bitboards

Bitboards are a compact data structure used to represent the state of board games, most notably chess. In a bitboard, the board is mapped to the bits of one or more integers; a set bit indicates the presence of a piece or occupancy on the corresponding square. On an 8x8 chessboard, a 64-bit unsigned integer can represent a single plane, and engines commonly allocate one bitboard per piece type and color (for example white pawns, black pawns, white knights, etc.), along with bitboards for occupancy of all pieces, white, and black.

Common practice in chess engines involves multiple bitboards to describe the full position. A typical configuration uses six piece-type bitboards per color (pawns, knights, bishops, rooks, queens, king) plus one or more occupancy bitboards. This allows efficient computation of moves and attacks via bitwise operations such as shifts, AND, OR, and XOR. For example, shifting a pawn bitboard by one rank can generate pseudo-legal advances, while masks prevent moves off the board or into occupied squares. Population count and trailing-zero operations enable fast iteration over the set bits to enumerate moves.

Advantages include fast move generation and evaluation, use of native CPU bitwise instructions, and compact memory usage. Bitboards also enable parallel evaluation of multiple squares and pieces within a single operation. Limitations include considerable programming complexity, especially for sliding pieces and legality checks, and the approach is most convenient for fixed-size boards (like 8x8). Larger or differently shaped boards require multiple bitboards or larger word sizes, which can reduce performance advantages.