tile(nmap, context->tile, ptdef));
  case REQ_RANGE_CITY:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      if (tile_matches_tiledef(ptdef, ptile)) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    return TRI_NO;
 
  case REQ_RANGE_TRADE_ROUTE:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      if (tile_matches_tiledef(ptdef, ptile)) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    ret = TRI_NO;
    trade_partners_iterate(context->city, trade_partner) {
      if (trade_partner == nullptr) {
        ret = TRI_MAYBE;
      } else {
        city_tile_iterate(nmap, city_map_radius_sq_get(trade_partner),
                          city_tile(trade_partner), ptile) {
          if (tile_matches_tiledef(ptdef, ptile)) {
            return TRI_YES;
          }
        } city_tile_iterate_end;
      }
    } trade_partners_iterate_end;
 
    return ret;
 
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_good_req_active(const struct civ_map *nmap,
                   const struct req_context *context,
                   const struct req_context *other_context,
                   const struct requirement *req)
{
  const struct goods_type *pgood;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_GOOD);
 
  pgood = req->source.value.good;
 
  switch (req->range) {
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_CITY:
    /* The requirement is filled if the city imports good of requested type. */
    if (!context->city) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(city_receives_goods(context->city, pgood)
                            || (goods_has_flag(pgood, GF_SELF_PROVIDED)
                                && goods_can_be_provided(context->city, pgood,
                                                         nullptr)));
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_action_req_active(const struct civ_map *nmap,
                     const struct req_context *context,
                     const struct req_context *other_context,
                     const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_ACTION);
 
  if (context->action) {
    return BOOL_TO_TRISTATE(action_number(context->action)
                            == action_number(req->source.value.action));
  }
 
  if (context->unit != nullptr && context->unit->action != ACTION_NONE) {
    log_normal("Unit action %s", action_id_rule_name(context->unit->action));
    return BOOL_TO_TRISTATE(context->unit->action
                            == action_number(req->source.value.action));
  }
 
  return TRI_NO;
}
 
/**********************************************************************/
static enum fc_tristate
is_outputtype_req_active(const struct civ_map *nmap,
                         const struct req_context *context,
                         const struct req_context *other_context,
                         const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_OTYPE);
 
  return BOOL_TO_TRISTATE(context->output
                          && context->output->index
                             == req->source.value.outputtype);
}
 
/**********************************************************************/
static enum fc_tristate
is_specialist_req_active(const struct civ_map *nmap,
                         const struct req_context *context,
                         const struct req_context *other_context,
                         const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_SPECIALIST);
 
  return BOOL_TO_TRISTATE(context->specialist
                          && context->specialist
                             == req->source.value.specialist);
}
 
/**********************************************************************/
static enum fc_tristate
is_terrain_req_active(const struct civ_map *nmap,
                      const struct req_context *context,
                      const struct req_context *other_context,
                      const struct requirement *req)
{
  const struct terrain *pterrain;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_TERRAIN);
 
  pterrain = req->source.value.terrain;
 
  switch (req->range) {
  case REQ_RANGE_TILE:
    /* The requirement is filled if the tile has the terrain. */
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return pterrain && tile_terrain(context->tile) == pterrain;
  case REQ_RANGE_CADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(pterrain && is_terrain_card_near(nmap, context->tile, pterrain, TRUE));
  case REQ_RANGE_ADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(pterrain && is_terrain_near_tile(nmap, context->tile, pterrain, TRUE));
  case REQ_RANGE_CITY:
    if (!context->city) {
      return TRI_MAYBE;
    }
    if (pterrain != nullptr) {
      city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                        city_tile(context->city), ptile) {
        if (tile_terrain(ptile) == pterrain) {
          return TRI_YES;
        }
      } city_tile_iterate_end;
    }
    return TRI_NO;
  case REQ_RANGE_TRADE_ROUTE:
    if (!context->city) {
      return TRI_MAYBE;
    }
    if (pterrain != nullptr) {
      enum fc_tristate ret;
 
      city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                        city_tile(context->city), ptile) {
        if (tile_terrain(ptile) == pterrain) {
          return TRI_YES;
        }
      } city_tile_iterate_end;
 
      ret = TRI_NO;
      trade_partners_iterate(context->city, trade_partner) {
        if (trade_partner == nullptr) {
          ret = TRI_MAYBE;
        } else {
          city_tile_iterate(nmap, city_map_radius_sq_get(trade_partner),
                            city_tile(trade_partner), ptile) {
            if (tile_terrain(ptile) == pterrain) {
              return TRI_YES;
            }
          } city_tile_iterate_end;
        }
      } trade_partners_iterate_end;
 
      return ret;
    }
 
    return TRI_MAYBE;
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_terrainclass_req_active(const struct civ_map *nmap,
                           const struct req_context *context,
                           const struct req_context *other_context,
                           const struct requirement *req)
{
  enum terrain_class pclass;
  enum fc_tristate ret;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_TERRAINCLASS);
 
  pclass = req->source.value.terrainclass;
 
  switch (req->range) {
  case REQ_RANGE_TILE:
    /* The requirement is filled if the tile has the terrain of correct class. */
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(terrain_type_terrain_class(tile_terrain(context->tile)) == pclass);
  case REQ_RANGE_CADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(terrain_type_terrain_class(tile_terrain(context->tile)) == pclass
                            || is_terrain_class_card_near(nmap, context->tile, pclass));
  case REQ_RANGE_ADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(terrain_type_terrain_class(tile_terrain(context->tile)) == pclass
                            || is_terrain_class_near_tile(nmap, context->tile, pclass));
  case REQ_RANGE_CITY:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      const struct terrain *pterrain = tile_terrain(ptile);
 
      if (pterrain != T_UNKNOWN
          && terrain_type_terrain_class(pterrain) == pclass) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    return TRI_NO;
  case REQ_RANGE_TRADE_ROUTE:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      const struct terrain *pterrain = tile_terrain(ptile);
 
      if (pterrain != T_UNKNOWN
          && terrain_type_terrain_class(pterrain) == pclass) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    ret = TRI_NO;
    trade_partners_iterate(context->city, trade_partner) {
      if (trade_partner == nullptr) {
        ret = TRI_MAYBE;
      } else {
        city_tile_iterate(nmap, city_map_radius_sq_get(trade_partner),
                          city_tile(trade_partner), ptile) {
          const struct terrain *pterrain = tile_terrain(ptile);
 
          if (pterrain != T_UNKNOWN
              && terrain_type_terrain_class(pterrain) == pclass) {
            return TRI_YES;
          }
        } city_tile_iterate_end;
      }
    } trade_partners_iterate_end;
 
    return ret;
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_terrainflag_req_active(const struct civ_map *nmap,
                          const struct req_context *context,
                          const struct req_context *other_context,
                          const struct requirement *req)
{
  enum terrain_flag_id terrflag;
  enum fc_tristate ret;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_TERRFLAG);
 
  terrflag = req->source.value.terrainflag;
 
  switch (req->range) {
  case REQ_RANGE_TILE:
    /* The requirement is fulfilled if the tile has a terrain with
     * correct flag. */
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(terrain_has_flag(tile_terrain(context->tile),
                                             terrflag));
  case REQ_RANGE_CADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(terrain_has_flag(tile_terrain(context->tile),
                                             terrflag)
                            || is_terrain_flag_card_near(nmap, context->tile,
                                                         terrflag));
  case REQ_RANGE_ADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(terrain_has_flag(tile_terrain(context->tile),
                                             terrflag)
                            || is_terrain_flag_near_tile(nmap, context->tile,
                                                         terrflag));
  case REQ_RANGE_CITY:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      const struct terrain *pterrain = tile_terrain(ptile);
 
      if (pterrain != T_UNKNOWN
          && terrain_has_flag(pterrain, terrflag)) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    return TRI_NO;
  case REQ_RANGE_TRADE_ROUTE:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      const struct terrain *pterrain = tile_terrain(ptile);
 
      if (pterrain != T_UNKNOWN
          && terrain_has_flag(pterrain, terrflag)) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    ret = TRI_NO;
    trade_partners_iterate(context->city, trade_partner) {
      if (trade_partner == nullptr) {
        ret = TRI_MAYBE;
      } else {
        city_tile_iterate(nmap, city_map_radius_sq_get(trade_partner),
                          city_tile(trade_partner), ptile) {
          const struct terrain *pterrain = tile_terrain(ptile);
 
          if (pterrain != T_UNKNOWN
              && terrain_has_flag(pterrain, terrflag)) {
            return TRI_YES;
          }
        } city_tile_iterate_end;
      }
    } trade_partners_iterate_end;
 
    return ret;
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_roadflag_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  enum road_flag_id roadflag;
  enum fc_tristate ret;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_ROADFLAG);
 
  roadflag = req->source.value.roadflag;
 
  switch (req->range) {
  case REQ_RANGE_LOCAL:
    {
      if (!context->extra) {
        return TRI_MAYBE;
      }
      struct road_type *r = extra_road_get(context->extra);
 
      return BOOL_TO_TRISTATE(
          r && road_has_flag(r, roadflag)
      );
    }
  case REQ_RANGE_TILE:
    /* The requirement is filled if the tile has a road with correct flag. */
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_has_road_flag(context->tile, roadflag));
  case REQ_RANGE_CADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_has_road_flag(context->tile, roadflag)
                            || is_road_flag_card_near(nmap, context->tile,
                                                      roadflag));
  case REQ_RANGE_ADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_has_road_flag(context->tile, roadflag)
                            || is_road_flag_near_tile(nmap, context->tile,
                                                      roadflag));
  case REQ_RANGE_CITY:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      if (tile_has_road_flag(ptile, roadflag)) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    return TRI_NO;
  case REQ_RANGE_TRADE_ROUTE:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      if (tile_has_road_flag(ptile, roadflag)) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    ret = TRI_NO;
    trade_partners_iterate(context->city, trade_partner) {
      if (trade_partner == nullptr) {
        ret = TRI_MAYBE;
      } else {
        city_tile_iterate(nmap, city_map_radius_sq_get(trade_partner),
                          city_tile(trade_partner), ptile) {
          if (tile_has_road_flag(ptile, roadflag)) {
            return TRI_YES;
          }
        } city_tile_iterate_end;
      }
    } trade_partners_iterate_end;
 
    return ret;
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_extraflag_req_active(const struct civ_map *nmap,
                        const struct req_context *context,
                        const struct req_context *other_context,
                        const struct requirement *req)
{
  enum extra_flag_id extraflag;
  enum fc_tristate ret;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_EXTRAFLAG);
 
  extraflag = req->source.value.extraflag;
 
  switch (req->range) {
  case REQ_RANGE_LOCAL:
    if (!context->extra) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(extra_has_flag(context->extra, extraflag));
  case REQ_RANGE_TILE:
    /* The requirement is filled if the tile has an extra with correct flag. */
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_has_extra_flag(context->tile, extraflag));
  case REQ_RANGE_CADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_has_extra_flag(context->tile, extraflag)
                            || is_extra_flag_card_near(nmap, context->tile, extraflag));
  case REQ_RANGE_ADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(tile_has_extra_flag(context->tile, extraflag)
                            || is_extra_flag_near_tile(nmap, context->tile, extraflag));
  case REQ_RANGE_CITY:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      if (tile_has_extra_flag(ptile, extraflag)) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    return TRI_NO;
  case REQ_RANGE_TRADE_ROUTE:
    if (!context->city) {
      return TRI_MAYBE;
    }
    city_tile_iterate(nmap, city_map_radius_sq_get(context->city),
                      city_tile(context->city), ptile) {
      if (tile_has_extra_flag(ptile, extraflag)) {
        return TRI_YES;
      }
    } city_tile_iterate_end;
 
    ret = TRI_NO;
    trade_partners_iterate(context->city, trade_partner) {
      if (trade_partner == nullptr) {
        ret = TRI_MAYBE;
      } else {
        city_tile_iterate(nmap, city_map_radius_sq_get(trade_partner),
                          city_tile(trade_partner), ptile) {
          if (tile_has_extra_flag(ptile, extraflag)) {
            return TRI_YES;
          }
        } city_tile_iterate_end;
      }
    } trade_partners_iterate_end;
 
    return ret;
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_terrainalter_req_active(const struct civ_map *nmap,
                           const struct req_context *context,
                           const struct req_context *other_context,
                           const struct requirement *req)
{
  enum terrain_alteration alteration;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_TERRAINALTER);
 
  alteration = req->source.value.terrainalter;
 
  if (!context->tile) {
    return TRI_MAYBE;
  }
 
  switch (req->range) {
  case REQ_RANGE_TILE:
    return BOOL_TO_TRISTATE(terrain_can_support_alteration(
                                tile_terrain(context->tile), alteration));
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT: /* XXX Could in principle support ADJACENT. */
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_gov_req_active(const struct civ_map *nmap,
                  const struct req_context *context,
                  const struct req_context *other_context,
                  const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_GOVERNMENT);
 
  if (context->player == nullptr) {
    return TRI_MAYBE;
  } else {
    return BOOL_TO_TRISTATE(government_of_player(context->player)
                            == req->source.value.govern);
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_govflag_req_active(const struct civ_map *nmap,
                      const struct req_context *context,
                      const struct req_context *other_context,
                      const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_GOVFLAG);
 
  if (context->player == nullptr) {
    return TRI_MAYBE;
  } else {
    return BOOL_TO_TRISTATE(BV_ISSET(government_of_player(context->player)->flags,
                                     req->source.value.govflag));
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_style_req_active(const struct civ_map *nmap,
                    const struct req_context *context,
                    const struct req_context *other_context,
                    const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_STYLE);
 
  if (context->player == nullptr) {
    return TRI_MAYBE;
  } else {
    return BOOL_TO_TRISTATE(context->player->style
                            == req->source.value.style);
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_mintechs_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MINTECHS);
 
  switch (req->range) {
  case REQ_RANGE_WORLD:
    /* "None" does not count */
    return ((game.info.global_advance_count - 1)
            >= req->source.value.min_techs);
  case REQ_RANGE_PLAYER:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    } else {
      /* "None" does not count */
      return BOOL_TO_TRISTATE(
                  (research_get(context->player)->techs_researched - 1)
                  >= req->source.value.min_techs
              );
    }
  default:
    return TRI_MAYBE;
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_futuretechs_req_active(const struct civ_map *nmap,
                          const struct req_context *context,
                          const struct req_context *other_context,
                          const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_FUTURETECHS);
 
  switch (req->range) {
  case REQ_RANGE_WORLD:
    players_iterate_alive(plr) {
      if (research_get(plr)->future_tech
          >= req->source.value.future_techs) {
        return TRI_YES;
      }
    } players_iterate_alive_end;
 
    return TRI_NO;
  case REQ_RANGE_PLAYER:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    } else {
      return BOOL_TO_TRISTATE(research_get(context->player)->future_tech
                              >= req->source.value.future_techs);
    }
  default:
    return TRI_MAYBE;
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_mincities_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MINCITIES);
 
  switch (req->range) {
  case REQ_RANGE_PLAYER:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    } else {
      /* "None" does not count */
      return BOOL_TO_TRISTATE(
                  city_list_size(context->player->cities)
                  >= req->source.value.min_cities
              );
    }
  default:
    return TRI_MAYBE;
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_ai_req_active(const struct civ_map *nmap,
                 const struct req_context *context,
                 const struct req_context *other_context,
                 const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_AI_LEVEL);
 
  if (context->player == nullptr) {
    return TRI_MAYBE;
  } else {
    return BOOL_TO_TRISTATE(is_ai(context->player)
                            && context->player->ai_common.skill_level
                               == req->source.value.ai_level);
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_nation_req_active(const struct civ_map *nmap,
                     const struct req_context *context,
                     const struct req_context *other_context,
                     const struct requirement *req)
{
  const struct nation_type *nation;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_NATION);
 
  nation = req->source.value.nation;
 
  switch (req->range) {
  case REQ_RANGE_PLAYER:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(nation_of_player(context->player) == nation);
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    }
    players_iterate_alive(plr2) {
      if (players_in_same_range(context->player, plr2, req->range)) {
        if (nation_of_player(plr2) == nation) {
          return TRI_YES;
        }
      }
    } players_iterate_alive_end;
    return TRI_NO;
  case REQ_RANGE_WORLD:
    /* NB: if a player is ever removed outright from the game
     * (e.g. via /remove), rather than just dying, this 'survives'
     * requirement will stop being true for their nation.
     * create_command_newcomer() can also cause this to happen. */
    return BOOL_TO_TRISTATE(nation->player != nullptr
                            && (req->survives || nation->player->is_alive));
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_nationgroup_req_active(const struct civ_map *nmap,
                          const struct req_context *context,
                          const struct req_context *other_context,
                          const struct requirement *req)
{
  const struct nation_group *ngroup;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_NATIONGROUP);
 
  ngroup = req->source.value.nationgroup;
 
  switch (req->range) {
  case REQ_RANGE_PLAYER:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(nation_is_in_group(
        nation_of_player(context->player), ngroup));
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    }
    players_iterate_alive(plr2) {
      if (players_in_same_range(context->player, plr2, req->range)) {
        if (nation_is_in_group(nation_of_player(plr2), ngroup)) {
          return TRI_YES;
        }
      }
    } players_iterate_alive_end;
    return TRI_NO;
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_nationality_req_active(const struct civ_map *nmap,
                          const struct req_context *context,
                          const struct req_context *other_context,
                          const struct requirement *req)
{
  const struct nation_type *nationality;
  enum fc_tristate ret;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_NATIONALITY);
 
  nationality = req->source.value.nationality;
 
  switch (req->range) {
  case REQ_RANGE_CITY:
    if (context->city == nullptr) {
     return TRI_MAYBE;
    }
    citizens_iterate(context->city, slot, count) {
      if (player_slot_get_player(slot)->nation == nationality) {
        return TRI_YES;
      }
    } citizens_iterate_end;
 
    return TRI_NO;
  case REQ_RANGE_TRADE_ROUTE:
    if (context->city == nullptr) {
      return TRI_MAYBE;
    }
    citizens_iterate(context->city, slot, count) {
      if (player_slot_get_player(slot)->nation == nationality) {
        return TRI_YES;
      }
    } citizens_iterate_end;
 
    ret = TRI_NO;
    trade_partners_iterate(context->city, trade_partner) {
      if (trade_partner == nullptr) {
        ret = TRI_MAYBE;
      } else {
        citizens_iterate(trade_partner, slot, count) {
          if (player_slot_get_player(slot)->nation == nationality) {
            return TRI_YES;
          }
        } citizens_iterate_end;
      }
    } trade_partners_iterate_end;
 
    return ret;
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_originalowner_req_active(const struct civ_map *nmap,
                            const struct req_context *context,
                            const struct req_context *other_context,
                            const struct requirement *req)
{
  const struct nation_type *nation;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_ORIGINAL_OWNER);
 
  nation = req->source.value.origowner;
 
  switch (req->range) {
  case REQ_RANGE_CITY:
    if (context->city == nullptr || context->city->original == nullptr) {
      return TRI_MAYBE;
    }
    if (player_nation(context->city->original) == nation) {
      return TRI_YES;
    }
 
    return TRI_NO;
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate is_diplrel_in_range(const struct player *target_player,
                                            const struct player *other_player,
                                            enum req_range range,
                                            int diplrel)
{
  switch (range) {
  case REQ_RANGE_PLAYER:
    if (target_player == nullptr) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(is_diplrel_to_other(target_player, diplrel));
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
    if (target_player == nullptr) {
      return TRI_MAYBE;
    }
    players_iterate_alive(plr2) {
      if (players_in_same_range(target_player, plr2, range)) {
        if (is_diplrel_to_other(plr2, diplrel)) {
          return TRI_YES;
        }
      }
    } players_iterate_alive_end;
    return TRI_NO;
  case REQ_RANGE_LOCAL:
    if (target_player == nullptr || other_player == nullptr) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(is_diplrel_between(target_player, other_player, diplrel));
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_diplrel_req_active(const struct civ_map *nmap,
                      const struct req_context *context,
                      const struct req_context *other_context,
                      const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_DIPLREL);
 
  return is_diplrel_in_range(context->player, other_context->player,
                             req->range, req->source.value.diplrel);
}
 
