If you want to write your own interface to GNU Go, or if you want to
create a go application using the GNU Go engine, this chapter is of
interest to you.

First an overview: GNU Go consists of two parts: the GNU Go @i{engine}
and a program (user interface) which uses this engine. These are linked
together into one binary. The current program implements the following
user modes:

@itemize @bullet
@item An interactive board playable on ASCII terminals
@item solo play - GNU Go plays against itself
@item replay - a mode which lets the user investigate moves in an existing
SGF file.
@item GMP - Go Modem Protocol, a protocol for automatic play between two
computers.
@item GTP - Go Text Protocol, a more general go protocol, @pxref{GTP}.
@end itemize
@cindex API

The GNU Go engine can be used in other applications. For example, supplied
with GNU Go is another program using the engine, called @file{debugboard},
in the directory @file{interface/debugboard/}. The program debugboard lets the
user load SGF files and can then interactively look at different properties of
the position such as group status and eye status.

The purpose of this Chapter is to show how to interface your own
program such as @code{debugboard} with the GNU Go engine.

Figure 1 describes the structure of a program using the GNU Go
engine.

@example
@group
                 +-----------------------------------+
                 |                                   |
                 |          Go application           |
                 |                                   |
                 +-----+----------+------+           |
                 |     |          |      |           |
                 |     |   Game   |      |           |
                 |     | handling |      |           |
                 |     |          |      |           |
                 |     +----+-----+      |           |
                 |   SGF    |    Move    |           |
                 | handling | generation |           |
                 |          |            |           |
                 +----------+------------+-----------+
                 |                                   |
                 |           Board handling          |
                 |                                   |
                 +-----------------------------------+
     
        Figure 1: The structure of a program using the GNU Go engine

@end group
@end example

The foundation is a library called @code{libboard.a} which provides 
efficient handling of a go board with rule checks for moves, with
incremental handling of connected strings of stones and with methods to
efficiently hash go positions.

On top of this, there is a library which helps the application use
Smart Game Format (SGF) files, with complete handling of game trees in
memory and in files. This library is called @code{libsgf.a}

The main part of the code within GNU Go is the move generation
library which given a position generates a move. This part of the
engine can also be used to manipulate a go position, add or remove
stones, do tactical and strategic reading and to query the engine for
legal moves. These functions are collected into @code{libengine.a}.

The game handling code helps the application programmer keep tracks
of the moves in a game. Games can be saved to
SGF files and then later be read back again. These are also within
@code{libengine.a}. 

The responsibility of the application is to provide the user with a
user interface, graphical or not, and let the user interact with the
engine.

@menu
* Getting Started::          How to use the engine in your program
* Basic Data Structures::    Basic Data Structures in the Engine
* The Board State::          The board_state `struct'
* Positional Functions::     Functions which manipulate a Position
@end menu

@node Getting Started

@section How to use the engine in your own program: getting started

To use the GNU Go engine in your own program you must include
the file @file{gnugo.h}. This file describes the whole public API. There is
another file, @file{liberty.h}, which describes the internal interface within
the engine. If you want to make a new module within the engine, e.g.  for
suggesting moves you will have to include this file also. In this section we
will only describe the public interface.

@findex init_gnugo
   Before you do anything else, you have to call the function
@code{init_gnugo()}. This function initializes everything within the engine.
It takes one parameter: the number of megabytes the engine can use for
the internal hash table. In addition to this the engine will use a few
megabytes for other purposes such as data describing groups (liberties,
life status, etc), eyes and so on.

@node Basic Data Structures
@section Basic Data Structures in the Engine
@cindex data structures
@cindex position struct


   There are some basic definitions in gnugo.h which are used
everywhere. The most important of these are the numeric declarations of
colors. Each intersection on the board is represented by one of these:

@example
@group

     color              value
     EMPTY                0
     WHITE                1
     BLACK                2

@end group
@end example

There is a macro, @code{OTHER_COLOR(color)} which can be used to get the
other color than the parameter. This macro can only be used on @code{WHITE}
or @code{BLACK}, but not on @code{EMPTY}.
@findex OTHER_COLOR

GNU Go uses two different representations of the board, for
most purposes a one-dimensional one, but for a few purposes a
two dimensional one (@pxref{Libboard}). The one-dimensional
board was introduced before GNU Go 3.2, while the two-dimensional
board dates back to the ancestral program written by Man Lung Li
before 1995. The API still uses the two-dimensional board, so
the API functions have not changed much since GNU Go 3.0.

@node The Board State

@section The board_state struct
@cindex board_state

A basic data structure in the engine is the @code{board_state} struct. 
This structure is internal to the engine and is defined in @file{liberty.h}.

