Position keys
Transposition table is used to avoid multiple searching of positions. Zobrist keys are then used for storing and comparing of positions inside the transposition table. In addition we use them for comparing threefold repetition of moves and creating opening books. We opted for 64-bit keys. The keys are generated by using bitwise XOR between random numbers in a 3-dimensional array which is populated at boot time. Array dimensions represent piece colour, its type, and its position. A very useful attribute of Zobrist keys is their ability to simply generate new keys from existing ones.