combat.c

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/***********************************************************************
 Freeciv - Copyright (C) 1996 - A Kjeldberg, L Gregersen, P Unold
   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 <math.h>
 
/* utility */
#include "bitvector.h"
#include "rand.h"
#include "log.h"
 
/* common */
#include "base.h"
#include "game.h"
#include "map.h"
#include "movement.h"
#include "packets.h"
#include "unit.h"
#include "unitlist.h"
#include "unittype.h"
 
#include "combat.h"
 
/*******************************************************************/
static bool can_player_attack_tile(const struct player *pplayer,
                            const struct tile *ptile)
{
  struct city *pcity = tile_city(ptile);
  
  /* 1. Is there anyone there at all? */
  if (!pcity && unit_list_size((ptile->units)) == 0) {
    return FALSE;
  }
 
  /* 2. If there is a city there, can we attack it? */
  if (pcity && !pplayers_at_war(city_owner(pcity), pplayer)) {
    return FALSE;
  }
 
  /* 3. Are we allowed to attack _all_ units there? */
  unit_list_iterate(ptile->units, aunit) {
    if (!pplayers_at_war(unit_owner(aunit), pplayer)) {
      /* Enemy hiding behind a human/diplomatic shield */
      return FALSE;
    }
  } unit_list_iterate_end;
 
  return TRUE;
}
 
/*******************************************************************/
static bool is_unit_reachable_by_unit(const struct unit *defender,
                                      const struct unit *attacker)
{
  struct unit_class *dclass = unit_class_get(defender);
  const struct unit_type *atype = unit_type_get(attacker);
 
  return BV_ISSET(atype->targets, uclass_index(dclass));
}
 
/*******************************************************************/
bool is_unit_reachable_at(const struct unit *defender,
                          const struct unit *attacker,
                          const struct tile *location)
{
  if (NULL != tile_city(location)) {
    return TRUE;
  }
 
  if (is_unit_reachable_by_unit(defender, attacker)) {
    return TRUE;
  }
 
  if (tile_has_native_base(location, unit_type_get(defender))) {
    return TRUE;
  }
 
  return FALSE;
}
 
/*******************************************************************/
enum unit_attack_result
unit_attack_unit_at_tile_result(const struct unit *punit,
                                const struct action *paction,
                                const struct unit *pdefender,
                                const struct tile *dest_tile)
{
  /* 1. Can we attack _anything_ ? */
  if (paction == NULL) {
    if (!(utype_can_do_action(unit_type_get(punit), ACTION_ATTACK)
          || utype_can_do_action(unit_type_get(punit),
                                 ACTION_SUICIDE_ATTACK)
          /* Needed because ACTION_NUKE_UNITS uses this when evaluating its
           * hard requirements. */
          || utype_can_do_action(unit_type_get(punit),
                                 ACTION_NUKE_UNITS))) {
      return ATT_NON_ATTACK;
    }
  } else {
    if (!utype_can_do_action(unit_type_get(punit), paction->id)) {
      return ATT_NON_ATTACK;
    }
  }
 
  /* 2. Can't attack with ground unit from ocean, except for marines */
  if (paction == NULL) {
    if (!is_native_tile(unit_type_get(punit), unit_tile(punit))
        && !utype_can_do_act_when_ustate(unit_type_get(punit),
                                         ACTION_ATTACK,
                                         USP_NATIVE_TILE, FALSE)
        && !utype_can_do_act_when_ustate(unit_type_get(punit),
                                         ACTION_SUICIDE_ATTACK,
                                         USP_NATIVE_TILE, FALSE)) {
      return ATT_NONNATIVE_SRC;
    }
  } else {
    if (!is_native_tile(unit_type_get(punit), unit_tile(punit))
        && !utype_can_do_act_when_ustate(unit_type_get(punit),
                                         paction->id,
                                         USP_NATIVE_TILE, FALSE)) {
      return ATT_NONNATIVE_SRC;
    }
  }
 
  /* 3. Most units can not attack non-native terrain.
   *    Most ships can attack land tiles (shore bombardment) */
  if (!is_native_tile(unit_type_get(punit), dest_tile)
      && !can_attack_non_native_hard_reqs(unit_type_get(punit))) {
    return ATT_NONNATIVE_DST;
  }
 
