                          Freeciv Hacker's Guide

This guide is intended to be a help for developers, wanting to mess with
Freeciv program.

Here and there, you'll see some comments marked as [...], containing more
personal thoughts on the design, why it looks like it does, and sometimes what
went wrong. I hope developers will find that interesting too.

To read about the AI, see README.AI

===========================================================================
Basic
===========================================================================
Freeciv is a client/server civilization style of game.
The client is pretty dumb. Almost all calculations are performed on the
server.

[It wasn't like this always. Originally more code was placed in the
common/ dir, allowing the client to do some of the world updates itself.
The end_of_turn city-refresh was for example performed both on the server
and on the client. However things got quite complex, more and more info
was needed on the client-side(security problem). Little by little we moved
more code to the server, and as of 1.5 the client is quite dumb -PU]

The source code has the following important directories:
dependencies: code from upstream projects
utility: utility functionality that is not freeciv-specific
common: data structures and code used by both the client and server.
server: (duh)
client: common client code
client/* (fx gui-gtk-3.22): a specific gui implementation of the client.
data: graphics, rulesets and stuff
translations: localization files
ai: the ai, later linked into the server
tools: freeciv support executables

Freeciv is written in C.  Header files should be compatible with C++ so
that C++ add-ons (particularly new clients) are possible.  See the
CodingStyle file for more.

===========================================================================
Server
===========================================================================
General:

The server main loop basically looks like:

  while (server_state == RUN_GAME_STATE) { /* looped once per turn */
    do_ai_stuff();   /* do the ai controlled players */
    sniff_packets(); /* get player requests and handle them */
    end_turn();      /* main turn update */
    game_next_year();
  }


Most time is spend in the sniff_packets() function, where a select()
call waits for packets or input on stdin(server-op commands).

===========================================================================
Server Autogame Testing
===========================================================================
Code changes should always be tested before submission for inclusion
into the git source tree. It is useful to run the client and server as
autogames to verify either a particular savegame no longer shows a
fixed bug, or as a random sequence of games in a while loop overnight.

To start a server game with all AI players, create a file (below named
civ.serv) with lines such as the following:

# set gameseed 42       # repeat a particular game (random) sequence
# set mapseed 42        # repeat a particular map generation sequence
# set timeout 3         # run a client/server autogame
set timeout -1          # run a server only autogame
set minplayers 0        # no human player needed
set ec_turns 0          # avoid timestamps in savegames
set aifill 7            # fill to 7 players
hard                    # make the AI do complex things
create Caesar           # first player (with known name) created and
                        # toggled to AI mode
start                   # start game

Note: The server prompt is unusable when game with timeout set to -1
is running. You can stop such game with single ctrl+c, and continue by
setting timeout to -1 again.

The commandline to run server-only games can be typed as variations
of:
$ while( time server/freeciv-server -r civ.serv ); do date; done
  ---  or  ---
$ server/freeciv-server -r civ.serv -f buggy1534.sav.gz

To attach one or more clients to an autogame, remove the "start"
command, start the server program and attach clients to created AI
players. Or type "aitoggle <player>" at the server command prompt for
each player that connects. Finally, type "start" when you are ready to
watch the show.

Note, that the server will eventually flood a client with updates
faster than they can be drawn to the screen, thus it should always be
throttled by setting a timeout value high enough to allow processing
of the large update loads near the end of the game.

The autogame mode with timeout -1 is only available in DEBUG versions
and should not be used with clients as it removes virtually all the
server gating controls.

If you plan to compare results of autogames the following changes can be
helpful:

- define __FC_LINE__ to a constant value in ./utility/log.h
- undef LOG_TIMERS in ./utility/timing.h
- deactivation of the event cache (set ec_turns 0)

===========================================================================
Data Structures
===========================================================================
For variable length list of fx units and cities freeciv uses a genlist,
which is implemented in utility/genlist.c. By some macro magic type specific
macros have been defined, avoiding much trouble.
For example a tile struct (the pointer to it we call ptile) has a unit
list, ptile->units; to iterate though all the units on the tile you would
do the following:

unit_list_iterate(ptile->units, punit) {
/* In here we could do something with punit, which is a pointer to a
    unit struct */
} unit_list_iterate_end;

Note that the macro itself declares the variable punit.
Similarly there is a

city_list_iterate(pplayer->cities, pcity) {
/* Do something with pcity, the pointer to a city struct */
} city_list_iterate_end;

There are other operations than iterating that can be performed on a list;
inserting, deleting, sorting etc. See utility/speclist.h
Note that the way the *_list_iterate macro is implemented means you can use
"continue" and "break" in the usual manner.

One thing you should keep in the back of your mind: Say you are iterating
through a unit list, and then somewhere inside the iteration decide to
disband a unit. In the server you would do this by calling
wipe_unit(punit), which would then remove the unit node from all the
relevant unit lists. But by the way unit_list_iterate works, if the removed
unit was the following node unit_list_iterate will already have saved the
pointer, and use it in a moment, with a segfault as the result. To avoid
this, use unit_list_iterate_safe instead.

You can also define your own lists with operations like iterating; read how
in utility/speclist.h.

=========================================================================
Network and Packets
=========================================================================
The basic netcode is located in server/sernet.c and client/clinet.c.

(FIXME: this section is a bit out of date. See also doc/README.delta.)

All information passed between the server and clients, must be sent
through the network as serialized packet structures.
These are defined in common/networking/packets.h.