@example 
@group

     typedef unsigned char Intersection;
     
     struct board_state @{
       int board_size;
     
       Intersection board[BOARDSIZE];
       int board_ko_pos;
       int black_captured;
       int white_captured;
     
       Intersection initial_board[BOARDSIZE];
       int initial_board_ko_pos;
       int initial_white_captured;
       int initial_black_captured;
       int move_history_color[MAX_MOVE_HISTORY];
       int move_history_pos[MAX_MOVE_HISTORY];
       int move_history_pointer;
     
       float komi;
       int move_number;
     @};

@end group
@end example

Here @code{Intersection} stores @code{EMPTY}, @code{WHITE} or
@code{BLACK}. It is currently defined as an @code{unsigned char} to make
it reasonably efficient in both storage and access time. The board state
contains an array of @code{Intersection}'s representing the board.
The move history is contained in the struct. Also contained in
the struct is the location of a ko (@code{EMPTY}) if the last
move was not a ko capture, the komi, the number of captures, and
corresponding data for the initial position at the beginning
of the move history.

@node Positional Functions

@section Functions which manipulate a Position

All the functions in the engine that manipulate Positions have names
prefixed by @code{gnugo_}. These functions still use the two-dimensional
representation of the board (@pxref{The Board Array}). Here is a complete
list, as prototyped in @file{gnugo.h}:

@itemize
@item @code{void init_gnugo(float memory)}
@findex init_gnugo
@quotation
Initialize the gnugo engine. This needs to be called 
once only.
@end quotation
@item @code{void gnugo_clear_board(int boardsize)}
@findex gnugo_clear_board
@quotation
Clear the board.
@end quotation
@item @code{void gnugo_set_komi(float new_komi)}
@findex gnugo_set_komi
@quotation
Set the komi.
@end quotation
@item @code{void gnugo_add_stone(int i, int j, int color)}
@findex gnugo_add_stone
@quotation
Place a stone on the board
@end quotation
@item @code{void gnugo_remove_stone(int i, int j)}
@findex gnugo_remove_stone
@quotation
Remove a stone from the board
@end quotation
@item @code{int gnugo_is_pass(int i, int j)}
@findex gnugo_is_pass
@quotation
Return true if (i,j) is PASS_MOVE
@end quotation
@item @code{void gnugo_play_move(int i, int j, int color)}
@findex gnugo_play_move
@quotation
Play a move and start the clock
@end quotation
@item @code{int gnugo_undo_move(int n)}
@findex gnugo_undo_move
@quotation
Undo n permanent moves. Returns 1 if successful and 0 if it fails.
If n moves cannot be undone, no move is undone.
@end quotation
@item @code{int gnugo_play_sgfnode(SGFNode *node, int to_move)}
@findex gnugo_play_sgfnode
@quotation
Perform the moves and place the stones from the SGF node on the 
board. Return the color of the player whose turn it is to move.
@end quotation
@item @code{int gnugo_play_sgftree(SGFNode *root, int *until, SGFNode **curnode)}
@findex gnugo_play_sgftree
@quotation
Play the moves in ROOT UNTIL movenumber is reached.  Return the color of the
player whose turn it is to move.  
@end quotation
@item @code{int gnugo_is_legal(int i, int j, int color)}
@findex gnugo_is_legal
@quotation
Interface to @code{is_legal()}.
@end quotation
@item @code{int gnugo_is_suicide(int i, int j, int color)}
@findex gnugo_is_suicide
@quotation
Interface to @code{is_suicide()}.
@end quotation
@item @code{int gnugo_placehand(int handicap)}
@findex gnugo_placehand
@quotation
Interface to placehand. Sets up handicap pieces and
returns the number of placed handicap stones.
@end quotation
@item @code{void gnugo_recordboard(SGFNode *root)}
@findex gnugo_recordboard
@quotation
Interface to @code{sgffile_recordboard()}
@end quotation
@item @code{int gnugo_sethand(int handicap, SGFNode *node)}
@findex gnugo_sethand
@quotation
Interface to placehand. Sets up handicap stones and
returns the number of placed handicap stones, updating the sgf file
@end quotation
@item @code{float gnugo_genmove(int *i, int *j, int color, int *resign)}
@findex gnugo_genmove
@quotation
Interface to @code{genmove()}.
@end quotation
@item @code{int gnugo_attack(int m, int n, int *i, int *j)}
@findex gnugo_attack
@quotation
Interface to @code{attack()}.
@end quotation
@item @code{int gnugo_find_defense(int m, int n, int *i, int *j)}
@findex gnugo_find_defense
@quotation
Interface to @code{find_defense()}.
@end quotation
@item @code{void gnugo_who_wins(int color, FILE *outfile)}
@findex gnugo_who_wins
@quotation
Interface to @code{who_wins()}.
@end quotation
@item @code{float gnugo_estimate_score(float *upper, float *lower)}
@findex gnugo_estimate_score
@quotation
Put upper and lower score estimates into @code{*upper}, @code{*lower} and
return the average. A positive score favors white. In computing
the upper bound, @code{CRITICAL} dragons are awarded to white; in
computing the lower bound, they are awarded to black.
@end quotation
@item @code{void gnugo_examine_position(int color, int how_much)}
@findex gnugo_examine_position
@quotation
Interface to @code{examine_position}.
@end quotation
@item @code{int gnugo_get_komi()}
@findex gnugo_get_komi
@quotation
Report the komi.
@end quotation
@item @code{void gnugo_get_board(int b[MAX_BOARD][MAX_BOARD])}
@findex gnugo_get_board
@quotation
Place the board into the @samp{b} array.
@end quotation
@item @code{int gnugo_get_boardsize()}
@findex gnugo_get_boardsize
@quotation
Report the board size.
@end quotation
@item @code{int gnugo_get_move_number()}
@findex gnugo_get_move_number
@quotation
Report the move number.
@end quotation
@end itemize