/**********************************************************************/
static enum fc_tristate
is_diplrel_tile_req_active(const struct civ_map *nmap,
                           const struct req_context *context,
                           const struct req_context *other_context,
                           const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_DIPLREL_TILE);
 
  return is_diplrel_in_range(context->tile ? tile_owner(context->tile)
                                           : nullptr,
                             context->player,
                             req->range,
                             req->source.value.diplrel);
}
 
/**********************************************************************/
static enum fc_tristate
is_diplrel_tile_o_req_active(const struct civ_map *nmap,
                             const struct req_context *context,
                             const struct req_context *other_context,
                             const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_DIPLREL_TILE_O);
 
  return is_diplrel_in_range(context->tile ? tile_owner(context->tile)
                                           : nullptr,
                             other_context->player,
                             req->range,
                             req->source.value.diplrel);
}
 
/**********************************************************************/
static enum fc_tristate
is_diplrel_unitany_in_range(const struct tile *target_tile,
                            const struct player *other_player,
                            enum req_range range,
                            int diplrel)
{
  enum fc_tristate out = TRI_NO;
 
  if (target_tile == nullptr) {
    return TRI_MAYBE;
  }
 
  unit_list_iterate(target_tile->units, target_unit) {
    enum fc_tristate for_target_unit = is_diplrel_in_range(
        unit_owner(target_unit), other_player, range, diplrel);
 
    out = fc_tristate_or(out, for_target_unit);
  } unit_list_iterate_end;
 
  return out;
}
 
/**********************************************************************/
static enum fc_tristate
is_diplrel_unitany_req_active(const struct civ_map *nmap,
                              const struct req_context *context,
                              const struct req_context *other_context,
                              const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_DIPLREL_UNITANY);
 
  return is_diplrel_unitany_in_range(context->tile, context->player,
                                     req->range,
                                     req->source.value.diplrel);
}
 
/**********************************************************************/
static enum fc_tristate
is_diplrel_unitany_o_req_active(const struct civ_map *nmap,
                                const struct req_context *context,
                                const struct req_context *other_context,
                                const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_DIPLREL_UNITANY_O);
 
  return is_diplrel_unitany_in_range(context->tile, other_context->player,
                                     req->range,
                                     req->source.value.diplrel);
}
 
/**********************************************************************/
static enum fc_tristate
is_unittype_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  const struct unit_type *punittype;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_UTYPE);
 
  punittype = req->source.value.utype;
 
  switch (req->range) {
  case REQ_RANGE_LOCAL:
    if (!context->unittype) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(context->unittype == punittype);
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
    if (context->tile == nullptr) {
      return TRI_MAYBE;
    }
 
    unit_list_iterate(context->tile->units, punit) {
      if (punit->utype == punittype) {
        return TRI_YES;
      }
    } unit_list_iterate_end;
 
    if (req->range == REQ_RANGE_TILE) {
      return TRI_NO;
    }
 
    if (req->range == REQ_RANGE_CADJACENT) {
      cardinal_adjc_iterate(nmap, context->tile, adjc_tile) {
        unit_list_iterate(adjc_tile->units, punit) {
          if (punit->utype == punittype) {
            return TRI_YES;
          }
        } unit_list_iterate_end;
      } cardinal_adjc_iterate_end;
    } else {
      fc_assert(req->range == REQ_RANGE_ADJACENT);
 
      adjc_iterate(nmap, context->tile, adjc_tile) {
        unit_list_iterate(adjc_tile->units, punit) {
          if (punit->utype == punittype) {
            return TRI_YES;
          }
        } unit_list_iterate_end;
      } adjc_iterate_end;
    }
 
    return TRI_NO;
 
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    fc_assert(FALSE);
    break;
  }
 
  return TRI_NO;
}
 
/**********************************************************************/
static enum fc_tristate
is_unitflag_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  enum unit_type_flag_id unitflag;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_UTFLAG);
 
  unitflag = req->source.value.unitflag;
 
  switch (req->range) {
  case REQ_RANGE_LOCAL:
    if (!context->unittype) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(utype_has_flag(context->unittype, unitflag));
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
    if (context->tile == nullptr) {
      return TRI_MAYBE;
    }
 
    unit_list_iterate(context->tile->units, punit) {
      if (unit_has_type_flag(punit, unitflag)) {
        return TRI_YES;
      }
    } unit_list_iterate_end;
 
    if (req->range == REQ_RANGE_TILE) {
      return TRI_NO;
    }
 
    if (req->range == REQ_RANGE_CADJACENT) {
      cardinal_adjc_iterate(nmap, context->tile, adjc_tile) {
        unit_list_iterate(adjc_tile->units, punit) {
          if (unit_has_type_flag(punit, unitflag)) {
            return TRI_YES;
          }
        } unit_list_iterate_end;
      } cardinal_adjc_iterate_end;
    } else {
      fc_assert(req->range == REQ_RANGE_ADJACENT);
 
      adjc_iterate(nmap, context->tile, adjc_tile) {
        unit_list_iterate(adjc_tile->units, punit) {
          if (unit_has_type_flag(punit, unitflag)) {
            return TRI_YES;
          }
        } unit_list_iterate_end;
      } adjc_iterate_end;
    }
 
     return TRI_NO;
 
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    fc_assert(FALSE);
    break;
  }
 
  return TRI_NO;
}
 
/**********************************************************************/
static enum fc_tristate
is_unitclass_req_active(const struct civ_map *nmap,
                        const struct req_context *context,
                        const struct req_context *other_context,
                        const struct requirement *req)
{
  const struct unit_class *pclass;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_UCLASS);
 
  pclass = req->source.value.uclass;
 
  switch (req->range) {
  case REQ_RANGE_LOCAL:
    if (!context->unittype) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(utype_class(context->unittype) == pclass);
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
    if (context->tile == nullptr) {
      return TRI_MAYBE;
    }
 
    unit_list_iterate(context->tile->units, punit) {
      if (unit_class_get(punit) == pclass) {
        return TRI_YES;
      }
    } unit_list_iterate_end;
 
    if (req->range == REQ_RANGE_TILE) {
      return TRI_NO;
    }
 
    if (req->range == REQ_RANGE_CADJACENT) {
      cardinal_adjc_iterate(nmap, context->tile, adjc_tile) {
        unit_list_iterate(adjc_tile->units, punit) {
          if (unit_class_get(punit) == pclass) {
            return TRI_YES;
          }
        } unit_list_iterate_end;
      } cardinal_adjc_iterate_end;
    } else {
      fc_assert(req->range == REQ_RANGE_ADJACENT);
 
      adjc_iterate(nmap, context->tile, adjc_tile) {
        unit_list_iterate(adjc_tile->units, punit) {
          if (unit_class_get(punit) == pclass) {
            return TRI_YES;
          }
        } unit_list_iterate_end;
      } adjc_iterate_end;
    }
 
     return TRI_NO;
 
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    fc_assert(FALSE);
    break;
  }
 
  return TRI_NO;
}
 
/**********************************************************************/
static enum fc_tristate
is_unitclassflag_req_active(const struct civ_map *nmap,
                            const struct req_context *context,
                            const struct req_context *other_context,
                            const struct requirement *req)
{
  enum unit_class_flag_id ucflag;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_UCFLAG);
 
  ucflag = req->source.value.unitclassflag;
 
  switch (req->range) {
  case REQ_RANGE_LOCAL:
    if (!context->unittype) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(uclass_has_flag(utype_class(context->unittype), ucflag));
  case REQ_RANGE_TILE:
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
    if (context->tile == nullptr) {
      return TRI_MAYBE;
    }
 
    unit_list_iterate(context->tile->units, punit) {
      if (unit_has_class_flag(punit, ucflag)) {
        return TRI_YES;
      }
    } unit_list_iterate_end;
 
    if (req->range == REQ_RANGE_TILE) {
      return TRI_NO;
    }
 
    if (req->range == REQ_RANGE_CADJACENT) {
      cardinal_adjc_iterate(nmap, context->tile, adjc_tile) {
        unit_list_iterate(adjc_tile->units, punit) {
          if (unit_has_class_flag(punit, ucflag)) {
            return TRI_YES;
          }
        } unit_list_iterate_end;
      } cardinal_adjc_iterate_end;
    } else {
      fc_assert(req->range == REQ_RANGE_ADJACENT);
 
      adjc_iterate(nmap, context->tile, adjc_tile) {
        unit_list_iterate(adjc_tile->units, punit) {
          if (unit_has_class_flag(punit, ucflag)) {
            return TRI_YES;
          }
        } unit_list_iterate_end;
      } adjc_iterate_end;
    }
 
    return TRI_NO;
 
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_WORLD:
  case REQ_RANGE_COUNT:
    fc_assert(FALSE);
    break;
  }
 
  return TRI_NO;
}
 
/**********************************************************************/
static enum fc_tristate
is_unitstate_req_active(const struct civ_map *nmap,
                        const struct req_context *context,
                        const struct req_context *other_context,
                        const struct requirement *req)
{
  enum ustate_prop uprop;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_UNITSTATE);
 
  uprop = req->source.value.unit_state;
 
  fc_assert_ret_val_msg(req->range == REQ_RANGE_LOCAL, TRI_NO,
                        "Unsupported range \"%s\"",
                        req_range_name(req->range));
 
  /* Could be asked with incomplete data.
   * is_req_active() will handle it based on prob_type. */
  if (context->unit == nullptr) {
    return TRI_MAYBE;
  }
 
  switch (uprop) {
  case USP_TRANSPORTED:
    return BOOL_TO_TRISTATE(context->unit->transporter != nullptr);
  case USP_LIVABLE_TILE:
    return BOOL_TO_TRISTATE(
          can_unit_exist_at_tile(nmap, context->unit,
                                 unit_tile(context->unit)));
    break;
  case USP_TRANSPORTING:
    return BOOL_TO_TRISTATE(0 < get_transporter_occupancy(context->unit));
  case USP_HAS_HOME_CITY:
    return BOOL_TO_TRISTATE(context->unit->homecity > 0);
  case USP_NATIVE_TILE:
    return BOOL_TO_TRISTATE(
        is_native_tile(unit_type_get(context->unit),
                       unit_tile(context->unit)));
    break;
  case USP_NATIVE_EXTRA:
    return BOOL_TO_TRISTATE(
        tile_has_native_base(unit_tile(context->unit),
                             unit_type_get(context->unit)));
    break;
  case USP_MOVED_THIS_TURN:
    return BOOL_TO_TRISTATE(context->unit->moved);
  case USP_COUNT:
    fc_assert_msg(uprop != USP_COUNT, "Invalid unit state property.");
    /* Invalid property is unknowable. */
    return TRI_NO;
  }
 
  /* Should never be reached */
  fc_assert_msg(FALSE, "Unsupported unit property %d", uprop);
 
  return TRI_NO;
}
 
/**********************************************************************/
static enum fc_tristate
is_activity_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  enum unit_activity activity;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_ACTIVITY);
 
  activity = req->source.value.activity;
 
  fc_assert_ret_val_msg(req->range == REQ_RANGE_LOCAL, TRI_NO,
                        "Unsupported range \"%s\"",
                        req_range_name(req->range));
 
  if (context->unit == nullptr) {
    /* FIXME: Excluding ACTIVITY_IDLE here is a bit ugly, but done because
     *        it's the zero value that context has by default - so many callers
     *        who meant not to set specific activity actually have ACTIVITY_IDLE
     *        instead of ACTIVITY_LAST */
    if (context->activity != ACTIVITY_LAST && context->activity != ACTIVITY_IDLE) {
      return BOOL_TO_TRISTATE(activity == context->activity);
    }
 
    /* Could be asked with incomplete data.
     * is_req_active() will handle it based on prob_type. */
    return TRI_MAYBE;
  }
 
  switch (context->unit->activity) {
  case ACTIVITY_IDLE:
  case ACTIVITY_SENTRY:
  case ACTIVITY_GOTO:
  case ACTIVITY_EXPLORE:
    /* Seen as idle. */
    return BOOL_TO_TRISTATE(activity == ACTIVITY_IDLE);
  default:
    /* Handled below. */
    break;
  }
 
  return BOOL_TO_TRISTATE(context->unit->activity == activity);
}
 
/**********************************************************************/
static enum fc_tristate
is_minveteran_req_active(const struct civ_map *nmap,
                         const struct req_context *context,
                         const struct req_context *other_context,
                         const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MINVETERAN);
 
  if (context->unit == nullptr) {
    return TRI_MAYBE;
  } else {
    return BOOL_TO_TRISTATE(context->unit->veteran
                            >= req->source.value.minveteran);
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_minmovefrags_req_active(const struct civ_map *nmap,
                           const struct req_context *context,
                           const struct req_context *other_context,
                           const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MINMOVES);
 
  if (context->unit == nullptr) {
    return TRI_MAYBE;
  } else {
    return BOOL_TO_TRISTATE(req->source.value.minmoves
                            <= context->unit->moves_left);
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_minhitpoints_req_active(const struct civ_map *nmap,
                           const struct req_context *context,
                           const struct req_context *other_context,
                           const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MINHP);
 
  if (context->unit == nullptr) {
    return TRI_MAYBE;
  } else {
    return BOOL_TO_TRISTATE(req->source.value.min_hit_points
                            <= context->unit->hp);
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_age_req_active(const struct civ_map *nmap,
                  const struct req_context *context,
                  const struct req_context *other_context,
                  const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_AGE);
 
  switch (req->range) {
  case REQ_RANGE_LOCAL:
    if (context->unit == nullptr || !is_server()) {
      return TRI_MAYBE;
    } else {
      return BOOL_TO_TRISTATE(
                req->source.value.age <=
                game.info.turn - context->unit->birth_turn);
    }
    break;
  case REQ_RANGE_CITY:
    if (context->city == nullptr) {
      return TRI_MAYBE;
    } else {
      return BOOL_TO_TRISTATE(
                req->source.value.age <=
                game.info.turn - context->city->turn_founded);
    }
    break;
  case REQ_RANGE_PLAYER:
    if (context->player == nullptr) {
      return TRI_MAYBE;
    } else {
      return BOOL_TO_TRISTATE(req->source.value.age
                              <= player_age(context->player));
    }
    break;
  default:
    return TRI_MAYBE;
    break;
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_form_age_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_FORM_AGE);
 
  switch (req->range) {
  case REQ_RANGE_LOCAL:
    if (context->unit == nullptr || !is_server()) {
      return TRI_MAYBE;
    } else {
      return BOOL_TO_TRISTATE(
                req->source.value.form_age <=
                game.info.turn - context->unit->current_form_turn);
    }
    break;
  default:
    return TRI_MAYBE;
    break;
  }
}
 
/**********************************************************************/
static inline enum fc_tristate
does_region_surrounder_match(Continent_id cont, Continent_id surrounder)
{
  Continent_id actual_surrounder;
  bool whole_known;
 
  if (cont > 0) {
    actual_surrounder = get_island_surrounder(cont);
    whole_known = is_whole_continent_known(cont);
 
    if (actual_surrounder > 0) {
      return TRI_NO;
    }
  } else if (cont < 0) {
    actual_surrounder = get_lake_surrounder(cont);
    whole_known = is_whole_ocean_known(-cont);
 
    if (actual_surrounder < 0) {
      return TRI_NO;
    }
  } else {
    return TRI_MAYBE;
  }
 
  if (actual_surrounder == 0 || surrounder == 0) {
    return TRI_MAYBE;
  } else if (actual_surrounder != surrounder) {
    return TRI_NO;
  } else if (!whole_known) {
    return TRI_MAYBE;
  } else {
    return TRI_YES;
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_tile_rel_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_TILE_REL);
 
  if (context->tile == nullptr || other_context->tile == nullptr) {
    /* Note: For some values, we might be able to give a definitive
     * TRI_NO answer even if one of the tiles is missing, but that's
     * probably not worth the added effort. */
    return TRI_MAYBE;
  }
 
  switch (req->source.value.tilerel) {
  case TREL_SAME_TCLASS:
    if (tile_terrain(other_context->tile) == T_UNKNOWN) {
      return TRI_MAYBE;
    }
    fc_assert_ret_val_msg((req->range == REQ_RANGE_TILE
                           || req->range == REQ_RANGE_CADJACENT
                           || req->range == REQ_RANGE_ADJACENT),
                          TRI_MAYBE,
                          "Invalid range %d for tile relation \"%s\" req",
                          req->range, tilerel_type_name(TREL_SAME_TCLASS));
    {
      enum terrain_class cls = terrain_type_terrain_class(
          tile_terrain(other_context->tile));
      bool seen_unknown = FALSE;
      const struct terrain *terr;
 
      if ((terr = tile_terrain(context->tile)) == T_UNKNOWN) {
        seen_unknown = TRUE;
      } else if (terrain_type_terrain_class(terr) == cls) {
        return TRUE;
      }
 
      range_adjc_iterate(nmap, context->tile, req->range, adj_tile) {
        if ((terr = tile_terrain(adj_tile)) == T_UNKNOWN) {
          seen_unknown = TRUE;
        } else if (terrain_type_terrain_class(terr) == cls) {
          return TRUE;
        }
      } range_adjc_iterate_end;
 
      if (seen_unknown) {
        return TRI_MAYBE;
      } else {
        return TRI_NO;
      }
    }
    break;
  case TREL_SAME_REGION:
    if (tile_continent(other_context->tile) == 0) {
      return TRI_MAYBE;
    }
    fc_assert_ret_val_msg((req->range == REQ_RANGE_TILE
                           || req->range == REQ_RANGE_CADJACENT
                           || req->range == REQ_RANGE_ADJACENT),
                          TRI_MAYBE,
                          "Invalid range %d for tile relation \"%s\" req",
                          req->range, tilerel_type_name(TREL_SAME_REGION));
 
    if (tile_continent(context->tile)
        == tile_continent(other_context->tile)) {
      return TRI_YES;
    } else {
      bool seen_unknown = (tile_continent(context->tile) == 0);
      Continent_id cont = tile_continent(other_context->tile);
 
      range_adjc_iterate(nmap, context->tile, req->range, adj_tile) {
        Continent_id adj_cont = tile_continent(adj_tile);
 
        if (adj_cont == cont) {
          return TRI_YES;
        } else if (adj_cont == 0) {
          seen_unknown = TRUE;
        }
      } range_adjc_iterate_end;
 
      if (seen_unknown) {
        return TRI_MAYBE;
      } else {
        return TRI_NO;
      }
    }
    break;
  case TREL_ONLY_OTHER_REGION:
    if (tile_continent(context->tile) == 0
        || tile_continent(other_context->tile) == 0) {
      /* Note: We could still give a definitive TRI_NO answer if there are
       * too many different adjacent continents, but that's probably not
       * worth the added effort. */
      return TRI_MAYBE;
    }
    fc_assert_ret_val_msg((req->range == REQ_RANGE_CADJACENT
                           || req->range == REQ_RANGE_ADJACENT),
                          TRI_MAYBE,
                          "Invalid range %d for tile relation \"%s\" req",
                          req->range,
                          tilerel_type_name(TREL_ONLY_OTHER_REGION));
 