  /* 4. Only fighters can attack planes, except in city or airbase attacks */
  if (!is_unit_reachable_at(pdefender, punit, dest_tile)) {
    return ATT_UNREACHABLE;
  }
 
  /* Unreachability check must remain the last check, as callers interpret
   * ATT_UNREACHABLE as "ATT_OK except that unreachable." */
 
  return ATT_OK;
}
 
/*******************************************************************/
static enum unit_attack_result
unit_attack_all_at_tile_result(const struct unit *punit,
                               const struct action *paction,
                               const struct tile *ptile)
{
  bool any_reachable_unit = FALSE;
  bool any_neverprotect_unit = FALSE;
 
  unit_list_iterate(ptile->units, aunit) {
    /* HACK: we don't count transported units here.  This prevents some
     * bugs like a submarine carrying a cruise missile being invulnerable
     * to other sea units.  However from a gameplay perspective it's a hack,
     * since players can load and unload their units manually to protect
     * their transporters. */
    if (!unit_transported(aunit)) {
      enum unit_attack_result result;
 
      result = unit_attack_unit_at_tile_result(punit, paction,
                                               aunit, ptile);
      if (result == ATT_UNREACHABLE
          && unit_has_type_flag(aunit, UTYF_NEVER_PROTECTS)) {
        /* Doesn't prevent us from attacking other units on the tile */
        any_neverprotect_unit = TRUE;
        continue;
      } else if (result != ATT_OK) {
        return result;
      }
      any_reachable_unit = TRUE;
    }
  } unit_list_iterate_end;
 
  /* If there are only unreachable, UTYF_NEVER_PROTECTS units, we still have
   * to return ATT_UNREACHABLE. */
  return (any_reachable_unit || !any_neverprotect_unit) ? ATT_OK : ATT_UNREACHABLE;
}
 
/*******************************************************************/
static enum unit_attack_result
unit_attack_any_at_tile_result(const struct unit *punit,
                               const struct action *paction,
                               const struct tile *ptile)
{
  enum unit_attack_result result = ATT_OK;
 
  unit_list_iterate(ptile->units, aunit) {
    /* HACK: we don't count transported units here.  This prevents some
     * bugs like a cargoplane carrying a land unit being vulnerable. */
    if (!unit_transported(aunit)) {
      result = unit_attack_unit_at_tile_result(punit, paction,
                                               aunit, ptile);
      if (result == ATT_OK) {
        return result;
      }
    }
  } unit_list_iterate_end;
 
  /* That's result from check against last unit on tile, not first.
   * Shouldn't matter. */
  return result;
}
 
/*******************************************************************/
enum unit_attack_result
unit_attack_units_at_tile_result(const struct unit *punit,
                                 const struct action *paction,
                                 const struct tile *ptile)
{
  if (game.info.unreachable_protects) {
    return unit_attack_all_at_tile_result(punit, paction, ptile);
  } else {
    return unit_attack_any_at_tile_result(punit, paction, ptile);
  }
}
 
/*******************************************************************/
enum unit_attack_result
unit_wipe_units_at_tile_result(const struct unit *punit,
                               const struct tile *ptile)
{
  /* Can unit wipe in general? */
  if (!utype_can_do_action(unit_type_get(punit), ACTION_WIPE_UNITS)) {
    return ATT_NON_ATTACK;
  }
 
  unit_list_iterate(ptile->units, target) {
    if (get_total_defense_power(punit, target) > 0) {
      return ATT_NOT_WIPABLE;
    }
 
    /* Can't wipe with ground unit from ocean, except for marines */
    if (!is_native_tile(unit_type_get(punit), unit_tile(punit))
        && !utype_can_do_act_when_ustate(unit_type_get(punit), ACTION_WIPE_UNITS,
                                         USP_NATIVE_TILE, FALSE)) {
      return ATT_NONNATIVE_SRC;
    }
 
    /* Most units can not wipe on non-native terrain.
     * Most ships can wipe on land tiles (shore bombardment) */
    if (!is_native_tile(unit_type_get(punit), ptile)
        && !can_attack_non_native_hard_reqs(unit_type_get(punit))) {
      return ATT_NONNATIVE_DST;
    }
 