For each 'foo' packet structure, there is one send and one receive function:

int		    send_packet_foo	(struct connection *pc,
					 struct packet_foo *packet);
struct packet_foo * receive_packet_foo	(struct connection *pc);

The send_packet_foo() function serializes a structure into a bytestream
and adds this to the send buffer in the connection struct.
The receive_packet_foo() function de-serializes a bytestream into a
structure and removes the bytestream from the input buffer in the
connection struct.
The connection struct is defined in common/networking/connection.h.

Each structure field in a structure is serialized using architecture
independent functions such as dio_put_uint32() and de-serialized with
functions like dio_get_uint32().

A packet is constituted by header followed by the serialized structure
data. The header contains the following fields (the sizes are defined in
common/packets.c:packet_header_set()):

uint16	:	length		(the length of the entire packet)
uint16	:	type		(e.g. PACKET_TILE_INFO)

For backward compatibility reasons, packets used for the initial protocol
(notably before checking the capabilities) have different header fields
sizes (defined in common/packets.c:packet_header_init()):

uint16	:	length		(the length of the entire packet)
uint8	:	type		(e.g. PACKET_SERVER_JOIN_REQ)

To demonstrate the route for a packet through the system, here's how
a unit disband is performed:

1)  A player disbands a unit.
2)  The client initializes a packet_unit_request structure, and calls the
    packet layer function send_packet_unit_request() with this structure and
    packet type: PACKET_UNIT_DISBAND.
3)  The packet layer serializes the structure, wraps it up in a packet
    containing the packetlength, type and the serialized data. Finally
    the data is send to the server.
4)  On the server the packet is read. Based on the type, the corresponding
    de-serialize function is called is called by get_packet_from_connection().
5)  A packet_unit_request is initialized with the bytestream.
6)  Since the incoming packet is a request (a request in this context
    is every packet sent from the client to the server) the server sends a
    PACKET_PROCESSING_STARTED packet to the client.
7)  Finally the corresponding packet-handler, handle_unit_disband() function,
    is called with the newly constructed structure.
8)  The handler function checks if the disband request is legal (is the sender
    really the owner of the unit) etc.
9)  The unit is disbanded => wipe_unit() => send_remove_unit().
10) Now an integer, containing the id of the disbanded unit is
    wrapped into a packet along with the type PACKET_REMOVE_UNIT:
    send_packet_generic_integer().
11) The packet is serialized and send across the network.
12) The packet-handler returns and the end of the processing is
    announced to the client with a PACKET_PROCESSING_FINISHED packet.
13) On the client the PACKET_REMOVE_UNIT packet is deserialized into
    a packet_generic_integer structure.
14) The corresponding client handler function is now called
    handle_remove_unit(), and finally the unit is disbanded.

Notice that the two packets (PACKET_UNIT_DISBAND and
PACKET_REMOVE_UNIT) were generic packets.  That means the packet
structures involved, are used by various requests.  The
packet_unit_request() is for example also used for the packets
PACKET_UNIT_BUILD_CITY and PACKET_UNIT_CHANGE_HOMECITY.

When adding a new packet type, check to see if you can reuse some of the
existing packet types. This saves you the trouble of
writing new serialize/deserialize functions.

The PACKET_PROCESSING_STARTED and PACKET_PROCESSING_FINISHED packets
from above serve two main purposes:

 - they allow the client to identify what causes a certain packet the
 client receives. If the packet is framed by PACKET_PROCESSING_STARTED
 and PACKET_PROCESSING_FINISHED packets it is the causes of the
 request. If not the received packet was not caused by this client
 (server operator, other clients, server at a new turn)

 - after a PACKET_PROCESSING_FINISHED packet the client can test if
 the requested action was performed by the server. If the server has
 sent some updates the client data structure will now hold other
 values.

The PACKET_FREEZE_HINT and PACKET_THAW_HINT packets serve two
purposes:

 - Packets send between these two packets may contain multiple
 information packets which may cause multiple updates of some GUI
 items. PACKET_FREEZE_HINT and PACKET_THAW_HINT can now be used to
 freeze the GUI at the time PACKET_FREEZE_HINT is received and only
 update the GUI after the PACKET_THAW_HINT packet is received.

 - Packets send between these two packets may contain contradicting
 information which may confuse a client-side AI (agents for
 example). So any updates send between these two packets are only
 processed after the PACKET_THAW_HINT packet is received.

The following areas are wrapped by PACKET_FREEZE_HINT and
PACKET_THAW_HINT:

 - the data send if a new game starts
 - the data send to a reconnecting player
 - the end turn activities

The GTK+ client uses gdk_input_add() to tell gtk to call the callback
functions, when something happens on the client socket.

=========================================================================
Network Improvements
=========================================================================

In previous versions when a connection send buffer in the server got full
we emptied the buffer contents and continued processing. Unfortunately this
caused incomplete packets to be sent to the client, which caused crashes
in either the client or the server, since the client cannot detect this
situation. This has been fixed by closing the client connection when the
buffer is emptied.

We also had (and still have) several problems related to flow control.
Basically the problem is the server can send packets much faster than the
client can process them. This is especially true when in the end of the
turn the AIs move all their units. Unit moves in particular take a long
time for the client to process since by default smooth unit moves is on.

There are 3 ways to solve this problem:
1) We wait for the send buffers to drain before continuing processing.
2) We cut the player's connection and empty the send buffer.
3) We lose packets (this is similar to 2) but can cause an incoherent
   state in the client).

We mitigated the problem by increasing the send buffer size on the server
and making it dynamic. We also added in strategic places in the code calls
to a new flush_packets() function that makes the server stall for some time
draining the send buffers. Strategic places include whenever we send the
whole map. The maximum amount of time spent per flush_packets() call is
specified by the 'netwait' variable.