    {
      bool seen_unknown = FALSE;
      Continent_id cont = tile_continent(context->tile);
      Continent_id other_cont = tile_continent(other_context->tile);
 
      range_adjc_iterate(nmap, context->tile, req->range, adj_tile) {
        Continent_id adj_cont = tile_continent(adj_tile);
 
        if (adj_cont == 0) {
          seen_unknown = TRUE;
        } else if (adj_cont != cont && adj_cont != other_cont) {
          return TRI_NO;
        }
      } range_adjc_iterate_end;
 
      if (seen_unknown) {
        return TRI_MAYBE;
      } else {
        return TRI_YES;
      }
    }
    break;
  case TREL_REGION_SURROUNDED:
    fc_assert_ret_val_msg((req->range == REQ_RANGE_TILE
                           || req->range == REQ_RANGE_CADJACENT
                           || req->range == REQ_RANGE_ADJACENT),
                          TRI_MAYBE,
                          "Invalid range %d for tile relation \"%s\" req",
                          req->range,
                          tilerel_type_name(TREL_REGION_SURROUNDED));
 
    {
      bool seen_maybe = FALSE;
      Continent_id wanted = tile_continent(other_context->tile);
 
      switch (does_region_surrounder_match(tile_continent(context->tile),
                                           wanted)) {
        case TRI_YES:
          return TRI_YES;
        case TRI_MAYBE:
          seen_maybe = TRUE;
          break;
        default:
          break;
      }
 
      range_adjc_iterate(nmap, context->tile, req->range, adj_tile) {
        switch (does_region_surrounder_match(tile_continent(adj_tile),
                                             wanted)) {
          case TRI_YES:
            return TRI_YES;
          case TRI_MAYBE:
            seen_maybe = TRUE;
            break;
          default:
            break;
        }
      } range_adjc_iterate_end;
 
      if (seen_maybe) {
        return TRI_MAYBE;
      } else {
        return TRI_NO;
      }
    }
    break;
  default:
    break;
  }
 
  fc_assert_msg(FALSE,
                "Illegal value %d for tile relationship requirement.",
                req->source.value.tilerel);
  return TRI_MAYBE;
}
 
/**********************************************************************/
static bool is_city_in_tile(const struct tile *ptile,
                            const struct city *pcity)
{
  if (pcity == nullptr) {
    return tile_city(ptile) != nullptr;
  } else {
    return is_city_center(pcity, ptile);
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_citytile_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  enum citytile_type citytile;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_CITYTILE);
 
  citytile = req->source.value.citytile;
 
  fc_assert_ret_val(req_range_is_valid(req->range), TRI_MAYBE);
  if (context->tile == nullptr) {
    return TRI_MAYBE;
  }
 
  switch (citytile) {
  case CITYT_CENTER:
    switch (req->range) {
    case REQ_RANGE_TILE:
      return BOOL_TO_TRISTATE(is_city_in_tile(context->tile,
                                              context->city));
    case REQ_RANGE_CADJACENT:
      if (is_city_in_tile(context->tile, context->city)) {
        return TRI_YES;
      }
      cardinal_adjc_iterate(nmap, context->tile, adjc_tile) {
        if (is_city_in_tile(adjc_tile, context->city)) {
          return TRI_YES;
        }
      } cardinal_adjc_iterate_end;
 
      return TRI_NO;
    case REQ_RANGE_ADJACENT:
      if (is_city_in_tile(context->tile, context->city)) {
        return TRI_YES;
      }
      adjc_iterate(nmap, context->tile, adjc_tile) {
        if (is_city_in_tile(adjc_tile, context->city)) {
          return TRI_YES;
        }
      } adjc_iterate_end;
 
      return TRI_NO;
    case REQ_RANGE_CITY:
    case REQ_RANGE_TRADE_ROUTE:
    case REQ_RANGE_CONTINENT:
    case REQ_RANGE_PLAYER:
    case REQ_RANGE_TEAM:
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_WORLD:
    case REQ_RANGE_LOCAL:
    case REQ_RANGE_COUNT:
      fc_assert_msg(FALSE, "Invalid range %d for citytile.", req->range);
      break;
    }
 
    return TRI_MAYBE;
  case CITYT_CLAIMED:
    switch (req->range) {
    case REQ_RANGE_TILE:
      return BOOL_TO_TRISTATE(context->tile->owner != nullptr);
    case REQ_RANGE_CADJACENT:
      if (context->tile->owner != nullptr) {
        return TRI_YES;
      }
      cardinal_adjc_iterate(nmap, context->tile, adjc_tile) {
        if (adjc_tile->owner != nullptr) {
          return TRI_YES;
        }
      } cardinal_adjc_iterate_end;
 
      return TRI_NO;
    case REQ_RANGE_ADJACENT:
      if (context->tile->owner != nullptr) {
        return TRI_YES;
      }
      adjc_iterate(nmap, context->tile, adjc_tile) {
        if (adjc_tile->owner != nullptr) {
          return TRI_YES;
        }
      } adjc_iterate_end;
 
      return TRI_NO;
    case REQ_RANGE_CITY:
    case REQ_RANGE_TRADE_ROUTE:
    case REQ_RANGE_CONTINENT:
    case REQ_RANGE_PLAYER:
    case REQ_RANGE_TEAM:
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_WORLD:
    case REQ_RANGE_LOCAL:
    case REQ_RANGE_COUNT:
      fc_assert_msg(FALSE, "Invalid range %d for citytile.", req->range);
      break;
    }
 
    return TRI_MAYBE;
  case CITYT_EXTRAS_OWNED:
    switch (req->range) {
    case REQ_RANGE_TILE:
      return BOOL_TO_TRISTATE(context->tile->extras_owner != nullptr);
    case REQ_RANGE_CADJACENT:
      if (context->tile->extras_owner != nullptr) {
        return TRI_YES;
      }
      cardinal_adjc_iterate(nmap, context->tile, adjc_tile) {
        if (adjc_tile->extras_owner != nullptr) {
          return TRI_YES;
        }
      } cardinal_adjc_iterate_end;
 
      return TRI_NO;
    case REQ_RANGE_ADJACENT:
      if (context->tile->extras_owner != nullptr) {
        return TRI_YES;
      }
      adjc_iterate(nmap, context->tile, adjc_tile) {
        if (adjc_tile->extras_owner != nullptr) {
          return TRI_YES;
        }
      } adjc_iterate_end;
 
      return TRI_NO;
    case REQ_RANGE_CITY:
    case REQ_RANGE_TRADE_ROUTE:
    case REQ_RANGE_CONTINENT:
    case REQ_RANGE_PLAYER:
    case REQ_RANGE_TEAM:
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_WORLD:
    case REQ_RANGE_LOCAL:
    case REQ_RANGE_COUNT:
      fc_assert_msg(FALSE, "Invalid range %d for citytile.", req->range);
      break;
    }
 
    return TRI_MAYBE;
  case CITYT_WORKED:
    switch (req->range) {
    case REQ_RANGE_TILE:
      return BOOL_TO_TRISTATE(context->tile->worked != nullptr);
    case REQ_RANGE_CADJACENT:
      if (context->tile->worked != nullptr) {
        return TRI_YES;
      }
      cardinal_adjc_iterate(nmap, context->tile, adjc_tile) {
        if (adjc_tile->worked != nullptr) {
          return TRI_YES;
        }
      } cardinal_adjc_iterate_end;
 
      return TRI_NO;
    case REQ_RANGE_ADJACENT:
      if (context->tile->worked != nullptr) {
        return TRI_YES;
      }
      adjc_iterate(nmap, context->tile, adjc_tile) {
        if (adjc_tile->worked != nullptr) {
          return TRI_YES;
        }
      } adjc_iterate_end;
 
      return TRI_NO;
    case REQ_RANGE_CITY:
    case REQ_RANGE_TRADE_ROUTE:
    case REQ_RANGE_CONTINENT:
    case REQ_RANGE_PLAYER:
    case REQ_RANGE_TEAM:
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_WORLD:
    case REQ_RANGE_LOCAL:
    case REQ_RANGE_COUNT:
      fc_assert_msg(FALSE, "Invalid range %d for citytile.", req->range);
      break;
    }
 
    return TRI_MAYBE;
  case CITYT_SAME_CONTINENT:
    {
      Continent_id cc;
      const struct tile *target_tile = context->tile, *cc_tile;
 
      if (!context->city) {
        return TRI_MAYBE;
      }
      cc_tile = city_tile(context->city);
      if (!cc_tile) {
        /* Unplaced virtual city */
        return TRI_MAYBE;
      }
      cc = tile_continent(cc_tile);
      /* Note: No special treatment of 0 == cc here*/
      switch (req->range) {
      case REQ_RANGE_TILE:
        return BOOL_TO_TRISTATE(tile_continent(target_tile) == cc);
      case REQ_RANGE_CADJACENT:
        if (tile_continent(target_tile) == cc) {
          return TRI_YES;
        }
        cardinal_adjc_iterate(nmap, target_tile, adjc_tile) {
          if (tile_continent(adjc_tile) == cc) {
            return TRI_YES;
          }
        } cardinal_adjc_iterate_end;
 
        return TRI_NO;
      case REQ_RANGE_ADJACENT:
        if (tile_continent(target_tile) == cc) {
          return TRI_YES;
        }
        adjc_iterate(nmap, target_tile, adjc_tile) {
          if (tile_continent(adjc_tile) == cc) {
            return TRI_YES;
          }
        } adjc_iterate_end;
 
        return TRI_NO;
      case REQ_RANGE_CITY:
      case REQ_RANGE_TRADE_ROUTE:
      case REQ_RANGE_CONTINENT:
      case REQ_RANGE_PLAYER:
      case REQ_RANGE_TEAM:
      case REQ_RANGE_ALLIANCE:
      case REQ_RANGE_WORLD:
      case REQ_RANGE_LOCAL:
      case REQ_RANGE_COUNT:
        fc_assert_msg(FALSE, "Invalid range %d for citytile.", req->range);
        break;
      }
    }
 
    return TRI_MAYBE;
  case CITYT_BORDERING_TCLASS_REGION:
    {
      int n = 0;
      Continent_id adjc_cont[8], cc;
      bool ukt = FALSE;
      const struct tile *target_tile = context->tile, *cc_tile;
 
      if (!context->city) {
        return TRI_MAYBE;
      }
      cc_tile = city_tile(context->city);
      if (!cc_tile) {
        /* Unplaced virtual city */
        return TRI_MAYBE;
      }
      cc = tile_continent(cc_tile);
      if (!cc) {
        /* Don't know the city center terrain class.
         * Maybe, the city floats? Even if the rules prohibit it... */
        return TRI_MAYBE;
      }
      adjc_iterate(nmap, cc_tile, adjc_tile) {
        Continent_id tc = tile_continent(adjc_tile);
 
        if (0 != tc) {
          bool seen = FALSE;
          int i = n;
 
          if (tc == cc) {
            continue;
          }
          while (--i >= 0) {
            if (adjc_cont[i] == tc) {
              seen = TRUE;
              break;
            }
          }
          if (seen) {
            continue;
          }
          adjc_cont[n++] = tile_continent(adjc_tile);
        } else {
          /* Likely, it's a black tile in client and we don't know
           * We possibly can calculate, but keep it simple. */
          ukt = TRUE;
        }
      } adjc_iterate_end;
      if (0 == n) {
        return ukt ? TRI_MAYBE : TRI_NO;
      }
 
      switch (req->range) {
      case REQ_RANGE_TILE:
        {
          Continent_id tc = tile_continent(target_tile);
 
          if (cc == tc) {
            return TRI_NO;
          }
          if (0 == tc || ukt) {
            return TRI_MAYBE;
          }
          for (int i = 0; i < n; i++) {
            if (tc == adjc_cont[i]) {
              return TRI_YES;
            }
          }
        }
 
        return TRI_NO;
      case REQ_RANGE_ADJACENT:
        if (ukt) {
          /* If ALL the tiles in range are on cc, we can say it's false */
          square_iterate(nmap, target_tile, 1, adjc_tile) {
            if (tile_continent(adjc_tile) != cc) {
             return TRI_MAYBE;
            }
          } square_iterate_end;
 
          return TRI_NO;
        } else {
          square_iterate(nmap, target_tile, 1, adjc_tile) {
            Continent_id tc = tile_continent(adjc_tile);
 
            if (0 == tc) {
              return TRI_MAYBE;
            }
            for (int i = 0; i < n; i++) {
              if (tc == adjc_cont[i]) {
                return TRI_YES;
              }
            }
          } square_iterate_end;
        }
 
        return TRI_NO;
      case REQ_RANGE_CADJACENT:
        if (ukt) {
          /* If ALL the tiles in range are on cc, we can say it's false */
          circle_iterate(nmap, target_tile, 1, cadjc_tile) {
            if (tile_continent(cadjc_tile) != cc) {
              return TRI_MAYBE;
            }
          } circle_iterate_end;
        } else {
          circle_iterate(nmap, target_tile, 1, cadjc_tile) {
            Continent_id tc = tile_continent(cadjc_tile);
 
            if (0 == tc) {
              return TRI_MAYBE;
            }
            for (int i = 0; i < n; i++) {
              if (tc == adjc_cont[i]) {
                return TRI_YES;
              }
            }
          } circle_iterate_end;
        }
 
        return TRI_NO;
      case REQ_RANGE_CITY:
      case REQ_RANGE_TRADE_ROUTE:
      case REQ_RANGE_CONTINENT:
      case REQ_RANGE_PLAYER:
      case REQ_RANGE_TEAM:
      case REQ_RANGE_ALLIANCE:
      case REQ_RANGE_WORLD:
      case REQ_RANGE_LOCAL:
      case REQ_RANGE_COUNT:
        fc_assert_msg(FALSE, "Invalid range %d for citytile.", req->range);
        break;
      }
    }
 
    return TRI_MAYBE;
  case CITYT_LAST:
    /* Handled below */
    break;
  }
 
  /* Not implemented */
  log_error("is_req_active(): citytile %d not supported.",
            citytile);
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_citystatus_req_active(const struct civ_map *nmap,
                         const struct req_context *context,
                         const struct req_context *other_context,
                         const struct requirement *req)
{
  enum citystatus_type citystatus;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_CITYSTATUS);
 
  citystatus = req->source.value.citystatus;
 
  if (context->city == nullptr) {
    return TRI_MAYBE;
  }
 
  switch (citystatus) {
  case CITYS_OWNED_BY_ORIGINAL:
    switch (req->range) {
    case REQ_RANGE_CITY:
      if (context->city->original == nullptr) {
        return TRI_MAYBE;
      }
      return BOOL_TO_TRISTATE(city_owner(context->city) == context->city->original);
    case REQ_RANGE_TRADE_ROUTE:
      {
        enum fc_tristate ret;
 
        if (city_owner(context->city) == context->city->original) {
          return TRI_YES;
        }
 
        ret = TRI_NO;
        trade_partners_iterate(context->city, trade_partner) {
          if (trade_partner == nullptr || trade_partner->original == nullptr) {
            ret = TRI_MAYBE;
          } else if (city_owner(trade_partner) == trade_partner->original) {
            return TRI_YES;
          }
        } trade_partners_iterate_end;
 
        return ret;
      }
    case REQ_RANGE_LOCAL:
    case REQ_RANGE_TILE:
    case REQ_RANGE_CADJACENT:
    case REQ_RANGE_ADJACENT:
    case REQ_RANGE_CONTINENT:
    case REQ_RANGE_PLAYER:
    case REQ_RANGE_TEAM:
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_WORLD:
    case REQ_RANGE_COUNT:
      break;
    }
 
    fc_assert_msg(FALSE, "Invalid range %d for citystatus OwnedByOriginal.",
                         req->range);
 
    return TRI_MAYBE;
 
  case CITYS_STARVED:
    switch (req->range) {
    case REQ_RANGE_CITY:
      return BOOL_TO_TRISTATE(context->city->had_famine);
    case REQ_RANGE_TRADE_ROUTE:
      {
        enum fc_tristate ret;
 
        if (context->city->had_famine) {
          return TRI_YES;
        }
 
        ret = TRI_NO;
        trade_partners_iterate(context->city, trade_partner) {
          if (trade_partner == nullptr) {
            ret = TRI_MAYBE;
          } else if (trade_partner->had_famine) {
            return TRI_YES;
          }
        } trade_partners_iterate_end;
 
        return ret;
      }
    case REQ_RANGE_LOCAL:
    case REQ_RANGE_TILE:
    case REQ_RANGE_CADJACENT:
    case REQ_RANGE_ADJACENT:
    case REQ_RANGE_CONTINENT:
    case REQ_RANGE_PLAYER:
    case REQ_RANGE_TEAM:
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_WORLD:
    case REQ_RANGE_COUNT:
      break;
    }
 
    fc_assert_msg(FALSE, "Invalid range %d for citystatus Starved.",
                         req->range);
 
    return TRI_MAYBE;
 
  case CITYS_DISORDER:
    switch (req->range) {
    case REQ_RANGE_CITY:
      return BOOL_TO_TRISTATE(context->city->anarchy > 0);
    case REQ_RANGE_TRADE_ROUTE:
      {
        enum fc_tristate ret;
 