    /* Only fighters can wipe planes, except in city or airbase attacks */
    if (!is_unit_reachable_at(target, punit, ptile)) {
      return ATT_UNREACHABLE;
    }
  } unit_list_iterate_end;
 
  return ATT_OK;
}
 
/*******************************************************************/
bool can_unit_attack_tile(const struct unit *punit,
                          const struct action *paction,
                          const struct tile *dest_tile)
{
  return (can_player_attack_tile(unit_owner(punit), dest_tile)
          && (unit_attack_units_at_tile_result(punit, paction, dest_tile)
              == ATT_OK));
}
 
/*******************************************************************/
double win_chance(int as, int ahp, int afp, int ds, int dhp, int dfp)
{
  /* number of rounds a unit can fight without dying */
  int att_N_lose = (ahp + dfp - 1) / dfp;
  int def_N_lose = (dhp + afp - 1) / afp;
  /* Probability of losing one round */
  double att_P_lose1 = (as + ds == 0) ? 0.5 : (double) ds / (as + ds);
  double def_P_lose1 = 1 - att_P_lose1;
 
  /*
    This calculates 
 
    binomial_coeff(def_N_lose-1 + lr, lr)
      * def_P_lose1^(def_N_lose-1)
      * att_P_lose1^(lr)
      * def_P_lose1
 
    for each possible number of rounds lost (rl) by the winning unit.
    rl is of course less than the number of rounds the winning unit
    should lose to lose all it's hit points.
    The probabilities are then summed.
 
    To see this is correct consider the set of series for all valid fights.
    These series are the type (win, lose, lose...). The possible lengths are
    def_N_lose to def_N_lose+att_N_lose-1. A series is not valid unless it
    contains def_N_lose wins, and one of the wins must be the last one, or
    the series would be equivalent the a shorter series (the attacker would
    have won one or more fights previously).
    So since the last fight is a win we disregard it while calculating. Now
    a series contains def_N_lose-1 wins. So for each possible length of a
    series we find the probability of every valid series and then sum.
    For a specific length (a "lr") every series have the probability
      def_P_lose1^(def_N_lose-1) * att_P_lose1^(lr)
    and then getting from that to the real series requires a win, ie factor
    def_N_lose. The number of series with length (def_N_lose-1 + lr) and
    "lr" lost fights is
      binomial_coeff(def_N_lose-1 + lr, lr)
    And by multiplying we get the formula on the top of this code block.
    Adding the cumulative probability for each valid length then gives the
    total probability.
 
    We clearly have all valid series this way. To see that we have counted
    none twice note that would require a series with a smaller series inbedded.
    But since the smaller series already included def_N_lose wins, and the
    larger series ends with a win, it would have too many wins and therefore
    cannot exist.
 
    In practice each binomial coefficient for a series length can be calculated
    from the previous. In the coefficient (n, k) n is increased and k is
    unchanged.
    The "* def_P_lose1" is multiplied on the sum afterwards.
 
    (lots of talk for so little code)
  */
 
  double binom_save = pow(def_P_lose1, (double)(def_N_lose - 1));
  double accum_prob = binom_save; /* lr = 0 */
 
  int lr; /* the number of Lost Rounds by the attacker */
  for (lr = 1; lr < att_N_lose; lr++) {
    /* update the coefficient */
    int n = lr + def_N_lose - 1;
    binom_save *= n;
    binom_save /= lr;
    binom_save *= att_P_lose1;
    /* use it for this lr */
    accum_prob += binom_save;
  }
  /* Every element of the sum needs a factor for the very last fight round */
  accum_prob *= def_P_lose1;
 
  return accum_prob;
}
 
/*******************************************************************/
void get_modified_firepower(const struct civ_map *nmap,
                            const struct unit *attacker,
                            const struct unit *defender,
                            int *att_fp, int *def_fp)
{
  struct city *pcity = tile_city(unit_tile(defender));
  const struct unit_type *att_type;
  const struct unit_type *def_type;
  struct tile *att_tile;
 
  att_type = unit_type_get(attacker);
  def_type = unit_type_get(defender);
 
  *att_fp = att_type->firepower;
  *def_fp = def_type->firepower;
 
  /* Check CityBuster flag */
  if (unit_has_type_flag(attacker, UTYF_CITYBUSTER) && pcity) {
    *att_fp *= 2;
  }
 