To disconnect unreachable clients we added two other features: the server
terminates a client connection if it doesn't accept writes for a period
of time (set using the 'tcptimeout' variable). It also pings the client
after a certain time elapses (set using the 'pingtimeout' variable). If
the client doesn't reply its connection is closed.

=========================================================================
Graphics
=========================================================================
Currently the graphics is stored in the PNG file format (other formats
may be readable by some clients).

If you alter the graphics, then make sure that the background remains
transparent.  Failing to do this means the mask-pixmaps will not be
generated properly, which will certainly not give any good results.

Each terrain tile is drawn in 16 versions, all the combinations with
with a green border in one of the main directions. Hills, mountains,
forests and rivers are treated in special cases.

Isometric tilesets are drawn in a similar way to how civ2 draws (that's
why civ2 graphics are compatible).  For each base terrain type there
exists one tile sprite for that terrain.  The tile is blended with
nearby tiles to get a nice-looking boundary.  This is erroneously called
"dither" in the code.

Non-isometric tilesets draw the tiles in the "original" freeciv way,
which is both harder and less pretty.  There are multiple copies of
each tile, so that a different copy can be drawn depending the terrain
type of the adjacent tiles.  It may eventually be worthwhile to convert
this to the civ2 system.

=========================================================================
Diplomacy
=========================================================================
A few words about the diplomacy system. When a diplomacy meeting is
established, a Treaty structure is created on both of the clients and
on the server. All these structures are updated concurrently as clauses
are added and removed.

=========================================================================
Map structure
=========================================================================
The map is maintained in a pretty straightforward C array, containing
X*Y tiles. You can use the function
struct tile *map_pos_to_tile(x, y)
to find a pointer to a specific tile.
A tile has various fields; see the struct in "common/map.h"

You may iterate tiles, you may use the following methods:

whole_map_iterate(tile_itr) {
  /* do something */
} whole_map_iterate_end;
for iterating all tiles of the map;

adjc_iterate(center_tile, tile_itr) {
  /* do something */
} adjc_iterate_end;
for iterating all tiles close to 'center_tile', in all *valid* directions
for the current topology (see below);

cardinal_adjc_iterate(center_tile, tile_itr) {
  /* do something */
} cardinal_adjc_iterate_end;
for iterating all tiles close to 'center_tile', in all *cardinal* directions
for the current topology (see below);

square_iterate(center_tile, radius, tile_itr) {
  /* do something */
} square_iterate_end;
for iterating all tiles in the radius defined 'radius' (in real distance,
see below), beginning by 'center_tile';

circle_iterate(center_tile, radius, tile_itr) {
  /* do something */
} square_iterate_end;
for iterating all tiles in the radius defined 'radius' (in square distance,
see below), beginning by 'center_tile';

iterate_outward(center_tile, real_dist, tile_itr) {
  /* do something */
} iterate_outward_end;
for iterating all tiles in the radius defined 'radius' (in real distance,
see below), beginning by 'center_tile'. (Actually, this is the duplicate
of square_iterate);

or various tricky ones defined in "common/map.h", which automatically
adjust the tile values. The defined macros should be used whenever
possible, the examples above were only included to give people the
knowledge of how things work.

Note that the following
for (x1 = x-1; x1 <= x+1; x1++) {
  for (y1 = y-1; y1 <= y+1; y1++) {
    /* do something */
  }
}
is not a reliable way to iterate all adjacent tiles for all topologies, so
such operations should be avoided.


Also available are the functions calculating distance between tiles. In
Freeciv, we are using 3 types of distance between tiles:

map_distance(ptile0, ptile1) returns the *Manhattan* distance between
tiles, i.e. the distance from 'ptile0' to 'ptile1', only using cardinal
directions, for example (abs(dx) + ads(dy)) for non-hexagonal topologies.

real_map_distance(ptile0, ptile1) returns the *normal* distance between
tiles, i.e. the minimal distance from 'ptile0' to 'ptile1' using all
valid directions for the current topology.

sq_map_distance(ptile0, ptile1) returns the *square* distance between
tiles. This is a simple way to make Pythagorean effects for making circles
on the map for example. For non-hexagonal topologies, it would be
(dx * dx + dy * dy). Only useless square root is missing.

=========================================================================
Different types of map topology
=========================================================================

Originally Freeciv supports only a simple rectangular map.  For instance
a 5x3 map would be conceptualized as

  <- XXXXX ->
  <- XXXXX ->
  <- XXXXX ->

and it looks just like that under "overhead" (non-isometric) view (the
arrows represent an east-west wrapping).  But under an isometric-view
client, the same map will look like

     X
    X X
   X X X
    X X X
     X X X
      X X
       x

where "north" is to the upper-right and "south" to the lower-left.  This
makes for a mediocre interface.

An isometric-view client will behave better with an isometric map.  This is
what Civ2, SMAC, Civ3, etc. all use.  A rectangular isometric map can be
conceptualized as

  <- X X X X X  ->
  <-  X X X X X ->
  <- X X X X X  ->
  <-  X X X X X ->

(north is up) and it will look just like that under an isometric-view client.
Of course under an overhead-view client it will again turn out badly.

Both types of maps can easily wrap in either direction: north-south or
east-west.  Thus there are four types of wrapping: flat-earth, vertical
cylinder, horizontal cylinder, and torus.  Traditionally Freeciv only wraps
in the east-west direction.

=========================================================================
Topology, cardinal directions and valid directions
=========================================================================

A *cardinal* direction connects tiles per a *side*.
Another *valid* direction connects tiles per a *corner*.