        if (context->city->anarchy > 0) {
          return TRI_YES;
        }
 
        ret = TRI_NO;
        trade_partners_iterate(context->city, trade_partner) {
          if (trade_partner == nullptr) {
            ret = TRI_MAYBE;
          } else if (trade_partner->anarchy > 0) {
            return TRI_YES;
          }
        } trade_partners_iterate_end;
 
        return ret;
      }
    case REQ_RANGE_LOCAL:
    case REQ_RANGE_TILE:
    case REQ_RANGE_CADJACENT:
    case REQ_RANGE_ADJACENT:
    case REQ_RANGE_CONTINENT:
    case REQ_RANGE_PLAYER:
    case REQ_RANGE_TEAM:
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_WORLD:
    case REQ_RANGE_COUNT:
      break;
    }
 
    fc_assert_msg(FALSE, "Invalid range %d for citystatus Disorder.",
                         req->range);
 
    return TRI_MAYBE;
 
  case CITYS_CELEBRATION:
    switch (req->range) {
    case REQ_RANGE_CITY:
      return BOOL_TO_TRISTATE(context->city->rapture > 0);
    case REQ_RANGE_TRADE_ROUTE:
      {
        enum fc_tristate ret;
 
        if (context->city->rapture > 0) {
          return TRI_YES;
        }
 
        ret = TRI_NO;
        trade_partners_iterate(context->city, trade_partner) {
          if (trade_partner == nullptr) {
            ret = TRI_MAYBE;
          } else if (trade_partner->rapture > 0) {
            return TRI_YES;
          }
        } trade_partners_iterate_end;
 
        return ret;
      }
    case REQ_RANGE_LOCAL:
    case REQ_RANGE_TILE:
    case REQ_RANGE_CADJACENT:
    case REQ_RANGE_ADJACENT:
    case REQ_RANGE_CONTINENT:
    case REQ_RANGE_PLAYER:
    case REQ_RANGE_TEAM:
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_WORLD:
    case REQ_RANGE_COUNT:
      break;
    }
 
    fc_assert_msg(FALSE, "Invalid range %d for citystatus Celebration.",
                         req->range);
 
    return TRI_MAYBE;
 
  case CITYS_TRANSFERRED:
    switch (req->range) {
    case REQ_RANGE_CITY:
      return BOOL_TO_TRISTATE(context->city->acquire_t != CACQ_FOUNDED);
    case REQ_RANGE_TRADE_ROUTE:
      {
        enum fc_tristate ret;
 
        if (context->city->acquire_t != CACQ_FOUNDED) {
          return TRI_YES;
        }
 
        ret = TRI_NO;
        trade_partners_iterate(context->city, trade_partner) {
          if (trade_partner == nullptr) {
            ret = TRI_MAYBE;
          } else if (trade_partner->acquire_t != CACQ_FOUNDED) {
            return TRI_YES;
          }
        } trade_partners_iterate_end;
 
        return ret;
      }
    case REQ_RANGE_LOCAL:
    case REQ_RANGE_TILE:
    case REQ_RANGE_CADJACENT:
    case REQ_RANGE_ADJACENT:
    case REQ_RANGE_CONTINENT:
    case REQ_RANGE_PLAYER:
    case REQ_RANGE_TEAM:
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_WORLD:
    case REQ_RANGE_COUNT:
      break;
    }
 
    fc_assert_msg(FALSE, "Invalid range %d for citystatus Transferred.",
                         req->range);
 
    return TRI_MAYBE;
 
  case CITYS_CAPITALCONNECTED:
    if (!is_server()) {
      /* Client has no idea. */
      return TRI_MAYBE;
    }
 
    switch (req->range) {
    case REQ_RANGE_CITY:
      return BOOL_TO_TRISTATE(context->city->server.aarea != nullptr
                              && context->city->server.aarea->capital);
    case REQ_RANGE_TRADE_ROUTE:
      {
        enum fc_tristate ret;
 
        if (context->city->server.aarea != nullptr
            && context->city->server.aarea->capital) {
          return TRI_YES;
        }
 
        ret = TRI_NO;
        trade_partners_iterate(context->city, trade_partner) {
          if (trade_partner == nullptr) {
            ret = TRI_MAYBE;
          } else if (trade_partner->server.aarea != nullptr
                     && trade_partner->server.aarea->capital) {
            return TRI_YES;
          }
        } trade_partners_iterate_end;
 
        return ret;
      }
    case REQ_RANGE_LOCAL:
    case REQ_RANGE_TILE:
    case REQ_RANGE_CADJACENT:
    case REQ_RANGE_ADJACENT:
    case REQ_RANGE_CONTINENT:
    case REQ_RANGE_PLAYER:
    case REQ_RANGE_TEAM:
    case REQ_RANGE_ALLIANCE:
    case REQ_RANGE_WORLD:
    case REQ_RANGE_COUNT:
      break;
    }
 
    fc_assert_msg(FALSE, "Invalid range %d for citystatus CapitalConnected.",
                         req->range);
 
    return TRI_MAYBE;
 
  case CITYS_LAST:
    break;
  }
 
  /* Not implemented */
  log_error("is_req_active(): citystatus %d not supported.",
            citystatus);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_minsize_req_active(const struct civ_map *nmap,
                      const struct req_context *context,
                      const struct req_context *other_context,
                      const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MINSIZE);
 
  if (context->city == nullptr) {
    return TRI_MAYBE;
  } else {
    if (req->range == REQ_RANGE_TRADE_ROUTE) {
      enum fc_tristate ret;
 
      if (city_size_get(context->city) >= req->source.value.minsize) {
        return TRI_YES;
      }
 
      ret = TRI_NO;
      trade_partners_iterate(context->city, trade_partner) {
        if (trade_partner == nullptr) {
          ret = TRI_MAYBE;
        } else if (city_size_get(trade_partner) >= req->source.value.minsize) {
          return TRI_YES;
        }
      } trade_partners_iterate_end;
 
      return ret;
    } else {
      return BOOL_TO_TRISTATE(city_size_get(context->city)
                              >= req->source.value.minsize);
    }
  }
}
 
/**********************************************************************/
static enum fc_tristate
is_counter_req_active(const struct civ_map *nmap,
                      const struct req_context *context,
                      const struct req_context *other_context,
                      const struct requirement *req)
{
  const struct counter *count;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_COUNTER);
 
  count = req->source.value.counter;
 
  if (context->city == nullptr) {
    return TRI_MAYBE;
  }
  return BOOL_TO_TRISTATE(count->checkpoint <=
                          context->city->counter_values[
                              counter_index(count)]);
}
 
/**********************************************************************/
static enum fc_tristate
is_achievement_req_active(const struct civ_map *nmap,
                          const struct req_context *context,
                          const struct req_context *other_context,
                          const struct requirement *req)
{
  const struct achievement *achievement;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_ACHIEVEMENT);
 
  achievement = req->source.value.achievement;
 
  if (req->range == REQ_RANGE_WORLD) {
    return BOOL_TO_TRISTATE(achievement_claimed(achievement));
  } else if (context->player == nullptr) {
    return TRI_MAYBE;
  } else if (req->range == REQ_RANGE_ALLIANCE
             || req->range == REQ_RANGE_TEAM) {
    players_iterate_alive(plr2) {
      if (players_in_same_range(context->player, plr2, req->range)
          && achievement_player_has(achievement, plr2)) {
        return TRI_YES;
      }
    } players_iterate_alive_end;
    return TRI_NO;
  } else if (req->range == REQ_RANGE_PLAYER) {
    if (achievement_player_has(achievement, context->player)) {
      return TRI_YES;
    } else {
      return TRI_NO;
    }
  }
 
  fc_assert_msg(FALSE, "Invalid range %d.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_latitude_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  int min = -MAP_MAX_LATITUDE, max = MAP_MAX_LATITUDE;
 
  fc_assert_ret_val(req != nullptr, TRI_MAYBE);
  fc_assert(context != nullptr);
 
  switch (req->source.kind) {
  case VUT_MINLATITUDE:
    min = req->source.value.latitude;
    break;
  case VUT_MAXLATITUDE:
    max = req->source.value.latitude;
    break;
  default:
    fc_assert(req->source.kind == VUT_MINLATITUDE
              || req->source.kind == VUT_MAXLATITUDE);
    break;
  }
 
  switch (req->range) {
  case REQ_RANGE_WORLD:
    return BOOL_TO_TRISTATE(min <= MAP_MAX_REAL_LATITUDE(wld.map)
                            && max >= MAP_MIN_REAL_LATITUDE(wld.map));
 
  case REQ_RANGE_TILE:
    if (context->tile == nullptr) {
      return TRI_MAYBE;
    } else {
      int tile_lat = map_signed_latitude(context->tile);
 
      return BOOL_TO_TRISTATE(min <= tile_lat && max >= tile_lat);
    }
 
  case REQ_RANGE_CADJACENT:
    if (context->tile == nullptr) {
      return TRI_MAYBE;
    }
 
    cardinal_adjc_iterate(nmap, context->tile, adjc_tile) {
      int tile_lat = map_signed_latitude(adjc_tile);
 
      if (min <= tile_lat && max >= tile_lat) {
        return TRI_YES;
      }
    } cardinal_adjc_iterate_end;
    return TRI_NO;
 
  case REQ_RANGE_ADJACENT:
    if (!context->tile) {
      return TRI_MAYBE;
    }
 
    adjc_iterate(nmap, context->tile, adjc_tile) {
      int tile_lat = map_signed_latitude(adjc_tile);
 
      if (min <= tile_lat && max >= tile_lat) {
        return TRI_YES;
      }
    } adjc_iterate_end;
    return TRI_NO;
 
  case REQ_RANGE_CITY:
  case REQ_RANGE_TRADE_ROUTE:
  case REQ_RANGE_CONTINENT:
  case REQ_RANGE_PLAYER:
  case REQ_RANGE_TEAM:
  case REQ_RANGE_ALLIANCE:
  case REQ_RANGE_LOCAL:
  case REQ_RANGE_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE,
                "Illegal range %d for latitude requirement.", req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_max_distance_sq_req_active(const struct civ_map *nmap,
                              const struct req_context *context,
                              const struct req_context *other_context,
                              const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MAX_DISTANCE_SQ);
 
  switch (req->range) {
  case REQ_RANGE_TILE:
    if (context->tile == nullptr || other_context->tile == nullptr) {
      return TRI_MAYBE;
    }
    return BOOL_TO_TRISTATE(
      sq_map_distance(context->tile, other_context->tile)
      <= req->source.value.distance_sq
    );
  default:
    break;
  }
 
  fc_assert_msg(FALSE,
                "Illegal range %d for max squared distance requirement.",
                req->range);
 
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_max_region_tiles_req_active(const struct civ_map *nmap,
                               const struct req_context *context,
                               const struct req_context *other_context,
                               const struct requirement *req)
{
  int max_tiles, min_tiles = 1;
 
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MAX_REGION_TILES);
 
  switch (req->range) {
  case REQ_RANGE_CADJACENT:
  case REQ_RANGE_ADJACENT:
    if (context->tile == nullptr) {
      /* The tile itself is included in the range */
      max_tiles = 1 + ((req->range == REQ_RANGE_CADJACENT)
                      ? nmap->num_cardinal_dirs
                      : nmap->num_valid_dirs);
 
      break;
    } else {
      Continent_id cont = tile_continent(context->tile);
 
      /* Count how many adjacent tiles there actually are as we go along */
      max_tiles = 1;
 
      range_adjc_iterate(nmap, context->tile, req->range, adj_tile) {
        Continent_id adj_cont = tile_continent(adj_tile);
 
        if (adj_cont == 0 || cont == 0) {
          max_tiles++;
        } else if (adj_cont == cont) {
          min_tiles++;
          max_tiles++;
        }
      } range_adjc_iterate_end;
    }
    break;
  case REQ_RANGE_CONTINENT:
    {
      Continent_id cont = context->tile ? tile_continent(context->tile) : 0;
 
      fc_assert_ret_val(cont <= nmap->num_continents, TRI_MAYBE);
      fc_assert_ret_val(-cont <= nmap->num_oceans, TRI_MAYBE);
 
      /* Note: We could come up with a better upper bound by subtracting
       * all other continent/ocean sizes, or all except the largest if we
       * don't know the tile.
       * We could even do a flood-fill count of the unknown area bordered
       * by known tiles of the continent.
       * Probably not worth the effort though. */
      max_tiles = nmap->xsize * nmap->ysize;
 
      if (cont > 0) {
        min_tiles = nmap->continent_sizes[cont];
        if (is_whole_continent_known(cont)) {
          max_tiles = min_tiles;
        }
      } else if (cont < 0) {
        min_tiles = nmap->ocean_sizes[-cont];
        if (is_whole_ocean_known(-cont)) {
          max_tiles = min_tiles;
        }
      }
    }
    break;
  default:
    fc_assert_msg(FALSE,
                  "Illegal range %d for max region tiles requirement.",
                  req->range);
    return TRI_MAYBE;
  }
 
  if (min_tiles > req->source.value.region_tiles) {
    return TRI_NO;
  } else if (max_tiles <= req->source.value.region_tiles) {
    return TRI_YES;
  }
  return TRI_MAYBE;
}
 
/**********************************************************************/
static enum fc_tristate
is_minyear_req_active(const struct civ_map *nmap,
                      const struct req_context *context,
                      const struct req_context *other_context,
                      const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MINYEAR);
 
  return BOOL_TO_TRISTATE(game.info.year >= req->source.value.minyear);
}
 
/**********************************************************************/
static enum fc_tristate
is_mincalfrag_req_active(const struct civ_map *nmap,
                         const struct req_context *context,
                         const struct req_context *other_context,
                         const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_MINCALFRAG);
 
  return BOOL_TO_TRISTATE(game.info.fragment_count
                          >= req->source.value.mincalfrag);
}
 
/**********************************************************************/
static enum fc_tristate
is_topology_req_active(const struct civ_map *nmap,
                       const struct req_context *context,
                       const struct req_context *other_context,
                       const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_TOPO);
 
  return BOOL_TO_TRISTATE(
      current_topo_has_flag(req->source.value.topo_property));
}
 
/**********************************************************************/
static enum fc_tristate
is_wrap_req_active(const struct civ_map *nmap,
                   const struct req_context *context,
                   const struct req_context *other_context,
                   const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_WRAP);
 
  return BOOL_TO_TRISTATE(
      current_wrap_has_flag(req->source.value.wrap_property));
}
 
/**********************************************************************/
static enum fc_tristate
is_serversetting_req_active(const struct civ_map *nmap,
                            const struct req_context *context,
                            const struct req_context *other_context,
                            const struct requirement *req)
{
  IS_REQ_ACTIVE_VARIANT_ASSERT(VUT_SERVERSETTING);
 
  return BOOL_TO_TRISTATE(ssetv_setting_has_value(
                              req->source.value.ssetval));
}
 
/* Not const for potential ruleset-related adjustment */
static struct req_def req_definitions[VUT_COUNT] = {
  [VUT_NONE] = {is_none_req_active, REQUCH_YES},
 
  /* Alphabetical order of enum constant */
  [VUT_ACHIEVEMENT] = {is_achievement_req_active, REQUCH_YES, REQUC_PRESENT},
  [VUT_ACTION] = {is_action_req_active, REQUCH_YES},
  [VUT_ACTIVITY] = {is_activity_req_active, REQUCH_NO},
  [VUT_ADVANCE] = {is_tech_req_active, REQUCH_NO},
  [VUT_AGE] = {is_age_req_active, REQUCH_YES, REQUC_PRESENT},
  [VUT_FORM_AGE] = {is_form_age_req_active, REQUCH_YES, REQUC_PRESENT},
  [VUT_AI_LEVEL] = {is_ai_req_active, REQUCH_HACK},
  [VUT_CITYSTATUS] = {is_citystatus_req_active, REQUCH_NO, REQUC_CITYSTATUS},
  [VUT_CITYTILE] = {is_citytile_req_active, REQUCH_NO, REQUC_CITYTILE},
  [VUT_COUNTER] = {is_counter_req_active, REQUCH_NO},
  [VUT_DIPLREL] = {is_diplrel_req_active, REQUCH_NO},
  [VUT_DIPLREL_TILE] = {is_diplrel_tile_req_active, REQUCH_NO},
  [VUT_DIPLREL_TILE_O] = {is_diplrel_tile_o_req_active, REQUCH_NO},
  [VUT_DIPLREL_UNITANY] = {is_diplrel_unitany_req_active, REQUCH_NO},
  [VUT_DIPLREL_UNITANY_O] = {is_diplrel_unitany_o_req_active, REQUCH_NO},
  [VUT_EXTRA] = {is_extra_req_active, REQUCH_NO, REQUC_LOCAL},
  [VUT_TILEDEF] = {is_tiledef_req_active, REQUCH_NO, REQUC_LOCAL},
  [VUT_EXTRAFLAG] = {is_extraflag_req_active, REQUCH_NO, REQUC_LOCAL},
  [VUT_FUTURETECHS] = {is_futuretechs_req_active, REQUCH_ACT, REQUC_WORLD},
  [VUT_GOOD] = {is_good_req_active, REQUCH_NO},
  [VUT_GOVERNMENT] = {is_gov_req_active, REQUCH_NO},
  [VUT_GOVFLAG] = {is_govflag_req_active, REQUCH_NO},
  [VUT_IMPROVEMENT] = {is_building_req_active, REQUCH_NO, REQUC_IMPR},
  [VUT_SITE] = {is_building_req_active, REQUCH_NO, REQUC_IMPR},
  [VUT_IMPR_GENUS] = {is_buildinggenus_req_active, REQUCH_YES},
  [VUT_IMPR_FLAG] = {is_buildingflag_req_active, REQUCH_YES},
  [VUT_PLAYER_FLAG] = {is_plr_flag_req_active, REQUCH_NO},
  [VUT_PLAYER_STATE] = {is_plr_state_req_active, REQUCH_NO},
  [VUT_MAX_DISTANCE_SQ] = {is_max_distance_sq_req_active, REQUCH_YES},
  [VUT_MAX_REGION_TILES] = {is_max_region_tiles_req_active, REQUCH_NO},
  [VUT_MAXLATITUDE] = {is_latitude_req_active, REQUCH_YES},
  [VUT_MAXTILETOTALUNITS] = {is_maxtotalunitsontile_req_active, REQUCH_NO},
  [VUT_MAXTILETOPUNITS] = {is_maxtopunitsontile_req_active, REQUCH_NO},
  [VUT_MINCALFRAG] = {is_mincalfrag_req_active, REQUCH_NO},
  [VUT_MINCULTURE] = {is_minculture_req_active, REQUCH_NO},
  [VUT_MINFOREIGNPCT] = {is_minforeignpct_req_active, REQUCH_NO},
  [VUT_MINHP] = {is_minhitpoints_req_active, REQUCH_NO},
  [VUT_MINLATITUDE] = {is_latitude_req_active, REQUCH_NO},
  [VUT_MINMOVES] = {is_minmovefrags_req_active, REQUCH_NO},
  [VUT_MINSIZE] = {is_minsize_req_active, REQUCH_NO},
  [VUT_MINTECHS] = {is_mintechs_req_active, REQUCH_ACT, REQUC_WORLD},
  [VUT_MINCITIES] = {is_mincities_req_active, REQUCH_NO},
  [VUT_MINVETERAN] = {is_minveteran_req_active, REQUCH_SCRIPTS, REQUC_PRESENT},
  [VUT_MINYEAR] = {is_minyear_req_active, REQUCH_HACK, REQUC_PRESENT},
  [VUT_NATION] = {is_nation_req_active, REQUCH_HACK, REQUC_NALLY},
  [VUT_NATIONALITY] = {is_nationality_req_active, REQUCH_NO},
  [VUT_NATIONGROUP] = {is_nationgroup_req_active, REQUCH_HACK, REQUC_NALLY},
  [VUT_ORIGINAL_OWNER] = {is_originalowner_req_active, REQUCH_HACK},
  [VUT_OTYPE] = {is_outputtype_req_active, REQUCH_YES},
  [VUT_ROADFLAG] = {is_roadflag_req_active, REQUCH_NO, REQUC_LOCAL},
  [VUT_SERVERSETTING] = {is_serversetting_req_active, REQUCH_HACK},
  [VUT_SPECIALIST] = {is_specialist_req_active, REQUCH_YES},
  [VUT_STYLE] = {is_style_req_active, REQUCH_HACK},
  [VUT_TECHFLAG] = {is_techflag_req_active, REQUCH_NO},
  [VUT_TERRAIN] = {is_terrain_req_active, REQUCH_NO},
  [VUT_TERRAINALTER] = {is_terrainalter_req_active, REQUCH_NO},
  [VUT_TERRAINCLASS] = {is_terrainclass_req_active, REQUCH_NO},
  [VUT_TERRFLAG] = {is_terrainflag_req_active, REQUCH_NO},
  [VUT_TILE_REL] = {is_tile_rel_req_active, REQUCH_NO},
  [VUT_TOPO] = {is_topology_req_active, REQUCH_YES},
  [VUT_WRAP] = {is_wrap_req_active, REQUCH_YES},
  [VUT_UCFLAG] = {is_unitclassflag_req_active, REQUCH_YES},
  [VUT_UCLASS] = {is_unitclass_req_active, REQUCH_YES},
  [VUT_UNITSTATE] = {is_unitstate_req_active, REQUCH_NO},
  [VUT_UTFLAG] = {is_unitflag_req_active, REQUCH_YES},
  [VUT_UTYPE] = {is_unittype_req_active, REQUCH_YES}
};
 