  /*
   * UTYF_BADWALLATTACKER reduces the firepower of the attacking unit to
   * badwallattacker firepower if an EFT_DEFEND_BONUS applies
   * (such as a land unit attacking a city with city walls).
   */
  if (unit_has_type_flag(attacker, UTYF_BADWALLATTACKER)
      && get_unittype_bonus(unit_owner(defender), unit_tile(defender),
                            att_type, NULL,
                            EFT_DEFEND_BONUS) > 0) {
    *att_fp = MIN(*att_fp, game.info.low_firepower_badwallattacker);
  }
 
  /* pearl harbour - defender's firepower is reduced,
   *                 attacker's is multiplied by two         */
  if (unit_has_type_flag(defender, UTYF_BADCITYDEFENDER)
      && tile_city(unit_tile(defender))) {
    *att_fp *= 2;
    *def_fp = MIN(*def_fp, game.info.low_firepower_pearl_harbour);
  }
 
  /* 
   * When attacked by fighters, helicopters have their firepower
   * reduced to low firepower bonus.
   */
  if (combat_bonus_against(att_type->bonuses, def_type,
                           CBONUS_LOW_FIREPOWER)) {
    *def_fp = MIN(*def_fp, game.info.low_firepower_combat_bonus);
  }
 
  att_tile = unit_tile(attacker);
 
  /* In land bombardment both units have their firepower reduced.
   * Land bombardment is always towards tile not native for attacker.
   * It's initiated either from a tile not native to defender (Ocean for Land unit)
   * or from a tile where attacker is despite non-native terrain (city, transport) */
  if (utype_has_class_flag(def_type, UCF_NONNAT_BOMBARD_TGT)
      && !is_native_tile(att_type, unit_tile(defender))
      && (!can_exist_at_tile(nmap, def_type, att_tile)
          || !is_native_tile(att_type, att_tile))) {
    *att_fp = MIN(*att_fp, game.info.low_firepower_nonnat_bombard);
    *def_fp = MIN(*def_fp, game.info.low_firepower_nonnat_bombard);
  }
}
 
/*******************************************************************/
double unit_win_chance(const struct civ_map *nmap,
                       const struct unit *attacker,
                       const struct unit *defender,
                       const struct action *paction)
{
  int def_power = get_total_defense_power(attacker, defender);
  int att_power = get_total_attack_power(attacker, defender, paction);
  double chance;
  int def_fp, att_fp;
 
  get_modified_firepower(nmap, attacker, defender, &att_fp, &def_fp);
 
  chance = win_chance(att_power, attacker->hp, att_fp,
                      def_power, defender->hp, def_fp);
 
  return chance;
}
 
/*******************************************************************/
struct city *sdi_try_defend(const struct civ_map *nmap,
                            const struct player *owner,
                            const struct tile *ptile)
{
  square_iterate(nmap, ptile, 2, ptile1) {
    struct city *pcity = tile_city(ptile1);
 
    if (pcity
        && fc_rand(100)
           < get_target_bonus_effects(NULL,
                                      &(const struct req_context) {
                                        .player = city_owner(pcity),
                                        .city = pcity,
                                        .tile = ptile,
                                      },
                                      owner, EFT_NUKE_PROOF)) {
      return pcity;
    }
  } square_iterate_end;
 
  return NULL;
}
 
/*******************************************************************/
bool is_tired_attack(int moves_left)
{
  return game.info.tired_attack && moves_left < SINGLE_MOVE;
}
 
/*******************************************************************/
int get_attack_power(const struct unit *punit)
{
  return base_get_attack_power(unit_type_get(punit), punit->veteran,
                               punit->moves_left);
}
 
/*******************************************************************/
int base_get_attack_power(const struct unit_type *punittype,
                          int veteran, int moves_left)
{
  int power;
  const struct veteran_level *vlevel;
 
  fc_assert_ret_val(punittype != NULL, 0);
 
  vlevel = utype_veteran_level(punittype, veteran);
  fc_assert_ret_val(vlevel != NULL, 0);
 
  power = punittype->attack_strength * POWER_FACTOR
          * vlevel->power_fact / 100;
 
  if (is_tired_attack(moves_left)) {
    power = (power * moves_left) / SINGLE_MOVE;
  }
 
  return power;
}
 
/*******************************************************************/
int base_get_defense_power(const struct unit *punit)
{
  const struct veteran_level *vlevel;
  const struct unit_type *ptype;
 
  fc_assert_ret_val(punit != NULL, 0);
 
  ptype = unit_type_get(punit);
  vlevel = utype_veteran_level(ptype, punit->veteran);
  fc_assert_ret_val(vlevel != NULL, 0);
 
  return ptype->defense_strength * POWER_FACTOR
         * vlevel->power_fact / 100;
}
 