In non-hexagonal topologies, there are 4 cardinal directions, and 4 other
valid directions.
In hexagonal topologies, there are 6 cardinal directions, which matches
exactly the 6 valid directions.

Note that with isometric view, the direction named "North" (DIR8_NORTH)
is actually not from the top to the bottom of the screen view. All
directions are turned a step on the left (pi/4 rotation with square tiles,
pi/3 rotation for hexagonal tiles).

=========================================================================
Different coordinate systems
=========================================================================

In Freeciv, we have the general concept of a "position" or "tile".  A tile
can be referred to in any of several coordinate systems.  The distinction
becomes important when we start to use non-standard maps (see above).

  Here is a diagram of coordinate conversions for a classical map.

      map        natural      native       index

      ABCD        ABCD         ABCD
      EFGH  <=>   EFGH     <=> EFGH   <=> ABCDEFGHIJKL
      IJKL        IJKL         IJKL

  Here is a diagram of coordinate conversions for an iso-map.

     map          natural     native       index

        CF        A B C         ABC
       BEIL  <=>   D E F   <=>  DEF   <=> ABCDEFGHIJKL
      ADHK        G H I         GJI
       GJ          J K L        JKL

Below each of the coordinate systems are explained in more detail.

Note that hexagonal topologies are always considered as isometric.

- Map (or "standard") coordinates.

  All of the code examples above are in map coordinates.  These preserve
  the local geometry of square tiles, but do not represent the global map
  geometry well.  In map coordinates, you are guaranteed (so long as we use
  square tiles) that the tile adjacency rules

      (map_x-1, map_y-1)    (map_x, map_y-1)   (map_x+1, map_y-1)
      (map_x-1, map_y)      (map_x, map_y)     (map_x+1, map_y)
      (map_x-1, map_y+1)    (map_x, map_y+1)   (map_x+1, map_y+1)

  are preserved, regardless of what the underlying map or drawing code
  looks like.  This is the definition of the system.

  With an isometric view, this looks like:

                              (map_x-1, map_y-1)
                 (map_x-1, map_y)            (map_x, map_y-1)
  (map_x-1, map_y+1)          (map_x, map_y)              (map_x+1, map_y-1)
                 (map_x, map_y+1)            (map_x+1, map_y)
                              (map_x+1, map_y+1)

  Map coordinates are easiest for local operations (like 'square_iterate'
  and friends, translations, rotations and any other scalar operation)
  but unwieldy for global operations.

  When performing operations in map coordinates (like a translation
  of tile (x, y) by (dx, dy) -> (x + dx, y + dy)), the new map coordinates
  may be unsuitable for the current map. In case, you should use one of
  the following functions/macros:

  map_pos_to_tile(): return NULL if normalization is not possible;

  normalize_map_pos(): return TRUE if normalization have been done (wrapping
  X and Y coordinates if the current topology allows it);

  is_normal_map_pos(): return TRUE if the map coordinates exist for the map;

  is_real_map_pos(): return TRUE if the map coordinates may exist if we
  perform normalization.

  CHECK_MAP_POS(): assert whether the map coordinates exist for the map.

  Map coordinates are quite central in the coordinate system, and they may
  be easily converted to any other coordinates: MAP_TO_NATURAL_POS(),
  MAP_TO_NATIVE_POS(), map_pos_to_index().

- Natural coordinates.

  Natural coordinates preserve the geometry of map coordinates, but also have
  the rectangular property of native coordinates.  They are unwieldy for
  most operations because of their sparseness - they may not have the same
  scale as map coordinates and, in the iso case, there are gaps in the
  natural representation of a map.

  With classical view, this looks like:

      (nat_x-1, nat_y-1)    (nat_x, nat_y-1)   (nat_x+1, nat_y-1)
      (nat_x-1, nat_y)      (nat_x, nat_y)     (nat_x+1, nat_y)
      (nat_x-1, nat_y+1)    (nat_x, nat_y+1)   (nat_x+1, nat_y+1)

  With an isometric view, this looks like:

                              (nat_x, nat_y-2)
               (nat_x-1, nat_y-1)          (nat_x+1, nat_y-1)
  (nat_x-2, nat_y)            (nat_x, nat_y)              (nat_x+2, nat_y)
               (nat_x-1, nat_y+1)          (nat_x+1, nat_y+1)
                              (nat_x, nat_y+2)

  Natural coordinates are mostly used for operations which concern the user
  view. It is the best way to determine the horizontal and the vertical
  axis of the view.

  The only coordinates conversion is done using NATURAL_TO_MAP_POS().

- Native coordinates.

  With classical view, this looks like:

      (nat_x-1, nat_y-1)    (nat_x, nat_y-1)   (nat_x+1, nat_y-1)
      (nat_x-1, nat_y)      (nat_x, nat_y)     (nat_x+1, nat_y)
      (nat_x-1, nat_y+1)    (nat_x, nat_y+1)   (nat_x+1, nat_y+1)

  With an isometric view, this looks like:

                              (nat_x, nat_y-2)
               (nat_x-1, nat_y-1)          (nat_x, nat_y-1)
  (nat_x-1, nat_y)            (nat_x, nat_y)            (nat_x+1, nat_y)
               (nat_x-1, nat_y+1)          (nat_x, nat_y+1)
                              (nat_x, nat_y+2)

  Neither is particularly good for a global map operation such as
  map wrapping or conversions to/from map indexes, something better
  is needed.