/**********************************************************************/
bool is_req_active(const struct req_context *context,
                   const struct req_context *other_context,
                   const struct requirement *req,
                   const enum   req_problem_type prob_type)
{
  const struct civ_map *nmap = &(wld.map);
  enum fc_tristate eval;
  fc_thread_id self = fc_thread_self();
  struct thr_req_data *trdata = NULL;
 
  fc_mutex_allocate(&trmutex);
  thr_req_data_list_iterate(trdatas, data) {
    if (fc_threads_equal(self, data->thr_id)) {
      trdata = data;
    }
  } thr_req_data_list_iterate_end;
 
  if (trdata == NULL) {
    trdata = fc_malloc(sizeof(struct thr_req_data));
    trdata->thr_id = self;
    thr_req_data_list_append(trdatas, trdata);
  }
  fc_mutex_release(&trmutex);
 
  eval = tri_req_present(nmap, context, other_context, req);
 
  if (eval == TRI_MAYBE) {
    if (prob_type == RPT_POSSIBLE) {
      return TRUE;
    } else {
      return FALSE;
    }
  }
 
  return req->present ? (eval != TRI_NO) : (eval != TRI_YES);
}
 
/**********************************************************************/
static
enum fc_tristate tri_req_present(const struct civ_map *nmap,
                                 const struct req_context *context,
                                 const struct req_context *other_context,
                                 const struct requirement *req)
{
  if (!context) {
    context = req_context_empty();
  }
  if (!other_context) {
    other_context = req_context_empty();
  }
 
  if (req->source.kind >= VUT_COUNT) {
    log_error("tri_req_present(): invalid source kind %d.",
              req->source.kind);
    return TRI_NO;
  }
 
  fc_assert_ret_val(req_definitions[req->source.kind].cb != nullptr, TRI_NO);
 
  return req_definitions[req->source.kind].cb(nmap, context,
                                              other_context, req);
}
 
/**********************************************************************/
enum fc_tristate tri_req_active(const struct req_context *context,
                                const struct req_context *other_context,
                                const struct requirement *req)
{
  const struct civ_map *nmap = &(wld.map);
  enum fc_tristate eval = tri_req_present(nmap, context, other_context, req);
 
  if (!req->present) {
    if (TRI_NO == eval) {
      return TRI_YES;
    }
    if (TRI_YES == eval) {
      return TRI_NO;
    }
  }
 
  return eval;
}
 
/**********************************************************************/
bool are_reqs_active(const struct req_context *context,
                     const struct req_context *other_context,
                     const struct requirement_vector *reqs,
                     const enum   req_problem_type prob_type)
{
  requirement_vector_iterate(reqs, preq) {
    if (!is_req_active(context, other_context, preq, prob_type)) {
      return FALSE;
    }
  } requirement_vector_iterate_end;
 
  return TRUE;
}
 
/**********************************************************************/
bool are_reqs_active_ranges(const enum req_range min_range,
                            const enum req_range max_range,
                            const struct req_context *context,
                            const struct req_context *other_context,
                            const struct requirement_vector *reqs,
                            const enum   req_problem_type prob_type)
{
  requirement_vector_iterate(reqs, preq) {
    if (preq->range >= min_range && preq->range <= max_range) {
      if (!is_req_active(context, other_context, preq, prob_type)) {
        return FALSE;
      }
    }
  } requirement_vector_iterate_end;
 
  return TRUE;
}
 
/**********************************************************************/
enum fc_tristate
tri_req_active_turns(int pass, int period,
                     const struct req_context *context,
                     const struct req_context *other_context,
                     const struct requirement *req)
{
  /* FIXME: Doubles code from calendar.c */
  int ypt = get_world_bonus(EFT_TURN_YEARS);
  int fpt = get_world_bonus(EFT_TURN_FRAGMENTS);
  int fragment = game.info.fragment_count;
  int fragment1 = fragment; /* If fragments don't advance */
  int year_inc, year_inc1;
  const int slowdown = (victory_enabled(VC_SPACERACE)
        ? get_world_bonus(EFT_SLOW_DOWN_TIMELINE) : 0);
  bool present, present1;
 
  fc_assert(pass >= 0 && period >= 0);
  if (slowdown >= 3) {
    if (ypt > 1) {
      ypt = 1;
    }
  } else if (slowdown >= 2) {
    if (ypt > 2) {
      ypt = 2;
    }
  } else if (slowdown >= 1) {
    if (ypt > 5) {
      ypt = 5;
    }
  }
  year_inc = ypt * pass;
  year_inc1 = year_inc + ypt * period;
  if (game.calendar.calendar_fragments) {
    int fragment_years;
 
    fragment += fpt * pass;
    fragment_years = fragment / game.calendar.calendar_fragments;
    year_inc += fragment_years;
    fragment -= fragment_years * game.calendar.calendar_fragments;
    fragment1 = fragment + fpt * period;
    fragment_years += fragment1 / game.calendar.calendar_fragments;
    year_inc1 += fragment_years;
    fragment1 -= fragment_years * game.calendar.calendar_fragments;
  }
  if (game.calendar.calendar_skip_0 && game.info.year < 0) {
    if (year_inc + game.info.year >= 0) {
      year_inc++;
      year_inc1++;
    } else if (year_inc1 + game.info.year >= 0) {
      year_inc1++;
    }
  }
 
  switch (req->source.kind) {
  case VUT_AGE:
    switch (req->range) {
    case REQ_RANGE_LOCAL:
      if (context->unit == nullptr || !is_server()) {
        return TRI_MAYBE;
      } else {
        int ua = game.info.turn + pass - context->unit->birth_turn;
 
        present = req->source.value.age <= ua;
        present1 = req->source.value.age <= ua + period;
      }
      break;
    case REQ_RANGE_CITY:
      if (context->city == nullptr) {
        return TRI_MAYBE;
      } else {
        int ca = game.info.turn + pass - context->city->turn_founded;
 
        present = req->source.value.age <= ca;
        present1 = req->source.value.age <= ca + period;
      }
      break;
    case REQ_RANGE_PLAYER:
      if (context->player == nullptr) {
        return TRI_MAYBE;
      } else {
        present = req->source.value.age
            <= player_age(context->player) + pass;
        present1 = req->source.value.age
            <= player_age(context->player) + pass + period;
      }
      break;
    default:
      return TRI_MAYBE;
    }
    break;
  case VUT_FORM_AGE:
    if (context->unit == nullptr || !is_server()) {
      return TRI_MAYBE;
    } else {
      int ua = game.info.turn + pass - context->unit->current_form_turn;
 
      present = req->source.value.form_age <= ua;
      present1 = req->source.value.form_age <= ua + period;
    }
    break;
  case VUT_MINYEAR:
    present = game.info.year + year_inc >= req->source.value.minyear;
    present1 = game.info.year + year_inc1 >= req->source.value.minyear;
    break;
  case VUT_MINCALFRAG:
    if (game.calendar.calendar_fragments <= period) {
      /* Hope that the requirement is valid and fragments advance fine */
      return TRI_YES;
    }
    present = fragment >= req->source.value.mincalfrag;
    present1 = fragment1 >= req->source.value.mincalfrag;
    break;
  default:
    /* No special handling invented */
    return tri_req_active(context, other_context, req);
  }
  return BOOL_TO_TRISTATE(req->present
                          ? present || present1 : !(present && present1));
}
 
/**********************************************************************/
enum fc_tristate default_tester_cb
   (const struct req_context *context,
    const struct req_context *other_context,
    const struct requirement *req,
    void *data, int n_data)
{
  int i;
 
  fc_assert_ret_val(data || n_data == 0, TRI_NO);
 
  for (i = 0; i < n_data; i++) {
    if (are_requirements_contradictions(&((struct requirement *) data)[i],
                                        req)) {
      return TRI_NO;
    } else if (req_implies_req(&((struct requirement *) data)[i], req)) {
      return TRI_YES;
    }
  }
 
  return tri_req_active(context, other_context, req);
}
 
/**********************************************************************/
enum fc_tristate
  tri_reqs_cb_active(const struct req_context *context,
                     const struct req_context *other_context,
                     const struct requirement_vector *reqs,
                     struct requirement_vector *maybe_reqs,
                     req_tester_cb tester,
                     void *data, int n_data)
{
  bool active = TRUE;
  bool certain = TRUE;
 
  fc_assert_ret_val(tester != nullptr, TRI_NO);
 
  requirement_vector_iterate(reqs, preq) {
    switch(tester(context, other_context, preq,
                  data, n_data)) {
    case TRI_NO:
      active = FALSE;
      certain = TRUE;
      break;
    case TRI_YES:
      break;
    case TRI_MAYBE:
      certain = FALSE;
      if (maybe_reqs) {
        requirement_vector_append(maybe_reqs, *preq);
      }
      break;
    default:
      fc_assert(FALSE);
      active = FALSE;
    }
    if (!active) {
      break;
    }
  } requirement_vector_iterate_end;
 
  return certain ? (active ? TRI_YES : TRI_NO) : TRI_MAYBE;
}
 
/**********************************************************************/
enum req_unchanging_status
  is_req_unchanging(const struct req_context *context,
                    const struct requirement *req)
{
  enum req_unchanging_status s;
  const struct civ_map *nmap = &(wld.map);
 
  fc_assert_ret_val(req, REQUCH_NO);
  fc_assert_ret_val_msg(universals_n_is_valid(req->source.kind), REQUCH_NO,
                        "Invalid source kind %d.", req->source.kind);
  s = req_definitions[req->source.kind].unchanging;
 
  if (req->survives) {
    /* Special case for surviving requirements */
    /* Buildings may obsolete even here */
    if (VUT_IMPROVEMENT == req->source.kind) {
      const struct impr_type *b = req->source.value.building;
 
      if (can_improvement_go_obsolete(b)) {
        if (improvement_obsolete(context->player, b, context->city)) {
          /* FIXME: Sometimes can unobsolete, but considering it
           * may sometimes put the function on endless recursion */
          return REQUCH_ACT; /* Mostly about techs */
        } else {
          /* NOTE: May obsoletion reqs be unchanging? Hardly but why not. */
          return REQUCH_NO;
        }
      }
    }
    s = unchanging_present(nmap, s, context, req);
    if (s != REQUCH_NO) {
      return unchanging_noally(nmap, s, context, req);
    }
  } else {
    req_unchanging_cond_cb cond
      = req_definitions[req->source.kind].unchanging_cond;
 
    if (cond) {
      return cond(nmap, s, context, req);
    }
  }
 
  return s;
}
 
/**********************************************************************/
enum req_unchanging_status
  is_req_preventing(const struct req_context *context,
                    const struct req_context *other_context,
                    const struct requirement *req,
                    enum req_problem_type prob_type)
{
  enum req_unchanging_status u = is_req_unchanging(context, req);
 
  if (REQUCH_NO != u) {
    /* Presence is precalculated */
    bool auto_present = (req->survives
         && !(VUT_IMPROVEMENT == req->source.kind
              && can_improvement_go_obsolete(req->source.value.building)))
      || REQUC_PRESENT == req_definitions[req->source.kind].unchanging_cond
      || REQUC_WORLD == req_definitions[req->source.kind].unchanging_cond;
 
    if (auto_present ? req->present
        : is_req_active(context, other_context, req, RPT_POSSIBLE)) {
      /* Unchanging but does not block */
      return REQUCH_NO;
    }
  }
 
  return u;
}
 
/**********************************************************************/
bool is_req_in_vec(const struct requirement *req,
                   const struct requirement_vector *vec)
{
  requirement_vector_iterate(vec, preq) {
    if (are_requirements_equal(req, preq)) {
      return TRUE;
    }
  } requirement_vector_iterate_end;
 
  return FALSE;
}
 
/**********************************************************************/
bool req_vec_wants_type(const struct requirement_vector *reqs,
                        enum universals_n kind)
{
  requirement_vector_iterate(reqs, preq) {
    if (preq->present && preq->source.kind == kind) {
      return TRUE;
    }
  } requirement_vector_iterate_end;
  return FALSE;
}
 
/**********************************************************************/
bool universal_never_there(const struct universal *source)
{
  switch (source->kind) {
  case VUT_ACTION:
    return !action_is_in_use(source->value.action);
  case VUT_UTFLAG:
    return !utype_flag_is_in_use(source->value.unitflag);
  case VUT_UCFLAG:
    return !uclass_flag_is_in_use(source->value.unitclassflag);
  case VUT_EXTRAFLAG:
    return !extra_flag_is_in_use(source->value.extraflag);
  case VUT_MINLATITUDE:
    return source->value.latitude > MAP_MAX_REAL_LATITUDE(wld.map);
  case VUT_MAXLATITUDE:
    return source->value.latitude < MAP_MIN_REAL_LATITUDE(wld.map);
  case VUT_COUNTER:
  case VUT_OTYPE:
  case VUT_SPECIALIST:
  case VUT_AI_LEVEL:
  case VUT_CITYTILE:
  case VUT_CITYSTATUS:
  case VUT_STYLE:
  case VUT_TOPO:
  case VUT_WRAP:
  case VUT_SERVERSETTING:
  case VUT_NATION:
  case VUT_NATIONGROUP:
  case VUT_ADVANCE:
  case VUT_TECHFLAG:
  case VUT_GOVERNMENT:
  case VUT_GOVFLAG:
  case VUT_ACHIEVEMENT:
  case VUT_IMPROVEMENT:
  case VUT_SITE:
  case VUT_IMPR_GENUS:
  case VUT_IMPR_FLAG:
  case VUT_PLAYER_FLAG:
  case VUT_PLAYER_STATE:
  case VUT_MINSIZE:
  case VUT_MINCULTURE:
  case VUT_MINFOREIGNPCT:
  case VUT_MINTECHS:
  case VUT_FUTURETECHS:
  case VUT_MINCITIES:
  case VUT_NATIONALITY:
  case VUT_ORIGINAL_OWNER: /* As long as midgame player creation or civil war possible */
  case VUT_DIPLREL:
  case VUT_DIPLREL_TILE:
  case VUT_DIPLREL_TILE_O:
  case VUT_DIPLREL_UNITANY:
  case VUT_DIPLREL_UNITANY_O:
  case VUT_MAXTILETOTALUNITS:
  case VUT_MAXTILETOPUNITS:
  case VUT_UTYPE:
  case VUT_UCLASS:
  case VUT_MINVETERAN:
  case VUT_UNITSTATE:
  case VUT_ACTIVITY:
  case VUT_MINMOVES:
  case VUT_MINHP:
  case VUT_AGE:
  case VUT_FORM_AGE:
  case VUT_ROADFLAG:
  case VUT_MINCALFRAG:
  case VUT_TERRAIN:
  case VUT_EXTRA:
  case VUT_TILEDEF:
  case VUT_GOOD:
  case VUT_TERRAINCLASS:
  case VUT_TERRFLAG:
  case VUT_TERRAINALTER:
  case VUT_MINYEAR:
  case VUT_MAX_DISTANCE_SQ:
  case VUT_MAX_REGION_TILES:
  case VUT_TILE_REL:
  case VUT_NONE:
  case VUT_COUNT:
    /* Not implemented. */
    break;
  }
 
  return FALSE;
}
 
/**********************************************************************/
bool req_is_impossible_to_fulfill(const struct requirement *req)
{
  /* Not known to be impossible to fulfill */
  return req->present && universal_never_there(&req->source);
}
 
/**********************************************************************/
bool req_vec_is_impossible_to_fulfill(const struct requirement_vector *reqs)
{
  requirement_vector_iterate(reqs, preq) {
    if (req_is_impossible_to_fulfill(preq)) {
      return TRUE;
    }
  } requirement_vector_iterate_end;
 
  /* Not known to be impossible to fulfill */
  return FALSE;
}
 
/**********************************************************************/
req_vec_num_in_item
req_vec_vector_number(const void *parent_item,
                      const struct requirement_vector *vec)
{
  if (vec) {
    return 0;
  } else {
    return -1;
  }
}
 
/********************************************************************/
struct requirement_vector *
req_vec_by_number(const void *parent_item, req_vec_num_in_item number)
{
  fc_assert_ret_val(number == 0, nullptr);
 
  return (struct requirement_vector *)parent_item;
}
 
/**********************************************************************/
const char *req_vec_change_translation(const struct req_vec_change *change,
                                       const requirement_vector_namer namer)
{
  const char *req_vec_description;
  static char buf[MAX_LEN_NAME * 3];
  struct astring astr;
 
  fc_assert_ret_val(change, nullptr);
  fc_assert_ret_val(req_vec_change_operation_is_valid(change->operation),
                    nullptr);
 