/*******************************************************************/
static int get_defense_power(const struct unit *punit)
{
  int db, power = base_get_defense_power(punit);
  struct tile *ptile = unit_tile(punit);
  struct unit_class *pclass = unit_class_get(punit);
 
  if (uclass_has_flag(pclass, UCF_TERRAIN_DEFENSE)) {
    db = 100 + tile_terrain(ptile)->defense_bonus;
    power = (power * db) / 100;
  }
 
  if (!is_native_tile_to_class(pclass, ptile)) {
    power = power * pclass->non_native_def_pct / 100;
  }
 
  return power;
}
 
/*******************************************************************/
int get_total_attack_power(const struct unit *attacker,
                           const struct unit *defender,
                           const struct action *paction)
{
  int mod;
  int attackpower = get_attack_power(attacker);
  struct tile *deftile = unit_tile(defender);
  struct city *pcity = tile_city(deftile);
 
  /* Note: get_unit_bonus() would use the tile unit is on,
   *       but we want defending tile. */
  mod = 100 + get_target_bonus_effects(NULL,
                                       &(const struct req_context) {
                                         .player = unit_owner(attacker),
                                         .city = pcity,
                                         .tile = deftile,
                                         .unit = attacker,
                                         .unittype = unit_type_get(attacker),
                                         .action = paction,
                                       },
                                       NULL, EFT_ATTACK_BONUS);
 
  return attackpower * mod / 100;
}
 
/*******************************************************************/
static int defense_multiplication(const struct unit_type *att_type,
                                  const struct unit *def,
                                  const struct player *def_player,
                                  const struct tile *ptile,
                                  int defensepower)
{
  const struct city *pcity = tile_city(ptile);
  const struct unit_type *def_type = unit_type_get(def);
 
  fc_assert_ret_val(NULL != def_type, 0);
 
  if (NULL != att_type) {
    int scramble_bonus = 0;
    int defense_divider_pct;
 
    if (pcity) {
      scramble_bonus = def_type->cache.scramble_coeff[utype_index(att_type)];
    }
 
    if (scramble_bonus) {
      /* Use type-specific city bonus,
       * already multiplied on common type-specific bonus */
      defensepower = defensepower * scramble_bonus / 10000;
      defensepower = MAX(0, defensepower);
    } else {
      /* Use city defense effect */
      int defense_multiplier_pct = 100
        + def_type->cache.defense_mp_bonuses_pct[utype_index(att_type)];
      int mod = 100 + get_unittype_bonus(def_player, ptile,
                                         att_type, NULL, EFT_DEFEND_BONUS);
 
      /* This applies even if pcity is NULL. */
      defensepower = defensepower * defense_multiplier_pct / 100;
      defensepower = MAX(0, defensepower * mod / 100);
    }
 
    defense_divider_pct = 100 + combat_bonus_against(att_type->bonuses,
                                    def_type, CBONUS_DEFENSE_DIVIDER_PCT)
        + 100 * combat_bonus_against(att_type->bonuses, def_type,
                                     CBONUS_DEFENSE_DIVIDER);
 
    defensepower = defensepower * 100 / defense_divider_pct;
  }
 
  defensepower +=
    defensepower * tile_extras_defense_bonus(ptile, def_type) / 100;
 
  defensepower = defensepower
    * (100
       + get_target_bonus_effects(NULL,
                                  &(const struct req_context) {
                                    .player = unit_owner(def),
                                    .city = pcity,
                                    .tile = ptile,
                                    .unit = def,
                                    .unittype = unit_type_get(def),
                                  },
                                  NULL,
                                  EFT_FORTIFY_DEFENSE_BONUS)) / 100;
 
  return defensepower;
}
 
/*******************************************************************/
int get_virtual_defense_power(const struct civ_map *nmap,
                              const struct unit_type *att_type,
                              const struct unit_type *def_type,
                              struct player *def_player,
                              struct tile *ptile,
                              bool fortified, int veteran)
{
  int defensepower = def_type->defense_strength;
  int db;
  const struct veteran_level *vlevel;
  struct unit_class *defclass;
  struct unit *vdef;
  int def;
 
  fc_assert_ret_val(def_type != NULL, 0);
 