  Native coordinates compress the map into a continuous rectangle; the
  dimensions are defined as map.xsize x map.ysize.  For instance the
  above iso-rectangular map is represented in native coordinates by
  compressing the natural representation in the X axis to get the
  3x3 iso-rectangle of

                    ABC       (0,0) (1,0) (2,0)
                    DEF  <=>  (0,1) (1,1) (2,1)
                    GHI       (0,2) (1,2) (3,2)

  The resulting coordinate system is much easier to use than map
  coordinates for some operations.  These include most internal topology
  operations (e.g., normalize_map_pos, whole_map_iterate) as well as
  storage (in map.tiles and savegames, for instance).

  In general, native coordinates can be defined based on this property:
  the basic map becomes a continuous (gap-free) cardinally-oriented
  rectangle when expressed in native coordinates.

  Native coordinates can be easily converted to map coordinates using
  NATIVE_TO_MAP_POS(), to index using native_pos_to_index() and
  to tile (shortcut) using native_pos_to_tile().

  After operations, such as FC_WRAP(x, map.xsize), the result may be checked
  with CHECK_NATIVE_POS().

- Index coordinates.

  Index coordinates simply reorder the map into a continuous (filled-in)
  one-dimensional system.  This coordinate system is closely tied to
  the ordering of the tiles in native coordinates, and is slightly
  easier to use for some operations (like storage) because it is
  one-dimensional.  In general you can't assume anything about the ordering
  of the positions within the system.

  Indexes can be easily converted to native coordinates using
  index_to_native_pos() or to map positions (shortcut) using
  index_to_map_pos().

  An map index can tested using the CHECK_INDEX macro.

With a classical rectangular map, the first three coordinate systems are
equivalent.  When we introduce isometric maps, the distinction becomes
important, as demonstrated above.  Many places in the code have
introduced "map_x/map_y" or "nat_x/nat_y" to help distinguish whether
map or native coordinates are being used.  Other places are not yet
rigorous in keeping them apart, and will often just name their variables
"x" and "y".  The latter can usually be assumed to be map coordinates.

Note that if you don't need to do some abstract geometry exploit, you
will mostly use tile pointers, and give to map.[ch] tools the ability
to perform what you want.

Note that map.xsize and map.ysize define the dimension of the map in
_native_ coordinates.

Of course, if a future topology does not fit these rules for coordinate
systems, they will have to be refined.

=========================================================================
Native coordinates on an isometric map
=========================================================================

An isometric map is defined by the operation that converts between map
(user) coordinates and native (internal) ones.  In native coordinates, an
isometric map behaves exactly the same way as a standard one.  (See
"native coordinates", above.

Converting from map to native coordinates involves a pi/2 rotation (which
scales in each dimension by sqrt(2)) followed by a compression in the X
direction by a factor of 2.  Then a translation is required since the
"normal set" of native coordinates is defined as
  {(x, y) | x: [0..map.xsize) and y: [0..map.ysize)}
while the normal set of map coordinates must satisfy x >= 0 and y >= 0.

Converting from native to map coordinates (a less cumbersome operation) is
the opposite.
                                        EJ
           ABCDE     A B C D E         DIO
  (native) FGHIJ <=>  F G H I J <=>   CHN  (map)
           KLMNO     K L M N O       BGM
                                    AFL
                                     K

Note that

  native_to_map_pos(0, 0) == (0, map.xsize-1)
  native_to_map_pos(map.xsize-1, 0) == (map.xsize-1, 0)
  native_to_map_pos(x, y+2) = native_to_map_pos(x,y) + (1,1)
  native_to_map_pos(x+1, y) = native_to_map_pos(x,y) + (1,-1)

The math then works out to

  map_x = ceiling(nat_y / 2) + nat_x
  map_y = floor(nat_y / 2) - nat_x + map.xsize - 1

  nat_y = map_x + map_y - map.xsize
  nat_x = floor(map_x - map_y + map.xsize / 2)

which leads to the macros NATIVE_TO_MAP_POS(), MAP_TO_NATIVE_POS() that are
defined in map.h.

=========================================================================
Unknown tiles and Fog of War
=========================================================================

In common/player.h, there are several fields:

struct player {
  ...
  struct dbv tile_known;

  union {
    struct {
      ...
    } server;

    struct {
      struct dbv tile_vision[V_COUNT];
    } client;
  };
};

While tile_get_known() returns:

/* network, order dependent */
enum known_type {
 TILE_UNKNOWN = 0,
 TILE_KNOWN_UNSEEN = 1,
 TILE_KNOWN_SEEN = 2,
};

The values TILE_UNKNOWN, TILE_KNOWN_SEEN are straightforward.
TILE_KNOWN_UNSEEN is a tile of which the user knows the terrain,
but not recent cities, roads, etc.

TILE_UNKNOWN tiles never are (nor should be) sent to the client.  In the
past, UNKNOWN tiles that were adjacent to UNSEEN or SEEN were sent to make
the drawing process easier, but this has now been removed.  This means
exploring new land may sometimes change the appearance of existing land (but
this is not fundamentally different from what might happen when you
transform land).  Sending the extra info, however, not only confused the
goto code but allowed cheating.

Fog of war is the fact that even when you have seen a tile once you are
not sent updates unless it is inside the sight range of one of your units
or cities.

