# Board Representation

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Home * Board Representation

Paul Klee, Überschach, 1937 [1]
A chess program needs an internal board representation to maintain chess positions for its search, evaluation and game-play. Beside modelizing the chessboard with its piece-placement, some additional information is required to fully specify a chess position, such as side to move, castling rights, possible en passant target square and the number of reversible moves to keep track on the fifty-move rule.

To begin with, we further elaborate on the pure data structures to represent the board and its piece-placement. There are piece centric and square centric representations as well as hybrid solutions.

# Piece Centric

A piece centric representation keeps lists, arrays or sets of all pieces still on the board - with the associated information which square they occupy. A popular piece centric representative is the set-wise bitboard-approach. One 64-bit word for each piece type, where one-bits associate their occupancy.

# Square Centric

The square centric representation implements the inverse association - is a square empty or is it occupied by a particular piece? The most popular square centric representations, mailbox or it's 0x88-enhancements - are basically arrays of direct piece-codes including the empty square and probably out of board codes. Hybrid solutions may further refer piece-list entries.

# Hybrid Solutions

While different algorithms and tasks inside a chess program might prefer one of these associations, it is quite common to use redundant board representations with elements of both. Bitboard approaches often keep a 8x8 board to determine a piece by square, while square centric board array approaches typically keep piece-lists and/or piece-sets to avoid scanning the board for move generation purposes.

# Move Generation

With a board representation, one big consideration is the generation of moves. This is essential to the game playing aspect of a chess program, and it must be completely correct. Writing a good move generator is often the first basic step of creating a chess program.