  /* Get rid of the previous. */
  buf[0] = '\0';
 
  if (namer == nullptr) {
    /* TRANS: default description of a requirement vector
     * (used in ruledit) */
    req_vec_description = _("the requirement vector");
  } else {
    req_vec_description = namer(change->vector_number);
  }
 
  switch (change->operation) {
  case RVCO_REMOVE:
    fc_snprintf(buf, sizeof(buf),
                /* TRANS: remove a requirement from a requirement vector
                 * (in ruledit).
                 * The first %s is the operation.
                 * The second %s is the requirement.
                 * The third %s is a description of the requirement vector,
                 * like "actor_reqs" */
                _("%s %s from %s"),
                req_vec_change_operation_name(change->operation),
                req_to_fstring(&change->req, &astr),
                req_vec_description);
    astr_free(&astr);
    break;
  case RVCO_APPEND:
    fc_snprintf(buf, sizeof(buf),
                /* TRANS: append a requirement to a requirement vector
                 * (in ruledit).
                 * The first %s is the operation.
                 * The second %s is the requirement.
                 * The third %s is a description of the requirement vector,
                 * like "actor_reqs" */
                _("%s %s to %s"),
                req_vec_change_operation_name(change->operation),
                req_to_fstring(&change->req, &astr),
                req_vec_description);
    astr_free(&astr);
    break;
  case RVCO_NOOP:
    fc_snprintf(buf, sizeof(buf),
                /* TRANS: do nothing to a requirement vector (in ruledit).
                 * The first %s is a description of the requirement vector,
                 * like "actor_reqs" */
                _("Do nothing to %s"), req_vec_description);
    break;
  }
 
  return buf;
}
 
/**********************************************************************/
bool req_vec_change_apply(const struct req_vec_change *modification,
                          requirement_vector_by_number getter,
                          const void *parent_item)
{
  struct requirement_vector *target
      = getter(parent_item, modification->vector_number);
  int i = 0;
 
  switch (modification->operation) {
  case RVCO_APPEND:
    requirement_vector_append(target, modification->req);
    return TRUE;
  case RVCO_REMOVE:
    requirement_vector_iterate(target, preq) {
      if (are_requirements_equal(&modification->req, preq)) {
        requirement_vector_remove(target, i);
        return TRUE;
      }
      i++;
    } requirement_vector_iterate_end;
    return FALSE;
  case RVCO_NOOP:
    return FALSE;
  }
 
  return FALSE;
}
 
/**********************************************************************/
struct req_vec_problem *
req_vec_problem_new_transl(int num_suggested_solutions,
                           const char *description,
                           const char *description_translated)
{
  struct req_vec_problem *out;
  int i;
 
  out = fc_malloc(sizeof(*out));
 
  fc_strlcpy(out->description, description, sizeof(out->description));
  fc_strlcpy(out->description_translated, _(description_translated),
             sizeof(out->description_translated));
 
  out->num_suggested_solutions = num_suggested_solutions;
  out->suggested_solutions = fc_malloc(out->num_suggested_solutions
                                       * sizeof(struct req_vec_change));
  for (i = 0; i < out->num_suggested_solutions; i++) {
    /* No suggestions are ready yet. */
    out->suggested_solutions[i].operation = RVCO_NOOP;
    out->suggested_solutions[i].vector_number = -1;
    out->suggested_solutions[i].req.source.kind = VUT_NONE;
  }
 
  return out;
}
 
/**********************************************************************/
struct req_vec_problem *req_vec_problem_new(int num_suggested_solutions,
                                            const char *descr, ...)
{
  char description[500];
  char description_translated[500];
  va_list ap;
 
  va_start(ap, descr);
  fc_vsnprintf(description, sizeof(description), descr, ap);
  va_end(ap);
 
  va_start(ap, descr);
  fc_vsnprintf(description_translated, sizeof(description_translated),
               _(descr), ap);
  va_end(ap);
 
  return req_vec_problem_new_transl(num_suggested_solutions,
                                    description, description_translated);
}
 
/**********************************************************************/
void req_vec_problem_free(struct req_vec_problem *issue)
{
  FC_FREE(issue->suggested_solutions);
  issue->num_suggested_solutions = 0;
 
  FC_FREE(issue);
}
 
/**********************************************************************/
struct req_vec_problem *
req_vec_get_first_contradiction(const struct requirement_vector *vec,
                                requirement_vector_number get_num,
                                const void *parent_item)
{
  int i, j;
  req_vec_num_in_item vec_num;
 
  if (vec == nullptr || requirement_vector_size(vec) == 0) {
    /* No vector. */
    return nullptr;
  }
 
  if (get_num == nullptr || parent_item == nullptr) {
    vec_num = 0;
  } else {
    vec_num = get_num(parent_item, vec);
  }
 
  /* Look for contradictions */
  for (i = 0; i < requirement_vector_size(vec); i++) {
    struct requirement *preq = requirement_vector_get(vec, i);
    for (j = 0; j < requirement_vector_size(vec); j++) {
      struct requirement *nreq = requirement_vector_get(vec, j);
 
      if (are_requirements_contradictions(preq, nreq)) {
        struct req_vec_problem *problem;
        struct astring astr;
        struct astring nastr;
 
        problem = req_vec_problem_new(2,
            N_("Requirements {%s} and {%s} contradict each other."),
            req_to_fstring(preq, &astr), req_to_fstring(nreq, &nastr));
 
        astr_free(&astr);
        astr_free(&nastr);
 
        /* The solution is to remove one of the contradictions. */
        problem->suggested_solutions[0].operation = RVCO_REMOVE;
        problem->suggested_solutions[0].vector_number = vec_num;
        problem->suggested_solutions[0].req = *preq;
 
        problem->suggested_solutions[1].operation = RVCO_REMOVE;
        problem->suggested_solutions[1].vector_number = vec_num;
        problem->suggested_solutions[1].req = *nreq;
 
        /* Only the first contradiction is reported. */
        return problem;
      }
    }
  }
 
  return nullptr;
}
 
/**********************************************************************/
struct req_vec_problem *
req_vec_suggest_repair(const struct requirement_vector *vec,
                       requirement_vector_number get_num,
                       const void *parent_item)
{
  /* Check for self contradictins. */
  return req_vec_get_first_contradiction(vec, get_num, parent_item);
}
 
/**********************************************************************/
struct req_vec_problem *
req_vec_get_first_missing_univ(const struct requirement_vector *vec,
                               requirement_vector_number get_num,
                               const void *parent_item)
{
  int i;
  req_vec_num_in_item vec_num;
  struct req_vec_problem *problem = nullptr;
 
  if (vec == nullptr || requirement_vector_size(vec) == 0) {
    /* No vector. */
    return nullptr;
  }
 
  if (get_num == nullptr || parent_item == nullptr) {
    vec_num = 0;
  } else {
    vec_num = get_num(parent_item, vec);
  }
 
  /* Look for contradictions */
  for (i = 0; i < requirement_vector_size(vec); i++) {
    struct requirement *preq = requirement_vector_get(vec, i);
 
    if (universal_never_there(&preq->source)) {
      struct astring astr;
 
      if (preq->present) {
        /* The requirement vector can never be fulfilled. Removing the
         * requirement makes it possible to fulfill it. This is a rule
         * change and shouldn't be "fixed" without thinking. Don't offer any
         * automatic solution to prevent mindless "fixes". */
        /* TRANS: Ruledit warns a user about an unused requirement vector
         * that never can be fulfilled because it asks for something that
         * never will be there. */
        if (problem == nullptr) {
          problem = req_vec_problem_new(0,
                       N_("Requirement {%s} requires %s but it will never be"
                          " there."),
                       req_to_fstring(preq, &astr), universal_rule_name(&preq->source));
          astr_free(&astr);
        }
 
        /* Continue to check if other problems have a solution proposal,
         * and prefer to return those. */
        continue;
      }
 
      if (problem != nullptr) {
        /* Free previous one (one with no solution proposals) */
        req_vec_problem_free(problem);
      }
 
      problem = req_vec_problem_new(1,
          N_("Requirement {%s} mentions %s but it will never be there."),
          req_to_fstring(preq, &astr), universal_rule_name(&preq->source));
 
      astr_free(&astr);
 
      /* The solution is to remove the reference to the missing
       * universal. */
      problem->suggested_solutions[0].operation = RVCO_REMOVE;
      problem->suggested_solutions[0].vector_number = vec_num;
      problem->suggested_solutions[0].req = *preq;
 
      /* Only the first missing universal is reported. */
      return problem;
    }
  }
 
  return problem;
}
 
/**********************************************************************/
struct req_vec_problem *
req_vec_get_first_redundant_req(const struct requirement_vector *vec,
                                requirement_vector_number get_num,
                                const void *parent_item)
{
  int i, j;
  req_vec_num_in_item vec_num;
 
  if (vec == nullptr || requirement_vector_size(vec) == 0) {
    /* No vector. */
    return nullptr;
  }
 
  if (get_num == nullptr || parent_item == nullptr) {
    vec_num = 0;
  } else {
    vec_num = get_num(parent_item, vec);
  }
 
  /* Look for repeated requirements */
  for (i = 0; i < requirement_vector_size(vec) - 1; i++) {
    struct requirement *preq = requirement_vector_get(vec, i);
    for (j = i + 1; j < requirement_vector_size(vec); j++) {
      struct requirement *nreq = requirement_vector_get(vec, j);
 
      if (are_requirements_equal(preq, nreq)) {
        struct req_vec_problem *problem;
        struct astring astr;
        struct astring nastr;
 
        problem = req_vec_problem_new(2,
            N_("Requirements {%s} and {%s} are the same."),
            req_to_fstring(preq, &astr), req_to_fstring(nreq, &nastr));
 
        astr_free(&astr);
        astr_free(&nastr);
 
        /* The solution is to remove one of the redundant requirements. */
        problem->suggested_solutions[0].operation = RVCO_REMOVE;
        problem->suggested_solutions[0].vector_number = vec_num;
        problem->suggested_solutions[0].req = *preq;
 
        problem->suggested_solutions[1].operation = RVCO_REMOVE;
        problem->suggested_solutions[1].vector_number = vec_num;
        problem->suggested_solutions[1].req = *nreq;
 
        /* Only the first redundancy is reported. */
        return problem;
      }
    }
  }
 
  return nullptr;
}
 
/**********************************************************************/
struct req_vec_problem *
req_vec_suggest_improvement(const struct requirement_vector *vec,
                            requirement_vector_number get_num,
                            const void *parent_item)
{
  struct req_vec_problem *out;
 
  out = req_vec_suggest_repair(vec, get_num, parent_item);
  if (out != nullptr) {
    /* A bug, not just a potential improvement */
    return out;
  }
 
  /* Check if a universal that never will appear in the game is checked. */
  out = req_vec_get_first_missing_univ(vec, get_num, parent_item);
  if (out != nullptr) {
    return out;
  }
 
  /* Check if a requirement is redundant. */
  out = req_vec_get_first_redundant_req(vec, get_num, parent_item);
  return out;
}
 
/**********************************************************************/
bool are_universals_equal(const struct universal *psource1,
                          const struct universal *psource2)
{
  if (psource1->kind != psource2->kind) {
    return FALSE;
  }
  switch (psource1->kind) {
  case VUT_NONE:
    return TRUE;
  case VUT_COUNTER:
    return psource1->value.counter == psource2->value.counter;
  case VUT_ADVANCE:
    return psource1->value.advance == psource2->value.advance;
  case VUT_TECHFLAG:
    return psource1->value.techflag == psource2->value.techflag;
  case VUT_GOVERNMENT:
    return psource1->value.govern == psource2->value.govern;
  case VUT_GOVFLAG:
    return psource1->value.govflag == psource2->value.govflag;
  case VUT_ACHIEVEMENT:
    return psource1->value.achievement == psource2->value.achievement;
  case VUT_STYLE:
    return psource1->value.style == psource2->value.style;
  case VUT_IMPROVEMENT:
  case VUT_SITE:
    return psource1->value.building == psource2->value.building;
  case VUT_IMPR_GENUS:
    return psource1->value.impr_genus == psource2->value.impr_genus;
  case VUT_IMPR_FLAG:
    return psource1->value.impr_flag == psource2->value.impr_flag;
  case VUT_PLAYER_FLAG:
    return psource1->value.plr_flag == psource2->value.plr_flag;
  case VUT_PLAYER_STATE:
    return psource1->value.plrstate == psource2->value.plrstate;
  case VUT_EXTRA:
    return psource1->value.extra == psource2->value.extra;
  case VUT_TILEDEF:
    return psource1->value.tiledef == psource2->value.tiledef;
  case VUT_GOOD:
    return psource1->value.good == psource2->value.good;
  case VUT_TERRAIN:
    return psource1->value.terrain == psource2->value.terrain;
  case VUT_TERRFLAG:
    return psource1->value.terrainflag == psource2->value.terrainflag;
  case VUT_NATION:
    return psource1->value.nation == psource2->value.nation;
  case VUT_NATIONGROUP:
    return psource1->value.nationgroup == psource2->value.nationgroup;
  case VUT_NATIONALITY:
    return psource1->value.nationality == psource2->value.nationality;
  case VUT_ORIGINAL_OWNER:
    return psource1->value.origowner == psource2->value.origowner;
  case VUT_DIPLREL:
  case VUT_DIPLREL_TILE:
  case VUT_DIPLREL_TILE_O:
  case VUT_DIPLREL_UNITANY:
  case VUT_DIPLREL_UNITANY_O:
    return psource1->value.diplrel == psource2->value.diplrel;
  case VUT_UTYPE:
    return psource1->value.utype == psource2->value.utype;
  case VUT_UTFLAG:
    return psource1->value.unitflag == psource2->value.unitflag;
  case VUT_UCLASS:
    return psource1->value.uclass == psource2->value.uclass;
  case VUT_UCFLAG:
    return psource1->value.unitclassflag == psource2->value.unitclassflag;
  case VUT_MINVETERAN:
    return psource1->value.minveteran == psource2->value.minveteran;
  case VUT_UNITSTATE:
    return psource1->value.unit_state == psource2->value.unit_state;
  case VUT_ACTIVITY:
    return psource1->value.activity == psource2->value.activity;
  case VUT_MINMOVES:
    return psource1->value.minmoves == psource2->value.minmoves;
  case VUT_MINHP:
    return psource1->value.min_hit_points == psource2->value.min_hit_points;
  case VUT_AGE:
    return psource1->value.age == psource2->value.age;
  case VUT_FORM_AGE:
    return psource1->value.form_age == psource2->value.form_age;
  case VUT_MINTECHS:
    return psource1->value.min_techs == psource2->value.min_techs;
  case VUT_FUTURETECHS:
    return psource1->value.future_techs == psource2->value.future_techs;
  case VUT_MINCITIES:
    return psource1->value.min_cities == psource2->value.min_cities;
  case VUT_ACTION:
    return (action_number(psource1->value.action)
            == action_number(psource2->value.action));
  case VUT_OTYPE:
    return psource1->value.outputtype == psource2->value.outputtype;
  case VUT_SPECIALIST:
    return psource1->value.specialist == psource2->value.specialist;
  case VUT_MINSIZE:
    return psource1->value.minsize == psource2->value.minsize;
  case VUT_MINCULTURE:
    return psource1->value.minculture == psource2->value.minculture;
  case VUT_MINFOREIGNPCT:
    return psource1->value.minforeignpct == psource2->value.minforeignpct;
  case VUT_AI_LEVEL:
    return psource1->value.ai_level == psource2->value.ai_level;
  case VUT_MAXTILETOTALUNITS:
    return psource1->value.max_tile_total_units == psource2->value.max_tile_total_units;
  case VUT_MAXTILETOPUNITS:
    return psource1->value.max_tile_top_units == psource2->value.max_tile_top_units;
  case VUT_TERRAINCLASS:
    return psource1->value.terrainclass == psource2->value.terrainclass;
  case VUT_ROADFLAG:
    return psource1->value.roadflag == psource2->value.roadflag;
  case VUT_EXTRAFLAG:
    return psource1->value.extraflag == psource2->value.extraflag;
  case VUT_MINYEAR:
    return psource1->value.minyear == psource2->value.minyear;
  case VUT_MINCALFRAG:
    return psource1->value.mincalfrag == psource2->value.mincalfrag;
  case VUT_TOPO:
    return psource1->value.topo_property == psource2->value.topo_property;
  case VUT_WRAP:
    return psource1->value.wrap_property == psource2->value.wrap_property;
  case VUT_SERVERSETTING:
    return psource1->value.ssetval == psource2->value.ssetval;
  case VUT_TERRAINALTER:
    return psource1->value.terrainalter == psource2->value.terrainalter;
  case VUT_CITYTILE:
    return psource1->value.citytile == psource2->value.citytile;
  case VUT_CITYSTATUS:
    return psource1->value.citystatus == psource2->value.citystatus;
  case VUT_TILE_REL:
    return psource1->value.tilerel == psource2->value.tilerel;
  case VUT_MINLATITUDE:
  case VUT_MAXLATITUDE:
    return psource1->value.latitude == psource2->value.latitude;
  case VUT_MAX_DISTANCE_SQ:
    return psource1->value.distance_sq == psource2->value.distance_sq;
  case VUT_MAX_REGION_TILES:
    return psource1->value.region_tiles == psource2->value.region_tiles;
  case VUT_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid source kind %d.", psource1->kind);
  return FALSE;
}
 