We keep track of fog of war by counting the number of units and cities
[and nifty future things like radar outposts] of each client that can
see the tile. This requires a number per player, per tile, so each player_tile
has a short[]. Every time a unit/city/miscellaneous can observe a tile
1 is added to its player's number at the tile, and when it can't observe
any more (killed/moved/pillaged) 1 is subtracted. In addition to the
initialization/loading of a game this array is manipulated with the
void unfog_area(struct player *pplayer, int x, int y, int len)
and
void fog_area(struct player *pplayer, int x, int y, int len)
functions. "int len" is the radius of the area that should be
fogged/unfogged, i.e. a len of 1 is a normal unit. In addition to keeping
track of fog of war, these functions also make sure to reveal TILE_UNKNOWN
tiles you get near, and send info about TILE_UNKNOWN tiles near that the
client needs for drawing. They then send the tiles to
void send_tile_info(struct player *dest, int x, int y)
which then sets the correct known_type and sends the tile to the client.

If you want to just show the terrain and cities of the square the
function show_area does this. The tiles remain fogged.
If you play without fog of war all the values of the seen arrays are
initialized to 1. So you are using the exact same code, you just never
get down to 0. As changes in the "fogginess" of the tiles are only sent
to the client when the value shifts between zero and non-zero, no
redundant packages are sent. You can even switch fog of war on/off
in game just by adding/subtracting 1 to all the tiles.

We only send city and terrain updates to the players who can see the
tile. So a city (or improvement) can exist in a square that is known and
fogged and not be shown on the map. Likewise, you can see a city in a
fogged square even if the city doesn't exist (it will be removed when
you see the tile again). This is done by 1) only sending info to players
who can see a tile 2) keeping track of what info has been sent so the
game can be saved. For the purpose of 2) each player has a map on the
server (consisting of player_tile's and dumb_city's) where the relevant
information is kept.

The case where a player p1 gives map info to another player p2: This
requires some extra info. Imagine a tile that neither player sees, but
which p1 have the most recent info on. In that case the age of the players'
info should be compared which is why the player tile has a last_updated
field.
This field is not kept up to date as long as the player can see the tile
and it is unfogged, but when the tile gets fogged the date is updated.

[An alternative solution would be to give each tile a list
of the units and cities that observe it. IMO this would not be any
easier than just counting, and would have no benefits. The current
solution also gives the possibility to reveal squares as you like,
say near a radar tower tile special. Very flexible.]

[The barbarians and the ai take their map info directly from the server,
so they can currently ignore fog of war, and they do so. I really think
that the ideal AI wouldn't be cheating like this.]

There is now a shared vision feature, meaning that if p1 gives shared
vision to p2, every time a function like show_area, fog_area, unfog_area
or give_tile_info_from_player_to_player is called on p1 p2 also gets the
info. Note that if p2 gives shared info to p3, p3 also gets the info.
This is controlled by p1's really_gives_vision bitvector, where the
dependencies will be kept.

If there is anything I have explained inadequately in this section you
can ask me on <thue@diku.dk>.
-Thue

=========================================================================
National borders
=========================================================================
For the display of national borders (similar to those used in Sid Meier's
Alpha Centauri) each map tile also has an "owner" field, to identify
which nation lays claim to it. If game.borders is non-zero, each city
claims a circle of tiles game.borders in radius (in the case of neighboring
enemy cities, tiles are divided equally, with the older city winning any
ties). Cities claim all immediately adjacent tiles, plus any other tiles
within the border radius on the same continent. Land cities also claim ocean
tiles if they are surrounded by 5 land tiles on the same continent (this is
a crude detection of inland seas or lakes, which should be improved upon).

Tile ownership is decided only by the server, and sent to the clients, which
draw border lines between tiles of differing ownership. Owner information is
sent for all tiles that are known by a client, whether or not they are fogged.
A patch to convert this to "semi-fogged" behavior, whereby clients receive
limited information about non-neighboring and unseen enemies, is available
at http://freecivac.sf.net/.

===========================================================================
Generalized actions
===========================================================================
An action is something a player can do to achieve something in the game.
Not all actions are enabler controlled yet.

===========================================================================
Generalized action meaning
===========================================================================
A design goal for the action sub system is to keep the meaning of action
game rules clear. To achieve this actions should keep having clear
semantics. There should not be a bunch of exceptions to how for example an
action enabler is interpreted based on what action it enables. This keeps
action related rules easy to understand for ruleset authors and easy to
automatically reason about - both for parts of Freeciv like menus, help
text generation and agents and for third party tools.

Please don't make non actions actions because they are similar to actions
or because some of the things Freeciv automatically does for actions would
be nice to have. Abstract out the stuff you want instead. Make it apply to
both actions and to the thing you wanted.

An action is something a player can order a game entity, the actor, to do.
An action does something in the game it self as defined by the game rules.
It should not matter if those game rules run on the Freeciv server or on a
human umpire. An action can be controlled by game rules. That control can
not be broken by a patched client or by a quick player. An action is at the
level where the rules apply. A sequence of actions isn't an action. Parts
of an action isn't an action.

Putting a unit in a group so the quickly can select it with the rest of the
units in the group and the server can save what group a unit belongs to
is server side client state, not an action. The rules don't care what group
a unit belongs to. Adding a unit to an army where the game rules treat
units in armies different from units outside an army - say by having them
attack as one unit - would be an action.

Putting a unit under the control of the autosettlers server side agent
isn't an action. The player could modify their client to automatically give
the same orders as autosettlers would have given or even give those orders
by hand.

Leaving a destroyed transport isn't an action. The player can't order a
unit to perform this action. Having a unit destroy its transport and then
leave it is an action. Leaving a transport "mid flight" (no matter if it
was destroyed or not) and having a certain probability of surviving to
show up somewhere else is an action.

Please don't add action (result) specific interpretations of requirements
in action enablers. If you need a custom interpretation define a new actor
kind or target kind.