  if (!can_exist_at_tile(nmap, def_type, ptile)) {
    /* Ground units on ship doesn't defend. */
    return 0;
  }
 
  vlevel = utype_veteran_level(def_type, veteran);
  fc_assert_ret_val(vlevel != NULL, 0);
 
  defclass = utype_class(def_type);
 
  vdef = unit_virtual_create(def_player, NULL, def_type, veteran);
  unit_tile_set(vdef, ptile);
  if (fortified) {
    vdef->activity = ACTIVITY_FORTIFIED;
  }
 
  db = POWER_FACTOR;
  if (uclass_has_flag(defclass, UCF_TERRAIN_DEFENSE)) {
    db += tile_terrain(ptile)->defense_bonus / (100 / POWER_FACTOR);
  }
  defensepower *= db;
  defensepower *= vlevel->power_fact / 100;
  if (!is_native_tile_to_class(defclass, ptile)) {
    defensepower = defensepower * defclass->non_native_def_pct / 100;
  }
 
  def = defense_multiplication(att_type, vdef, def_player,
                               ptile, defensepower);
 
  unit_virtual_destroy(vdef);
 
  return def;
}
 
/*******************************************************************/
int get_total_defense_power(const struct unit *attacker,
                            const struct unit *defender)
{
  return defense_multiplication(unit_type_get(attacker),
                                defender,
                                unit_owner(defender), unit_tile(defender),
                                get_defense_power(defender));
}
 
/*******************************************************************/
int get_fortified_defense_power(const struct unit *attacker,
                                struct unit *defender)
{
  const struct unit_type *att_type = NULL;
  enum unit_activity real_act;
  int def;
  const struct unit_type *utype;
 
  if (attacker != NULL) {
    att_type = unit_type_get(attacker);
  }
 
  real_act = defender->activity;
 
  utype = unit_type_get(defender);
  if (utype_can_do_action_result(utype, ACTRES_FORTIFY)) {
    defender->activity = ACTIVITY_FORTIFIED;
  }
 
  def = defense_multiplication(att_type, defender,
                               unit_owner(defender), unit_tile(defender),
                               get_defense_power(defender));
 
  defender->activity = real_act;
 
  return def;
}
 
/*******************************************************************/
static int get_defense_rating(const struct civ_map *nmap,
                              const struct unit *attacker,
                              const struct unit *defender)
{
  int afp, dfp;
  int rating = get_total_defense_power(attacker, defender);
 
  get_modified_firepower(nmap, attacker, defender, &afp, &dfp);
 
  /* How many rounds the defender will last */
  rating *= (defender->hp + afp-1)/afp;
 
  rating *= dfp;
 
  return rating;
}
 
/*******************************************************************/
struct unit *get_defender(const struct civ_map *nmap,
                          const struct unit *attacker,
                          const struct tile *ptile,
                          const struct action *paction)
{
  struct unit *bestdef = NULL;
  int bestvalue = -99, best_cost = 0, rating_of_best = 0;
 
  /* Simply call unit_win_chance() with all the possible defenders in turn, and
   * take the best one. It currently uses build cost as a tiebreaker in
   * case 2 units are identical, but this is crude as build cost does not
   * necessarily have anything to do with the value of a unit. This function
   * could be improved to take the value of the unit into account. It would
   * also be nice if the function was a bit more fuzzy about prioritizing,
   * making it able to fx choose a 1a/9d unit over a 10a/10d unit. It should
   * also be able to spare units without full hp's to some extent, as these
   * could be more valuable later. */
  unit_list_iterate(ptile->units, defender) {
    /* We used to skip over allied units, but the logic for that is
     * complicated and is now handled elsewhere. */
    if (unit_can_defend_here(nmap, defender)
        && (unit_attack_unit_at_tile_result(attacker, NULL, defender, ptile)
            == ATT_OK)) {
      bool change = FALSE;
      int build_cost = unit_build_shield_cost_base(defender);
      int defense_rating = get_defense_rating(nmap, attacker, defender);
      /* This will make units roughly evenly good defenders look alike. */
      int unit_def
        = (int) (100000 * (1 - unit_win_chance(nmap, attacker,
                                               defender, paction)));
 
      fc_assert_action(0 <= unit_def, continue);
 
      if (unit_has_type_flag(defender, UTYF_GAMELOSS)
          && !is_stack_vulnerable(unit_tile(defender))) {
        unit_def = -1; /* Then always use leader as last defender. */
        /* FIXME: Multiple gameloss units with varying defense value
         *        not handled. */
      }
 