@section Game handling

The functions (in @pxref{Positional Functions}) are all that are needed to
create a fully functional go program.  But to make the life easier for the
programmer, there is a small set of functions specially designed for handling
ongoing games.

The data structure describing an ongoing game is the @code{Gameinfo}. It
is defined as follows:

@example
@group

typedef struct @{
  int       handicap;

  int       to_move;            /* whose move it currently is */
  SGFTree   game_record;        /* Game record in sgf format. */

  int       computer_player;    /* BLACK, WHITE, or EMPTY (used as BOTH) */

  char      outfilename[128];   /* Trickle file */
  FILE      *outfile;
@} Gameinfo;

@end group
@end example

The meaning of @code{handicap} should be obvious. @code{to_move} is the
color of the side whose turn it is to move.

The SGF tree @code{game_record} is used to store all the moves in the entire
game, including a header node which contains, among other things, komi
and handicap.

If one or both of the opponents is the computer, the field
@code{computer_player} is used. Otherwise it can be ignored.

GNU Go can use a trickle file to continuously save all the moves of an
ongoing game. This file can also contain information about internal
state of the engine such as move reasons for various locations or move
valuations. The name of this file should
be stored in @code{outfilename} and the file pointer to the open file is
stored in @code{outfile}. If no trickle file is used,
@code{outfilename[0]} will contain a null character and @code{outfile}
will be set to @code{NULL}.

@subsection Functions which manipulate a Gameinfo

All the functions in the engine that manipulate Gameinfos have names
prefixed by @code{gameinfo_}.  Here is a complete list, as prototyped in
@file{gnugo.h}:

@itemize
@item @code{void gameinfo_clear(Gameinfo *ginfo, int boardsize, float komi)}
@findex gameinfo_clear
@quotation
Initialize the @code{Gameinfo} structure.
@end quotation
@item @code{void gameinfo_print(Gameinfo *ginfo)}
@findex gameinfo_print
@quotation
Print a gameinfo.
@end quotation
@item @code{void gameinfo_load_sgfheader(Gameinfo *gameinfo, SGFNode *head)}
@findex gameinfo_load_sgfheader
@quotation
Reads header info from sgf structure and sets the appropriate variables.
@end quotation
@item @code{void gameinfo_play_move(Gameinfo *ginfo, int i, int j, int color)}
@findex gameinfo_play_move
@quotation
Make a move in the game. Return 1 if the move was legal. In that
case the move is actually done. Otherwise return 0.
@end quotation
@item @code{int gameinfo_play_sgftree_rot(Gameinfo *gameinfo, SGFNode *head, const char *untilstr, int orientation)}
@findex gameinfo_play_sgftree_rot
@quotation
Play the moves in an SGF tree. Walk the main variation, actioning the
properties into the playing board. Returns the color of the next move to be
made. Head is an sgf tree. Untilstr is an optional string of the form either
'L12' or '120' which tells it to stop playing at that move or move
number. When debugging, this is the location of the move being examined.
@end quotation
@item @code{int gameinfo_play_sgftree(Gameinfo *gameinfo, SGFNode *head, const char *untilstr)}
@findex gameinfo_play_sgftree
@quotation
Same as previous function, using standard orientation.
@end quotation
@end itemize