/**********************************************************************/
const char *universal_rule_name(const struct universal *psource)
{
  static char buffer[10];
 
  switch (psource->kind) {
  case VUT_NONE:
    return "(none)";
  case VUT_COUNTER:
    return counter_rule_name(psource->value.counter);
  case VUT_CITYTILE:
    return citytile_type_name(psource->value.citytile);
  case VUT_CITYSTATUS:
    return citystatus_type_name(psource->value.citystatus);
  case VUT_TILE_REL:
    return tilerel_type_name(psource->value.tilerel);
  case VUT_MINYEAR:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.minyear);
 
    return buffer;
  case VUT_MINCALFRAG:
    /* Rule name is 0-based number, not pretty name from ruleset */
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.mincalfrag);
 
    return buffer;
  case VUT_TOPO:
    return topo_flag_name(psource->value.topo_property);
  case VUT_WRAP:
    return wrap_flag_name(psource->value.wrap_property);
  case VUT_SERVERSETTING:
    return ssetv_rule_name(psource->value.ssetval);
  case VUT_ADVANCE:
    return advance_rule_name(psource->value.advance);
  case VUT_TECHFLAG:
    return tech_flag_id_name(psource->value.techflag);
  case VUT_GOVERNMENT:
    return government_rule_name(psource->value.govern);
  case VUT_GOVFLAG:
    return gov_flag_id_name(psource->value.govflag);
  case VUT_ACHIEVEMENT:
    return achievement_rule_name(psource->value.achievement);
  case VUT_STYLE:
    return style_rule_name(psource->value.style);
  case VUT_IMPROVEMENT:
  case VUT_SITE:
    return improvement_rule_name(psource->value.building);
  case VUT_IMPR_GENUS:
    return impr_genus_id_name(psource->value.impr_genus);
  case VUT_IMPR_FLAG:
    return impr_flag_id_name(psource->value.impr_flag);
  case VUT_PLAYER_FLAG:
    return plr_flag_id_name(psource->value.plr_flag);
  case VUT_PLAYER_STATE:
    return plrstate_type_name(psource->value.plrstate);
  case VUT_EXTRA:
    return extra_rule_name(psource->value.extra);
  case VUT_TILEDEF:
    return tiledef_rule_name(psource->value.tiledef);
  case VUT_GOOD:
    return goods_rule_name(psource->value.good);
  case VUT_TERRAIN:
    return terrain_rule_name(psource->value.terrain);
  case VUT_TERRFLAG:
    return terrain_flag_id_name(psource->value.terrainflag);
  case VUT_NATION:
    return nation_rule_name(psource->value.nation);
  case VUT_NATIONGROUP:
    return nation_group_rule_name(psource->value.nationgroup);
  case VUT_DIPLREL:
  case VUT_DIPLREL_TILE:
  case VUT_DIPLREL_TILE_O:
  case VUT_DIPLREL_UNITANY:
  case VUT_DIPLREL_UNITANY_O:
    return diplrel_rule_name(psource->value.diplrel);
  case VUT_NATIONALITY:
    return nation_rule_name(psource->value.nationality);
  case VUT_ORIGINAL_OWNER:
    return nation_rule_name(psource->value.origowner);
  case VUT_UTYPE:
    return utype_rule_name(psource->value.utype);
  case VUT_UTFLAG:
    return unit_type_flag_id_name(psource->value.unitflag);
  case VUT_UCLASS:
    return uclass_rule_name(psource->value.uclass);
  case VUT_UCFLAG:
    return unit_class_flag_id_name(psource->value.unitclassflag);
  case VUT_MINVETERAN:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.minveteran);
 
    return buffer;
  case VUT_UNITSTATE:
    return ustate_prop_name(psource->value.unit_state);
  case VUT_ACTIVITY:
    return unit_activity_name(psource->value.activity);
  case VUT_MINMOVES:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.minmoves);
 
    return buffer;
  case VUT_MINHP:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.min_hit_points);
 
    return buffer;
  case VUT_AGE:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.age);
 
    return buffer;
  case VUT_FORM_AGE:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.form_age);
 
    return buffer;
  case VUT_MINTECHS:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.min_techs);
 
    return buffer;
  case VUT_FUTURETECHS:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.future_techs);
 
    return buffer;
  case VUT_MINCITIES:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.min_cities);
 
    return buffer;
  case VUT_ACTION:
    return action_rule_name(psource->value.action);
  case VUT_OTYPE:
    return get_output_identifier(psource->value.outputtype);
  case VUT_SPECIALIST:
    return specialist_rule_name(psource->value.specialist);
  case VUT_MINSIZE:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.minsize);
 
    return buffer;
  case VUT_MINCULTURE:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.minculture);
 
    return buffer;
  case VUT_MINFOREIGNPCT:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.minforeignpct);
 
    return buffer;
  case VUT_AI_LEVEL:
    return ai_level_name(psource->value.ai_level);
  case VUT_MAXTILETOTALUNITS:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.max_tile_total_units);
    return buffer;
  case VUT_MAXTILETOPUNITS:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.max_tile_top_units);
    return buffer;
  case VUT_TERRAINCLASS:
    return terrain_class_name(psource->value.terrainclass);
  case VUT_ROADFLAG:
    return road_flag_id_name(psource->value.roadflag);
  case VUT_EXTRAFLAG:
    return extra_flag_id_name(psource->value.extraflag);
  case VUT_TERRAINALTER:
    return terrain_alteration_name(psource->value.terrainalter);
  case VUT_MINLATITUDE:
  case VUT_MAXLATITUDE:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.latitude);
 
    return buffer;
  case VUT_MAX_DISTANCE_SQ:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.distance_sq);
 
    return buffer;
  case VUT_MAX_REGION_TILES:
    fc_snprintf(buffer, sizeof(buffer), "%d", psource->value.region_tiles);
 
    return buffer;
  case VUT_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid source kind %d.", psource->kind);
 
  return nullptr;
}
 
/**********************************************************************/
const char *universal_name_translation(const struct universal *psource,
                                       char *buf, size_t bufsz)
{
  buf[0] = '\0'; /* to be safe. */
  switch (psource->kind) {
  case VUT_NONE:
    /* TRANS: missing value */
    fc_strlcat(buf, _("(none)"), bufsz);
    return buf;
  case VUT_ADVANCE:
    fc_strlcat(buf, advance_name_translation(psource->value.advance), bufsz);
    return buf;
  case VUT_COUNTER:
    fc_strlcat(buf, counter_name_translation(psource->value.counter), bufsz);
    return buf;
  case VUT_TECHFLAG:
    cat_snprintf(buf, bufsz, _("\"%s\" tech"),
                 tech_flag_id_translated_name(psource->value.techflag));
    return buf;
  case VUT_GOVERNMENT:
    fc_strlcat(buf, government_name_translation(psource->value.govern),
               bufsz);
    return buf;
  case VUT_GOVFLAG:
    cat_snprintf(buf, bufsz, _("\"%s\" gov"),
                 gov_flag_id_translated_name(psource->value.govflag));
    return buf;
  case VUT_ACHIEVEMENT:
    fc_strlcat(buf, achievement_name_translation(psource->value.achievement),
               bufsz);
    return buf;
  case VUT_STYLE:
    fc_strlcat(buf, style_name_translation(psource->value.style),
               bufsz);
    return buf;
  case VUT_IMPROVEMENT:
    fc_strlcat(buf, improvement_name_translation(psource->value.building),
               bufsz);
    return buf;
  case VUT_SITE:
    {
      char local_buf[1024];
 
      fc_snprintf(local_buf, sizeof(local_buf), _("%s site"),
                  improvement_name_translation(psource->value.building));
      fc_strlcat(buf, local_buf, bufsz);
    }
 
    return buf;
  case VUT_IMPR_GENUS:
    fc_strlcat(buf,
               impr_genus_id_translated_name(psource->value.impr_genus),
               bufsz);
    return buf;
  case VUT_IMPR_FLAG:
    fc_strlcat(buf,
               impr_flag_id_translated_name(psource->value.impr_flag),
               bufsz);
    return buf;
  case VUT_PLAYER_FLAG:
    fc_strlcat(buf,
               plr_flag_id_translated_name(psource->value.plr_flag),
               bufsz);
    return buf;
  case VUT_PLAYER_STATE:
    fc_strlcat(buf,
               plrstate_type_translated_name(psource->value.plrstate),
               bufsz);
    return buf;
  case VUT_EXTRA:
    fc_strlcat(buf, extra_name_translation(psource->value.extra), bufsz);
    return buf;
  case VUT_TILEDEF:
    fc_strlcat(buf, tiledef_name_translation(psource->value.tiledef), bufsz);
    return buf;
  case VUT_GOOD:
    fc_strlcat(buf, goods_name_translation(psource->value.good), bufsz);
    return buf;
  case VUT_TERRAIN:
    fc_strlcat(buf, terrain_name_translation(psource->value.terrain), bufsz);
    return buf;
  case VUT_NATION:
    fc_strlcat(buf, nation_adjective_translation(psource->value.nation),
               bufsz);
    return buf;
  case VUT_NATIONGROUP:
    fc_strlcat(buf, nation_group_name_translation(psource->value.nationgroup),
               bufsz);
    return buf;
  case VUT_NATIONALITY:
    cat_snprintf(buf, bufsz, _("%s citizens"),
                 nation_adjective_translation(psource->value.nationality));
    return buf;
  case VUT_ORIGINAL_OWNER:
    /* TRANS: Keep short. City founding nation. */
    cat_snprintf(buf, bufsz, _("%s original owner"),
                 nation_adjective_translation(psource->value.origowner));
    return buf;
  case VUT_DIPLREL:
  case VUT_DIPLREL_TILE:
  case VUT_DIPLREL_TILE_O:
  case VUT_DIPLREL_UNITANY:
  case VUT_DIPLREL_UNITANY_O:
    fc_strlcat(buf, diplrel_name_translation(psource->value.diplrel),
               bufsz);
    return buf;
  case VUT_UTYPE:
    fc_strlcat(buf, utype_name_translation(psource->value.utype), bufsz);
    return buf;
  case VUT_UTFLAG:
    cat_snprintf(buf, bufsz,
                 /* TRANS: Unit type flag */
                 Q_("?utflag:\"%s\" units"),
                 unit_type_flag_id_translated_name(
                   psource->value.unitflag));
    return buf;
  case VUT_UCLASS:
    cat_snprintf(buf, bufsz,
                 /* TRANS: Unit class */
                 _("%s units"),
                 uclass_name_translation(psource->value.uclass));
    return buf;
  case VUT_UCFLAG:
    cat_snprintf(buf, bufsz,
                 /* TRANS: Unit class flag */
                 Q_("?ucflag:\"%s\" units"),
                 unit_class_flag_id_translated_name(
                   psource->value.unitclassflag));
    return buf;
  case VUT_MINVETERAN:
    /* FIXME */
    cat_snprintf(buf, bufsz, _("Veteran level >=%d"),
                 psource->value.minveteran);
    return buf;
  case VUT_UNITSTATE:
    switch (psource->value.unit_state) {
    case USP_TRANSPORTED:
      /* TRANS: unit state. (appears in strings like "Missile+Transported") */
      cat_snprintf(buf, bufsz, _("Transported"));
      break;
    case USP_LIVABLE_TILE:
      cat_snprintf(buf, bufsz,
                   /* TRANS: unit state. (appears in strings like
                    * "Missile+On livable tile") */
                   _("On livable tile"));
      break;
    case USP_TRANSPORTING:
      /* TRANS: unit state. (appears in strings like "Missile+Transported") */
      cat_snprintf(buf, bufsz, _("Transporting"));
      break;
    case USP_HAS_HOME_CITY:
      /* TRANS: unit state. (appears in strings like "Missile+Has a home city") */
      cat_snprintf(buf, bufsz, _("Has a home city"));
      break;
    case USP_NATIVE_TILE:
      cat_snprintf(buf, bufsz,
                   /* TRANS: unit state. (appears in strings like
                    * "Missile+On native tile") */
                   _("On native tile"));
      break;
    case USP_NATIVE_EXTRA:
      cat_snprintf(buf, bufsz,
                   /* TRANS: unit state. (appears in strings like
                    * "Missile+In native extra") */
                   _("In native extra"));
      break;
    case USP_MOVED_THIS_TURN:
      /* TRANS: unit state. (appears in strings like
       * "Missile+Has moved this turn") */
      cat_snprintf(buf, bufsz, _("Has moved this turn"));
      break;
    case USP_COUNT:
      fc_assert_msg(psource->value.unit_state != USP_COUNT,
                    "Invalid unit state property.");
      break;
    }
    return buf;
  case VUT_ACTIVITY:
    cat_snprintf(buf, bufsz, _("%s activity"),
                 Q_(unit_activity_name(psource->value.activity)));
    return buf;
  case VUT_MINMOVES:
    /* TRANS: Minimum unit movement points left for requirement to be met
     * (%s is a string like "1" or "2 1/3") */
    cat_snprintf(buf, bufsz, _("%s MP"),
                 move_points_text(psource->value.minmoves, TRUE));
    return buf;
  case VUT_MINHP:
    /* TRANS: HP = hit points */
    cat_snprintf(buf, bufsz, _("%d HP"),
                 psource->value.min_hit_points);
    return buf;
  case VUT_AGE:
    cat_snprintf(buf, bufsz, _("Age %d"),
                 psource->value.age);
    return buf;
  case VUT_FORM_AGE:
    cat_snprintf(buf, bufsz, _("Form age %d"),
                 psource->value.form_age);
    return buf;
  case VUT_MINTECHS:
    cat_snprintf(buf, bufsz, _("%d Techs"),
                 psource->value.min_techs);
    return buf;
  case VUT_FUTURETECHS:
    cat_snprintf(buf, bufsz, _("%d Future techs"),
                 psource->value.future_techs);
    return buf;
  case VUT_MINCITIES:
    cat_snprintf(buf, bufsz, _("%d Cities"),
                 psource->value.min_cities);
    return buf;
  case VUT_ACTION:
    fc_strlcat(buf, action_name_translation(psource->value.action),
               bufsz);
    return buf;
  case VUT_OTYPE:
    /* FIXME */
    fc_strlcat(buf, get_output_name(psource->value.outputtype), bufsz);
    return buf;
  case VUT_SPECIALIST:
    fc_strlcat(buf, specialist_plural_translation(psource->value.specialist),
               bufsz);
    return buf;
  case VUT_MINSIZE:
    cat_snprintf(buf, bufsz, _("Size %d"),
                 psource->value.minsize);
    return buf;
  case VUT_MINCULTURE:
    cat_snprintf(buf, bufsz, _("Culture %d"),
                 psource->value.minculture);
    return buf;
  case VUT_MINFOREIGNPCT:
    cat_snprintf(buf, bufsz, _("%d%% Foreigners"),
                 psource->value.minforeignpct);
    return buf;
  case VUT_AI_LEVEL:
    /* TRANS: "Hard AI" */
    cat_snprintf(buf, bufsz, _("%s AI"),
                 ai_level_translated_name(psource->value.ai_level)); /* FIXME */
    return buf;
  case VUT_MAXTILETOTALUNITS:
    /* TRANS: here <= means 'less than or equal' */
    cat_snprintf(buf, bufsz, PL_("<=%d total unit",
                                 "<=%d total units",
                                 psource->value.max_tile_total_units),
                 psource->value.max_tile_total_units);
    return buf;
  case VUT_MAXTILETOPUNITS:
    /* TRANS: here <= means 'less than or equal' */
    cat_snprintf(buf, bufsz, PL_("<=%d unit",
                                 "<=%d units", psource->value.max_tile_top_units),
                 psource->value.max_tile_top_units);
    return buf;
  case VUT_TERRAINCLASS:
    /* TRANS: Terrain class: "Land terrain" */
    cat_snprintf(buf, bufsz, _("%s terrain"),
                 terrain_class_name_translation(psource->value.terrainclass));
    return buf;
  case VUT_TERRFLAG:
    cat_snprintf(buf, bufsz,
                 /* TRANS: Terrain flag */
                 Q_("?terrflag:\"%s\" terrain"),
                 terrain_flag_id_translated_name(
                   psource->value.terrainflag));
    return buf;
  case VUT_ROADFLAG:
    cat_snprintf(buf, bufsz,
                 /* TRANS: Road flag */
                 Q_("?roadflag:\"%s\" road"),
                 road_flag_id_translated_name(psource->value.roadflag));
    return buf;
  case VUT_EXTRAFLAG:
    cat_snprintf(buf, bufsz,
                 /* TRANS: Extra flag */
                 Q_("?extraflag:\"%s\" extra"),
                 extra_flag_id_translated_name(psource->value.extraflag));
    return buf;
  case VUT_MINYEAR:
    cat_snprintf(buf, bufsz, _("After %s"),
                 textyear(psource->value.minyear));
    return buf;
  case VUT_MINCALFRAG:
    /* TRANS: here >= means 'greater than or equal'.
     * %s identifies a calendar fragment (may be bare number). */
    cat_snprintf(buf, bufsz, _(">=%s"),
                 textcalfrag(psource->value.mincalfrag));
    return buf;
  case VUT_TOPO:
    /* TRANS: topology flag name ("Hex", "ISO") */
    cat_snprintf(buf, bufsz, _("%s map"),
                 _(topo_flag_name(psource->value.topo_property)));
    return buf;
  case VUT_WRAP:
    /* TRANS: wrap flag name ("WrapX", "WrapY") */
    cat_snprintf(buf, bufsz, _("%s map"),
                 _(wrap_flag_name(psource->value.wrap_property)));
    return buf;
  case VUT_SERVERSETTING:
    fc_strlcat(buf, ssetv_human_readable(psource->value.ssetval, TRUE),
               bufsz);
    return buf;
  case VUT_TERRAINALTER:
    /* TRANS: "Irrigation possible" */
    cat_snprintf(buf, bufsz, _("%s possible"),
                 Q_(terrain_alteration_name(psource->value.terrainalter)));
    return buf;
  case VUT_CITYTILE:
    switch (psource->value.citytile) {
    case CITYT_CENTER:
      fc_strlcat(buf, _("City center"), bufsz);
      break;
    case CITYT_CLAIMED:
      fc_strlcat(buf, _("Tile claimed"), bufsz);
      break;
    case CITYT_EXTRAS_OWNED:
      fc_strlcat(buf, _("Extras owned"), bufsz);
      break;
    case CITYT_WORKED:
      fc_strlcat(buf, _("Worked tile"), bufsz);
      break;
    case CITYT_SAME_CONTINENT:
      fc_strlcat(buf, _("Same continent tile"), bufsz);
      break;
    case CITYT_BORDERING_TCLASS_REGION:
      /* TRANS: Short for "a tile of other terrain class mass near city" */
      fc_strlcat(buf, _("Port reachable tile"), bufsz);
      break;
    case CITYT_LAST:
      fc_assert(psource->value.citytile != CITYT_LAST);
      fc_strlcat(buf, "error", bufsz);
      break;
    }
    return buf;
  case VUT_CITYSTATUS:
    switch (psource->value.citystatus) {
    case CITYS_OWNED_BY_ORIGINAL:
      fc_strlcat(buf, _("Owned by original"), bufsz);
      break;
    case CITYS_STARVED:
      fc_strlcat(buf, _("Starved"), bufsz);
      break;
    case CITYS_DISORDER:
      fc_strlcat(buf, _("Disorder"), bufsz);
      break;
    case CITYS_CELEBRATION:
      fc_strlcat(buf, _("Celebration"), bufsz);
      break;
    case CITYS_TRANSFERRED:
      /* TRANS: CityStatus value - city has changed hands */
      fc_strlcat(buf, _("Transferred"), bufsz);
      break;
    case CITYS_CAPITALCONNECTED:
      fc_strlcat(buf, _("CapitalConnected"), bufsz);
      break;
    case CITYS_LAST:
      fc_assert(psource->value.citystatus != CITYS_LAST);
      fc_strlcat(buf, "error", bufsz);
      break;
    }
    return buf;
  case VUT_TILE_REL:
    switch (psource->value.tilerel) {
    case TREL_SAME_TCLASS:
      fc_strlcat(buf, _("Same terrain class"), bufsz);
      break;
    case TREL_SAME_REGION:
      fc_strlcat(buf, _("Same continent/ocean"), bufsz);
      break;
    case TREL_ONLY_OTHER_REGION:
      fc_strlcat(buf, _("Only other continent/ocean"), bufsz);
      break;
    case TREL_REGION_SURROUNDED:
      fc_strlcat(buf, _("Lake/island surrounded"), bufsz);
      break;
    case TREL_COUNT:
      fc_assert(psource->value.tilerel != TREL_COUNT);
      fc_strlcat(buf, "error", bufsz);
      break;
    }
    return buf;
  case VUT_MINLATITUDE:
    /* TRANS: here >= means 'greater than or equal'. */
    cat_snprintf(buf, bufsz, _("Latitude >= %d"),
                 psource->value.latitude);
    return buf;
  case VUT_MAXLATITUDE:
    /* TRANS: here <= means 'less than or equal'. */
    cat_snprintf(buf, bufsz, _("Latitude <= %d"),
                 psource->value.latitude);
    return buf;
  case VUT_MAX_DISTANCE_SQ:
    /* TRANS: here <= means 'less than or equal'. */
    cat_snprintf(buf, bufsz, _("Squared distance <= %d"),
                 psource->value.distance_sq);
    return buf;
  case VUT_MAX_REGION_TILES:
    cat_snprintf(buf, bufsz, _("%d or fewer region tiles"),
                 psource->value.region_tiles);
    return buf;
  case VUT_COUNT:
    break;
  }
 
  fc_assert_msg(FALSE, "Invalid source kind %d.", psource->kind);
  return buf;
}
 
/**********************************************************************/
const char *universal_type_rule_name(const struct universal *psource)
{
  return universals_n_name(psource->kind);
}
 