      if (unit_def > bestvalue) {
        change = TRUE;
      } else if (unit_def == bestvalue) {
        if (build_cost < best_cost) {
          change = TRUE;
        } else if (build_cost == best_cost) {
          if (rating_of_best < defense_rating) {
            change = TRUE;
          }
        }
      }
 
      if (change) {
        bestvalue = unit_def;
        bestdef = defender;
        best_cost = build_cost;
        rating_of_best = defense_rating;
      }
    }
  } unit_list_iterate_end;
 
  return bestdef;
}
 
/*******************************************************************/
struct unit *get_attacker(const struct civ_map *nmap,
                          const struct unit *defender,
                          const struct tile *ptile)
{
  struct unit *bestatt = 0;
  int bestvalue = -1, unit_a, best_cost = 0;
 
  unit_list_iterate(ptile->units, attacker) {
    int build_cost = unit_build_shield_cost_base(attacker);
 
    if (pplayers_allied(unit_owner(defender), unit_owner(attacker))) {
      return NULL;
    }
    unit_a = (int) (100000 * (unit_win_chance(nmap, attacker,
                                              defender, NULL)));
    if (unit_a > bestvalue
        || (unit_a == bestvalue && build_cost < best_cost)) {
      bestvalue = unit_a;
      bestatt = attacker;
      best_cost = build_cost;
    }
  } unit_list_iterate_end;
 
  return bestatt;
}
 
/**********************************************************************/
struct unit *get_diplomatic_defender(const struct unit *act_unit,
                                     const struct unit *pvictim,
                                     const struct tile *tgt_tile,
                                     const struct action *paction)
{
  fc_assert_ret_val(act_unit, NULL);
  fc_assert_ret_val(tgt_tile, NULL);
 
  unit_list_iterate(tgt_tile->units, punit) {
    if (unit_owner(punit) == unit_owner(act_unit)) {
      /* I can't confirm if we won't deny that we weren't involved.
       * (Won't defend against its owner.) */
      continue;
    }
 
    if (punit == pvictim
        && !unit_has_type_flag(punit, UTYF_SUPERSPY)) {
      /* The victim unit is defenseless unless it's a SuperSpy.
       * Rationalization: A regular diplomat don't mind being bribed. A
       * SuperSpy is high enough up the chain that accepting a bribe is
       * against their own interests. */
      continue;
    }
 
    if (!(unit_has_type_flag(punit, UTYF_DIPLOMAT)
          || unit_has_type_flag(punit, UTYF_SUPERSPY))) {
      /* A UTYF_SUPERSPY unit may not actually be a spy, but a superboss
       * which we cannot allow puny diplomats from getting the better
       * of. UTYF_SUPERSPY vs UTYF_SUPERSPY in a diplomatic contest always
       * kills the attacker. */
 
      /* The unit can't defend in a diplomatic battle. */
      continue;
    }
 
    /* The first potential defender found is chosen. No priority is given
     * to the best defender. */
    return punit;
  } unit_list_iterate_end;
 
  /* No diplomatic defender found. */
  return NULL;
}
 
/*******************************************************************/
bool is_stack_vulnerable(const struct tile *ptile)
{
  return (game.info.killstack
          && !tile_has_extra_flag(ptile, EF_NO_STACK_DEATH)
          && NULL == tile_city(ptile));
}
 
/*******************************************************************/
int combat_bonus_against(const struct combat_bonus_list *list,
                         const struct unit_type *enemy,
                         enum combat_bonus_type type)
{
  int value = 0;
 
  combat_bonus_list_iterate(list, pbonus) {
    if (pbonus->type == type && utype_has_flag(enemy, pbonus->flag)) {
      value += pbonus->value;
    }
  } combat_bonus_list_iterate_end;
 
  return value;
}
 
/*******************************************************************/
int unit_bombard_rate(struct unit *punit)
{
  const struct unit_type *utype = unit_type_get(punit);
  int base_bombard_rate = utype->bombard_rate;
 
  if (game.info.damage_reduces_bombard_rate && base_bombard_rate > 0) {
    int rate = (base_bombard_rate * punit->hp) / utype->hp;
 
    return MAX(rate, 1);
  }
 
  return base_bombard_rate;
}