Internal Board Representation

chessengine uses bitboards to represent a chess board internally. A bitboard is just a 64-bit integer in which each bit corresponds to one square on the chess board (since a chess board also has 64 squares). A 1 at a certain position means that a piece is present on that square, and a 0 means that no piece is present on that square.

The board is defined in the chessengine.bitboard.Board class. It stores the bitboards of all pieces for each side as attributes with the naming

convention <side>_<piece>s. For example, you can access the bitboard corresponding to black bishops by accessing the black_bishops attribute on your Board object.

Available Attributes

The following attributes are available on the chessengine.bitboard.Board class -

Attributes For Chess Board

Attribute	Type	Description
Board.all_pieces	int	A bitboard representing the positions of all pieces on the board
Board.board	dict[tuple[str,str], int]	A dictionary mapping tuples of the format (side, piece) to the corresponding bitboard. For example, board[("white", "pawns")] returns the bitboard corresponding to white pawns.
Board.moves	list[tuple]	A list of all moves made on the board. Moves are stored as a tuple containing the fields listed below - start is the position the piece started from end is the position the piece landed on captured_side is the side of the piece captured (if any, else None) captured_piece is the piece captured (if any, else None) captured_bitboard is the bitboard of the captured piece before it was captured (used to restore it when we want to undo moves). castle_type is a string representing the castle type the move was (if it was a castle), else None. castle_type can take values "white_kingside", "white_queenside", "black_kingside", "black_queenside". score is the score (current static evaluation) of the board white_king_side_castle is a flag indicating whether white could castle king side white_queen_side_castle is a flag indicating whether white could castle queen side black_king_side_castle is a flag indicating whether black could castle king side black_queen_side_castle is a flag indicating whether black could castle queen side
Board.side	str	The side of the board. Can be "black" or "white".
Board.opponent_side	str	The side of the opponent. Can be "black" or "white"
Board.en_passant_position	int	The position on the board to which a pawn can move to capture a pawn of the opposite side by en passant. Set to 0 if no en passant move can be made in the current turn.

Attributes For White Side

Attribute	Description
Board.white_bishops	A bitboard representing the positions of all white bishops on the board
Board.white_kings	A bitboard representing the position of the white king on the board
Board.white_knights	A bitboard representing the positions of all white knights on the board
Board.white_pawns	A bitboard representing the positions of all white pawns on the board
Board.white_queens	A bitboard representing the positions of all white queens on the board
Board.white_rooks	A bitboard representing the positions of all white rooks on the board
Board.all_white	A bitboard representing the positions of all white pieces on the board

Attributes For Black Side

Attribute	Description
Board.black_bishops	A bitboard representing the positions of all black bishops on the board
Board.black_kings	A bitboard representing the position of the black king on the board
Board.black_knights	A bitboard representing the positions of all black knights on the board
Board.black_pawns	A bitboard representing the positions of all black pawns on the board
Board.black_queens	A bitboard representing the positions of all black queens on the board
Board.black_rooks	A bitboard representing the positions of all black rooks on the board
Board.all_black	A bitboard representing the positions of all black pieces on the board

Representing Positions On The Board

The squares on the chessboard are numbered as follows -

All positions on the board are specified as a power of 2, where the power is the index according to the diagram above. For example, to refer to the square c2, you would pass 2**10 to whichever function you are working with, and to refer to the square e5, you would pass 2**36.

All functions in chessengine.moves accept position as an argument, which is specified as mentioned here. In general, all functions that accept a position argument require it to be specified as mentioned here. Moreover, the Board.move function also requires this format for its start and end arguments.

To help with converting between coordinates on the board, positions, and powers of 2, you can use the chessengine.lookup_tables module.