/**********************************************************************/
int universal_build_shield_cost(const struct city *pcity,
                                const struct universal *target)
{
  switch (target->kind) {
  case VUT_IMPROVEMENT:
  case VUT_SITE:
    return impr_build_shield_cost(pcity, target->value.building);
  case VUT_UTYPE:
    return utype_build_shield_cost(pcity, nullptr, target->value.utype);
  default:
    break;
  }
  return FC_INFINITY;
}
 
/**********************************************************************/
bool universal_replace_in_req_vec(struct requirement_vector *reqs,
                                  const struct universal *to_replace,
                                  const struct universal *replacement)
{
  bool changed = FALSE;
 
  requirement_vector_iterate(reqs, preq) {
    if (universal_is_mentioned_by_requirement(preq, to_replace)) {
      preq->source = *replacement;
      changed = TRUE;
    }
  } requirement_vector_iterate_end;
 
  return changed;
}
 
/**********************************************************************/
bool universal_is_mentioned_by_requirements(
    const struct requirement_vector *reqs,
    const struct universal *psource)
{
  requirement_vector_iterate(reqs, preq) {
    if (universal_is_mentioned_by_requirement(preq, psource)) {
      return TRUE;
    }
  } requirement_vector_iterate_end;
 
  return FALSE;
}
 
/**********************************************************************/
bool universals_mean_unfulfilled(struct requirement_vector *reqs,
                                 struct universal *unis,
                                 size_t n_unis)
{
  int i;
 
  for (i = 0; i < n_unis; i++) {
    if (!universal_fulfills_requirements(FALSE, reqs, &unis[i])) {
      /* This universal makes it impossible to fulfill the specified
       * requirement vector */
      return TRUE;
    }
  }
 
  /* No specified universal is known to guarantee that the requirement
   * vector never will be fulfilled. */
  return FALSE;
}
 
/**********************************************************************/
bool universals_say_everything(struct requirement_vector *reqs,
                               struct universal *unis,
                               size_t n_unis)
{
  requirement_vector_iterate(reqs, preq) {
    int i;
    bool req_mentioned_a_source = FALSE;
 
    for (i = 0; i < n_unis; i++) {
      switch (universal_fulfills_requirement(preq, &(unis[i]))) {
      case ITF_NO:
      case ITF_YES:
        /* this req matched this source */
        req_mentioned_a_source = TRUE;
        break;
      case ITF_NOT_APPLICABLE:
        /* Not a mention. */
        break;
      }
    }
 
    if (!req_mentioned_a_source) {
      /* A requirement not relevant to any of the specified universals was
       * found in the requirement vector. */
      return FALSE;
    }
  } requirement_vector_iterate_end;
 
  /* No requirement not relevant to any of the specified universals was
   * found in the requirement vector. */
  return TRUE;
}
 
/**********************************************************************/
enum req_item_found
universal_fulfills_requirement(const struct requirement *preq,
                               const struct universal *source)
{
  fc_assert_ret_val_msg(universal_found_function[source->kind],
                        ITF_NOT_APPLICABLE,
                        "No req item found function for %s",
                        universal_type_rule_name(source));
 
  return (*universal_found_function[source->kind])(preq, source);
}
 
/**********************************************************************/
bool universal_fulfills_requirements(bool check_necessary,
                                     const struct requirement_vector *reqs,
                                     const struct universal *source)
{
  bool necessary = FALSE;
 
  fc_assert_ret_val_msg(universal_found_function[source->kind],
                        !check_necessary,
                        "No req item found function for %s",
                        universal_type_rule_name(source));
 
  requirement_vector_iterate(reqs, preq) {
    switch ((*universal_found_function[source->kind])(preq, source)) {
    case ITF_NOT_APPLICABLE:
      continue;
    case ITF_NO:
      if (preq->present) {
        return FALSE;
      }
      break;
    case ITF_YES:
      if (preq->present) {
        necessary = TRUE;
      } else {
        return FALSE;
      }
      break;
    }
  } requirement_vector_iterate_end;
 
  return (!check_necessary || necessary);
}
 
/**********************************************************************/
bool universal_is_relevant_to_requirement(const struct requirement *req,
                                          const struct universal *source)
{
  switch (universal_fulfills_requirement(req, source)) {
  case ITF_NOT_APPLICABLE:
    return FALSE;
  case ITF_NO:
  case ITF_YES:
    return TRUE;
  }
 
  log_error("Unhandled item_found value");
  return FALSE;
}
 
/**********************************************************************/
static enum req_item_found nation_found(const struct requirement *preq,
                                        const struct universal *source)
{
  fc_assert(source->value.nation);
 
  switch (preq->source.kind) {
  case VUT_NATION:
    return preq->source.value.nation == source->value.nation ? ITF_YES
                                                             : ITF_NO;
  case VUT_NATIONGROUP:
    return nation_is_in_group(source->value.nation,
                              preq->source.value.nationgroup) ? ITF_YES
                                                              : ITF_NO;
  default:
    break;
  }
 
  return ITF_NOT_APPLICABLE;
}
 
/**********************************************************************/
static enum req_item_found government_found(const struct requirement *preq,
                                            const struct universal *source)
{
  fc_assert(source->value.govern);
 
  if (preq->source.kind == VUT_GOVERNMENT) {
    return preq->source.value.govern == source->value.govern ? ITF_YES
                                                             : ITF_NO;
  } else if (preq->source.kind == VUT_GOVFLAG) {
    return BV_ISSET(source->value.govern->flags, preq->source.value.govflag)
      ? ITF_YES : ITF_NO;
  }
 
  return ITF_NOT_APPLICABLE;
}
 
/**********************************************************************/
static enum req_item_found mincities_found(const struct requirement *preq,
                                           const struct universal *source)
{
  fc_assert(source->value.min_cities);
 
  if (preq->source.kind == VUT_MINCITIES) {
    return preq->source.value.min_cities <= source->value.min_cities ? ITF_YES
                                                                     : ITF_NO;
  }
 
  return ITF_NOT_APPLICABLE;
}
 
/**********************************************************************/
static enum req_item_found improvement_found(const struct requirement *preq,
                                             const struct universal *source)
{
  fc_assert(source->value.building);
 
  /* We only ever return ITF_YES, because requiring a different
   * improvement does not mean that the improvement under consideration
   * cannot fulfill the requirements. This is necessary to allow
   * requirement vectors to specify multiple required improvements. */
 
  switch (preq->source.kind) {
  case VUT_IMPROVEMENT:
  case VUT_SITE:
    if (source->value.building == preq->source.value.building) {
      return ITF_YES;
    }
    break;
  case VUT_IMPR_GENUS:
    if (source->value.building->genus == preq->source.value.impr_genus) {
      return ITF_YES;
    }
    break;
  case VUT_IMPR_FLAG:
    if (improvement_has_flag(source->value.building,
                             preq->source.value.impr_flag)) {
      return ITF_YES;
    }
    break;
  default:
    break;
  }
 
  return ITF_NOT_APPLICABLE;
}
 
/**********************************************************************/
static enum req_item_found unit_class_found(const struct requirement *preq,
                                            const struct universal *source)
{
  fc_assert(source->value.uclass);
 
  switch (preq->source.kind) {
  case VUT_UCLASS:
    return source->value.uclass == preq->source.value.uclass ? ITF_YES
                                                             : ITF_NO;
  case VUT_UCFLAG:
    return uclass_has_flag(source->value.uclass,
                           preq->source.value.unitclassflag) ? ITF_YES
                                                             : ITF_NO;
 
  default:
    /* Not found and not relevant. */
    return ITF_NOT_APPLICABLE;
  };
}
 
/**********************************************************************/
static enum req_item_found unit_type_found(const struct requirement *preq,
                                           const struct universal *source)
{
  fc_assert(source->value.utype);
 
  switch (preq->source.kind) {
  case VUT_UTYPE:
    return source->value.utype == preq->source.value.utype ? ITF_YES : ITF_NO;
  case VUT_UCLASS:
    return utype_class(source->value.utype) == preq->source.value.uclass
                                            ? ITF_YES : ITF_NO;
  case VUT_UTFLAG:
    return utype_has_flag(source->value.utype,
                          preq->source.value.unitflag) ? ITF_YES : ITF_NO;
  case VUT_UCFLAG:
    return uclass_has_flag(utype_class(source->value.utype),
                           preq->source.value.unitclassflag) ? ITF_YES
                                                             : ITF_NO;
  default:
    /* Not found and not relevant. */
    return ITF_NOT_APPLICABLE;
  };
}
 
/**********************************************************************/
static enum req_item_found
unit_activity_found(const struct requirement *preq,
                    const struct universal *source)
{
  fc_assert_ret_val(unit_activity_is_valid(source->value.activity),
                    ITF_NOT_APPLICABLE);
 
  switch (preq->source.kind) {
  case VUT_ACTIVITY:
    return source->value.activity == preq->source.value.activity ? ITF_YES
                                                                 : ITF_NO;
  default:
    /* Not found and not relevant. */
    return ITF_NOT_APPLICABLE;
  };
}
 
/**********************************************************************/
static enum req_item_found terrain_type_found(const struct requirement *preq,
                                              const struct universal *source)
{
  fc_assert(source->value.terrain);
 
  switch (preq->source.kind) {
  case VUT_TERRAIN:
    return source->value.terrain == preq->source.value.terrain ? ITF_YES : ITF_NO;
  case VUT_TERRAINCLASS:
    return terrain_type_terrain_class(source->value.terrain) == preq->source.value.terrainclass
      ? ITF_YES : ITF_NO;
  case VUT_TERRFLAG:
    return terrain_has_flag(source->value.terrain,
                            preq->source.value.terrainflag) ? ITF_YES : ITF_NO;
  case VUT_TERRAINALTER:
    return (terrain_can_support_alteration(source->value.terrain,
                                           preq->source.value.terrainalter)
            ? ITF_YES : ITF_NO);
  default:
    /* Not found and not relevant. */
    return ITF_NOT_APPLICABLE;
  };
}
 
/**********************************************************************/
static enum req_item_found city_tile_found(const struct requirement *preq,
                                           const struct universal *source)
{
  fc_assert_ret_val(citytile_type_is_valid(source->value.citytile),
                    ITF_NOT_APPLICABLE);
 
  switch (preq->source.kind) {
  case VUT_CITYTILE:
    return (source->value.citytile == preq->source.value.citytile
            ? ITF_YES
            /* The presence of one tile state doesn't block another */
            : ITF_NOT_APPLICABLE);
  default:
    /* Not found and not relevant. */
    return ITF_NOT_APPLICABLE;
  };
}
 
/**********************************************************************/
static enum req_item_found extra_type_found(const struct requirement *preq,
                                            const struct universal *source)
{
  fc_assert(source->value.extra);
 
  switch (preq->source.kind) {
  case VUT_EXTRA:
    return source->value.extra == preq->source.value.extra ? ITF_YES : ITF_NO;
  case VUT_EXTRAFLAG:
    return extra_has_flag(source->value.extra,
                          preq->source.value.extraflag) ? ITF_YES : ITF_NO;
  case VUT_ROADFLAG:
    {
      struct road_type *r = extra_road_get(source->value.extra);
      return r && road_has_flag(r, preq->source.value.roadflag)
        ? ITF_YES : ITF_NO;
    }
  default:
    /* Not found and not relevant. */
    return ITF_NOT_APPLICABLE;
  }
}
 
/**********************************************************************/
static enum req_item_found action_found(const struct requirement *preq,
                                        const struct universal *source)
{
  fc_assert(source->value.action);
 
  if (preq->source.kind == VUT_ACTION) {
    return preq->source.value.action == source->value.action ? ITF_YES
                                                             : ITF_NO;
  }
 
  return ITF_NOT_APPLICABLE;
}
 
/**********************************************************************/
static enum req_item_found diplrel_found(const struct requirement *preq,
                                         const struct universal *source)
{
  fc_assert_ret_val((source->kind == VUT_DIPLREL
                     || source->kind == VUT_DIPLREL_TILE
                     || source->kind == VUT_DIPLREL_TILE_O
                     || source->kind == VUT_DIPLREL_UNITANY
                     || source->kind == VUT_DIPLREL_UNITANY_O),
                    ITF_NOT_APPLICABLE);
 
  if (preq->source.kind == source->kind) {
    if (preq->source.value.diplrel == source->value.diplrel) {
      /* The diplrel itself. */
      return ITF_YES;
    }
    if (preq->source.value.diplrel == DRO_FOREIGN
        && source->value.diplrel < DS_LAST) {
      /* All diplstate_type values are to foreigners. */
      return ITF_YES;
    }
    if (preq->source.value.diplrel == DRO_HOSTS_EMBASSY
        && source->value.diplrel == DRO_HOSTS_REAL_EMBASSY) {
      /* A real embassy is an embassy. */
      return ITF_YES;
    }
    if (preq->source.value.diplrel == DRO_HAS_EMBASSY
        && source->value.diplrel == DRO_HAS_REAL_EMBASSY) {
      /* A real embassy is an embassy. */
      return ITF_YES;
    }
    if (preq->source.value.diplrel < DS_LAST
        && source->value.diplrel < DS_LAST
        && preq->range == REQ_RANGE_LOCAL) {
      fc_assert_ret_val(preq->source.value.diplrel != source->value.diplrel,
                        ITF_YES);
      /* Can only have one diplstate_type to a specific player. */
      return ITF_NO;
    }
    /* Can't say this diplrel blocks the other diplrel. */
    return ITF_NOT_APPLICABLE;
  }
 
  /* Not relevant. */
  return ITF_NOT_APPLICABLE;
}
 
/**********************************************************************/
static enum req_item_found output_type_found(const struct requirement *preq,
                                             const struct universal *source)
{
  switch (preq->source.kind) {
  case VUT_OTYPE:
    return source->value.outputtype == preq->source.value.outputtype ? ITF_YES
                                                                     : ITF_NO;
  default:
    /* Not found and not relevant. */
    return ITF_NOT_APPLICABLE;
  }
}
 
/**********************************************************************/
static enum req_item_found ustate_found(const struct requirement *preq,
                                        const struct universal *source)
{
  if (preq->range != REQ_RANGE_LOCAL) {
    return ITF_NOT_APPLICABLE;
  }
 
  if (preq->source.kind == VUT_UNITSTATE) {
    switch (source->value.unit_state) {
    case USP_TRANSPORTED:
    case USP_TRANSPORTING:
      /* USP_TRANSPORTED and USP_TRANSPORTING don't exclude each other. */
    case USP_LIVABLE_TILE:
    case USP_NATIVE_TILE:
    /* USP_NATIVE_TILE isn't a strict subset of USP_LIVABLE_TILE. See
     * UTYF_COAST_STRICT. */
    case USP_HAS_HOME_CITY:
    case USP_NATIVE_EXTRA:
    case USP_MOVED_THIS_TURN:
      if (source->value.unit_state == preq->source.value.unit_state) {
        /* The other unit states doesn't contradict */
        return ITF_YES;
      }
      break;
    case USP_COUNT:
      fc_assert_ret_val(source->value.unit_state != USP_COUNT,
                        ITF_NOT_APPLICABLE);
    }
  }
 
  /* Not found and not relevant. */
  return ITF_NOT_APPLICABLE;
}
 
/**********************************************************************/
void universal_found_functions_init(void)
{
  universal_found_function[VUT_GOVERNMENT] = &government_found;
  universal_found_function[VUT_NATION] = &nation_found;
  universal_found_function[VUT_IMPROVEMENT] = &improvement_found;
  universal_found_function[VUT_SITE] = &improvement_found;
  universal_found_function[VUT_UCLASS] = &unit_class_found;
  universal_found_function[VUT_UTYPE] = &unit_type_found;
  universal_found_function[VUT_ACTIVITY] = &unit_activity_found;
  universal_found_function[VUT_TERRAIN] = &terrain_type_found;
  universal_found_function[VUT_CITYTILE] = &city_tile_found;
  universal_found_function[VUT_EXTRA] = &extra_type_found;
  universal_found_function[VUT_OTYPE] = &output_type_found;
  universal_found_function[VUT_ACTION] = &action_found;
  universal_found_function[VUT_DIPLREL] = &diplrel_found;
  universal_found_function[VUT_DIPLREL_TILE] = &diplrel_found;
  universal_found_function[VUT_DIPLREL_TILE_O] = &diplrel_found;
  universal_found_function[VUT_DIPLREL_UNITANY] = &diplrel_found;
  universal_found_function[VUT_DIPLREL_UNITANY_O] = &diplrel_found;
  universal_found_function[VUT_UNITSTATE] = &ustate_found;
  universal_found_function[VUT_MINCITIES] = &mincities_found;
}
 
/**********************************************************************/
int requirement_kind_ereq(const int value,
                          const enum req_range range,
                          const bool present,
                          const int max_value)
{
  /* The enumerators in each range starts with present for every possible
   * value followed by !present for every possible value. */
  const int pres_start = (present ? 0 : max_value);
 
  /* The enumerators for every range follows all the positions of the
   * previous range(s). */
  const int range_start = ((max_value - 1) * 2) * range;
 
  return range_start + pres_start + value;
}