===========================================================================
Misc - The idea trashcan
===========================================================================
[Currently all of the major entities - units, cities, players, contains
an unique id. This id is really only required when a reference to an entity
is to be serialized(saved or distributed over the net). However in the
current code, the id is also used for comparing, looking up and in general
referencing entities. This results in a lot of mess and unnecessary duplicate
functions. Often when programming, one wonders if some function needs
the id or a pointer when referring to an entity. -PU]

The paragraph above isn't true anymore for player, units and cities. -RF

===========================================================================

Player-related entities in Freeciv - by Reinier Post <reinpost@win.tue.nl>
+ by dwp@mso.anu.edu.au

Freeciv is confused about the meaning of 'player'.  As a participant in
Freeciv games, you'll notice that the participants are called 'players'.
At the same time, players seem to be identified with civilizations.
On the other hand, civilizations seem to be identified by 'nation':
every player chooses a nation at the start of the game.

In the data structures, a 'player' identifies a civilization, not a user.

----
  THE PLAN:

There are four player-related entities:

+ player
  A civilization, with a capital, cities, units, an income, etc.

+ nation
  A type of civilization (except that very little actually depends on
  nation, and the oddity exists that each player must be of different
  nation)

+ user
  Identifies 'someone out there', usually a human user running
  freeciv client.

+ connection
  Records a client connection; like a user, but disappears when the user
  disconnects, whereas for real users we may want to remember them between
  connections.  See Connections section below.

Where do these entities exist?

Nations aren't actually used for anything that matters; for them,
so the question isn't very interesting.

Players (more aptly named, 'civilizations'), exist in games.  Except in
the context of a running game, the entity makes no sense.  Players and
their status are part of savefiles.  A game can be saved and restarted
on a different server; the players will be the same.  A new game will
have new players.  Players exist in common/ (even games do) but a
client only has one instantiated player.

The reason to introduce users is client-side server commands.  It must
be possible to assign different levels of access to commands to different
users.  Attaching it to players is not good enough: the information must
survive the addition and removal of other players, the start or restart
of a game, reconnections by the same user even from different computers,
or transferral of the game to a different server.  However, a user
may have different levels of access in different games.

While they last, connections are sufficient identification for users.
The user entity will allow users to be identified when they reconnect.

Ideally, users would be identified with unique global ids, handed out
by a 'registry service' similar to the metaserver, but this would be
too cumbersome in practice.  So the plan is to make users persist in
a server session (even when a game is started, or restarted when that
option is added) and make them persist across games (when a saved
game is loaded in a different server session).

Users will be created when they first connect to a server, remembered by
the running server and in savefiles.  Upon connecting, the client will
be sent a unique session id, generated when the server started, plus a
fresh user id; it will store them in a ~/.civcookie file, and send it
back when trying to reconnect.  This will allow the identity of users
to be protected.  'Protected' players will only allow the same user to
reconnect; 'unprotected' ones allow anyone to connect; server commands
and probably client options will be available to control this.

Player names will be assigned to users, not players.

The server maintains a default access level, which is used for new
users and unprotected ones.

----
  THE PRESENT IMPLEMENTATION:

Currently access levels are stored in the connection struct.  This allows
access levels to be assigned to each individual connected player, which
would not be the case if they were directly assigned to the player struct
(due to the fact that the players array changes when players are added or
removed).

But that's it.

Players are still created before the game is started, and player names
still belong to players.  Access levels exist in client and server,
but only the server uses them.  User ids are not yet implemented;
Server ids do not exist at all.

Commands to protect/unprotect users do not yet exist; they would serve
no useful purpose.

Access levels can set for individual users, including AI players with
a connected observer, but only while someone is connected; they will not
be remembered when the user disconnects.

===========================================================================
Connections
===========================================================================

The code is currently transitioning from 1 or 0 connections per player
only, to allowing multiple connections for each player (recall
'player' means a civilization, see above), where each connection may
be either an "observer" or "controller".

This discussion is mostly about connection in the server.  The client
only has one real connection (client.conn) -- its connection to the
server -- though it does use some other connection structs (currently
pplayer->conn) to store information about other connected clients
(eg, capability strings).

In the old paradigm, server code would usually send information to a
single player, or to all connected players (usually represented by
destination being a NULL player pointer).  With multiple connections
per player things become more complicated.  Sometimes information
should be sent to a single connection, or to all connections for a
single player, or to all (established) connections, etc.  To handle
this, "destinations" should now be specified as a pointer to a struct
conn_list (list of connections).  For convenience the following
commonly applicable lists are maintained:
   game.all_connections   -  all connections
   game.est_connections   -  established connections
   game.game_connections  -  connections observing and/or involved in game
   pplayer->connections   -  connections for specific player
   pconn->self            -  single connection (as list)

Connections can be classified as follows:  (first match applies)

1. (pconn->used == 0) Not a real connection (closed/unused), should
   not exist in any list of have any information sent to it.

(All following cases exist in game.all_connections.)

2. (pconn->established == 0) TCP connection has been made, but initial
   Freeciv packets have not yet been negotiated (join_game etc).
   Exists in game.all_connections only.  Should not be sent any
   information except directly as result of join_game etc packets,
   or server shutdown, or connection close, etc.

(All following cases exist in game.est_connections.)

3. (pconn->player == NULL) Connection has been established, but is not
   yet associated with a player.  Currently this is not possible, but
   the plan is to allow this in future, so clients can connect and
   then see list of players to choose from, or just control the server
   or observe etc. Two subcases:

   3a. (pconn->observer == 0) Not observing the game.  Should receive
       information about other clients, game status etc, but not map,
       units, cities, etc.

