Chess Engine

A chess engine powered by lookup tables and bad code.

Basic Overview

The chess board is represented by a series of 16 bits per square, although only 9 bits are needed per piece. This holds the piece type, color, and whether any special moves may involve said piece (namely castling or en passant). The bit combinations, defined in include/ChessEngine.h, are as follows:

code-block:: cpp

enum Piece {

Empty = 0x0000, Bishop = 0x0001, King = 0x0002, Knight = 0x0004, Pawn = 0x0008, Queen = 0x0010, Rook = 0x0020,

};

enum Color {

Black = 0x0040, White = 0x0080,

};

enum Special {

Normal = 0x0100, Special = 0x0200, /* Denotes castling or en passant */

};

Pseudolegal and legal moves are validated based on their "signature". Each square of importance compares the AND between its bitflags and an inverse bitmask of consistent of all possible flags. If all squares return 0, then the move is considered pseudolegal. Special consideration is given to squares that cannot match an empty square to the inverse bitmask and therefore must be verified using different patterns.

Creating a Game Loop

Initializing Components

In order to use a game loop, it is first necessary to create a ChessGame and a MovesList. The former is responsible for managing the chess game, while the latter is responsible for storing all possible moves. void init_game(ChessGame* game), as defined in src/chessengine.c, initializes a new chess game with the black player starting on the first two rows and the white player on the bottom two.

If using the history module, a ChessHistory object should also be created and allocated using int init_history(ChessHistory* history, size_t capacity), as defined in src/chesshistory.c.

The Game Loop

Although no functions check for the existence of checkmate or stalemate, it is possible to assume the game is in one of two such positions if unsigned char generate_moves_list(ChessGame* game, MovesList* moves_list), as defined in src/chessengine.c, returns 0 - meaning that there are no possible legal moves.

Restrictions based on the number of turns or half moves may also be applied by referencing ChessGame.turn_counter or ChessGame.hm_clock, respectively. Note that ChessGame.hm_clock resets every time a piece is captured or a pawn moves.

Within the loop, the function void update_antemove(ChessGame* game, MoveOp* move_obj), as defined in src/chessengine.c, updates the half-move clock and updates what piece should be marked as en passant, if any. This also removes any special flags granted to a piece previously marked as en passant. The next function, void do_move( ChessGame* game, MoveOp* move_obj), as defined in src/chessengine.c, executes the described move. The final required function, void update( ChessGame* game) updates the active player, turn counter, and check status for the game.

A basic game loop that always make the last generated move would appear as follows:

code-block:: cpp

ChessGame game; MovesList moves;

init_game(&game);

while(

game.hm_clock < 50 && generate_moves_list(&game, &moves) > 0

) {

update_antemove(&game, moves.moves + moves.size - 1); do_move(&game, moves.moves + moves.size - 1); update_postmove(&game);

}

When using the history module, void record_move(ChessGame* game, MoveOp* move_obj, ChessHistory* history), as described in src/chesshistory.c, should be called before update_antemove(...). This records the move before making any changes to the ChessGame. Moves can be unmade using size_t undo_move(ChessGame* game, ChessHistory* history), as defined in src/chesshistory.c. This function additionally returns the number of moves that still remain within the ChessHistory.

An example of the chess engine using the history module can be seen in examples/perft.c.

Using This in a Project

You can use this within your projects by including:

code-block:: cpp

#include <ChessEngine.h>

at the top of the appropriate file(s), and adding -I$(PROJDIR)/include -L $(PROJDIR)/lib -lchessengine.a to your make command, where $(PROJDIR) is the location of this folder (or more precisely, the location of the include/ and lib/ folders from this project).

If you wish to use the history module to record and unmake moves, also include:

code-block:: cpp

#include <ChessHistory.h>

at the top of the appropriate file(s).

make install will install libraries to /usr/local/lib by default, which may not be automatically searched by your distribution. If not, please update your /etc/ld.so.conf file to include this directory (or create a config file in /etc/ld.so.conf.d/ that contains this path). Make sure to run ldconfig with root permissions afterwards for this change to take effect.

Contributing

Any contributions are welcome! However, please refrain from directly modifying src/moveinfo.c, as this file is generated by generators/moves.py as opposed to being manually written.