To determine this rule, a complete history of the game from the last irreversible action (capture, pawn movement, or castling) needs to be maintained, and so, is generally tracked in separate data structures.
Without this information, models may repeat the position despite having a winning advantage, resulting in an excessive amount of draws.
Some of the very earliest chess programs working with extremely limited amounts of memory maintained serial lists (arrays) of the pieces in a conveniently searchable order, like largest to smallest; associated with each piece was its location on the board as well as other information, such as squares representing its legal moves.
Each move has to be checked to ensure it does not wrap around an edge of the board, which significantly slows down the process.
[1][3] Better memory usage can be achieved with a 10x12 array, which provides the same functionalities as a 12x12 one by overlapping the leftmost and rightmost edge files (which are marked as off-the-board).
A bitboard is a 64-bit sequence of bits (0 or 1), which indicates the absence or presence (false or true) of some state of each space on the board.
The advantage to this representation is the ability to use bit parallel operations upon the 64-bit entities instead of iteration to manipulate and derive information about the state of the board.
These bits can be extracted and used as an index into a table to obtain the map of spaces attacked by these pieces.
Rotating a chessboard is conceptually challenging, and rotating a bitboard is computationally inelegant, but the transformation avoids serially enumerating the piece moves, or a lengthy sequence of shifting and masking a bitboard of the attack map of the piece to take into account the board configuration.
One such scheme that uses a perfect hash function along with tricks to minimize the potential size of the table that must be stored in memory, is called "magic bitboards".
[5] Other methods such as Compact Chessboard Representation (CCR) have been proposed,[citation needed] but none has gained acceptance.