(All following cases exist in game.game_connections.)

   3b. (pconn->observer == 1) Observing the game.  Exists in
       game.game_connections.  Should receive game information about
       map, units, cities, etc.

4. (pconn->player != NULL) Connected to specific player, either as
   "observer" or "controller".  (But observers not yet implemented.)
   Exists in game.game_connections, and in pconn->player->connections.


===========================================================================
Starting a Server from Within a Client
===========================================================================

After many years of complaints regarding the ease (or lack thereof) of
starting a game of Freeciv [start a server, input settings on a command line,
start a client, and connect, etc], a method has been developed for starting
and playing a complete game using only the client. This has been called the
"extended" or "new connect dialog". This is perhaps a misnomer, but there it
is.

It works by forking a server from within the client and then controlling that
server via chatline messages. The guts of the machinery to do this can be
found in these files:

client/connectdlg_common.[ch]
client/gui-*/connectdlg.[ch]
common/networking/packets.def
server/gamehand.[ch]
server/stdinhand.[ch]

When a player starts a client, they are presented with several options:
start a new game, continue a saved game and connect to a networked game.
For the latter option, connect_to_server() is called and login proceeds as
normal. The first two options, connectdlg_common.c:client_start_server() is
called. Here, a server is spawned, standard input and outputs to that process
are closed, and then connect_to_server() is called so the client connects to
that server.

At this point everything regarding the client/server connection is as usual;
however, for the client to control the server, it must have access level HACK,
so it must verify to the server that it is local (on the same machine or at
least has access to the same disk as the server). The procedure is:

1. the server creates a file using gamehand.c:create_challenge_filename() and
   puts the name of this file in packet_server_join_reply that it sends back
   to the client. The name of the file is semi-random.
2. The client, upon receiving confirmation that it can join the server,
   creates a file using the name the server selected and places a random number
   inside that file.
3. The client sends a packet [packet_single_want_hack_req] with that random
   number back to the server.
4. The server upon receiving the packet [packet_single_want_hack_req], opens
   the file and compares the two numbers. If the file exists and the numbers
   are equal the server grants that client HACK access level and sends a
   packet [packet_single_want_hack_reply] informing the client of its elevated
   access level.

Only one other packet is used --- packet_single_playerlist_req --- which asks
the server to send a player list when a savegame is loaded. This list is used
for player selection.


===========================================================================
Macros and inline functions
===========================================================================

For a long time Freeciv had no inline functions, only macros.  With the
use of other C99 features and some new requirements by the code, this has
changed.  Now both macros and inline functions are used.

This causes problems because one coder may prefer to use a macro while
another prefers an inline function.  Of course there was always some
discretion to the author about whether to use a function or a macro; all
we've done is add even more choices.

Therefore the following guidelines should be followed:

- Functions should only be put into header files when doing so makes a
  measurable impact on speed.  Functions should not be turned into macros or
  inlined unless there is a reason to do so.

- Macros that take function-like parameters should evaluate each parameter
  exactly once.  Any macro that doesn't follow this convention should be
  named in all upper-case letters as a MACRO.

- Iterator macros should respect "break".

- In header files macros are preferred to inline functions, but inline
  functions are better than MACROS.

- Functions or macros that are currently in one form do not have to be
  changed to the other form.

Note that many existing macros do not follow these guidelines.


===========================================================================
Style Guide
===========================================================================

See CodingStyle in this directory.

- If you send patches, use "diff -u" (or "diff -r -u").  For further
  information, see <https://www.freeciv.org/wiki/How_to_Contribute>.
  Also, name patch files descriptively (e.g. "fix-foo-bug-0.patch" is good,
  but "freeciv.patch" is not).

- When doing a "diff" for a patch, be sure to exclude unnecessary files
  by using the "-X" argument to "diff".  E.g.:

    % diff -ruN -Xdiff_ignore freeciv_git freeciv >/tmp/fix-foo-bug-0.patch

  A suggested "diff_ignore" file is included in the Freeciv distribution.

===========================================================================
Internationalization (I18N)
===========================================================================

Messages and text in general which are shown in the GUI should be
translated by using the "_()" macro. In addition log_normal() and
some log_warn() messages should be translated. In most cases, the other
log levels (log_fatal(), log_error(), log_verbose(), log_debug()) should
NOT be localised.

See utility/fciconv.h for details of how Freeciv handles character sets
and encodings. Briefly:

The data_encoding is used in all data files and network transactions.
This is UTF-8.

The internal_encoding is used internally within freeciv.  This is always
UTF-8 at the server, but can be configured by the GUI client.  (When your
charset is the same as your GUI library, GUI writing is easier.)

The local_encoding is the one supported on the command line.  This is not
under our control, and all output to the command line must be converted.

===========================================================================
Debugging and Profiling
===========================================================================

Debugging has to be activated on compile time. That's done by adding
the option '-Ddebug=true' to the meson configure.
When debugging is enabled:
  - FREECIV_DEBUG is defined
  - Debugging log level available ("debug", log_debug())

On autotools based builds profiling is enabled by '--enable-gprof'.
After the compilation run freeciv normally. It will be 5 to 10 times
slower due to the added calls to the profiling functions.
After freeciv quits normally (not after a crash) the file gmon.out
is written. It can be analyzed by calling
'gprof ./server/freeciv-server gmon.out > gprof.txt'.
More information can be found at
https://sourceware.org/binutils/docs/gprof/index.html.

===========================================================================
