TalkChess.com

[Vincent Diepeveen]

Note that to store FENs, you can store chesspositions pretty compact.
A simple method is to use say for example:

64 bits : bitmap that basically says whether a piece is on a square or not

now we have for 32 pieces at most which can have 12 values, so with arithmetic encoding that's :

log 12 / log 2 = 3.58 bits
So for 32 pieces that's 114.7 = 115 bits.
So we have 147 bits for the position.
now let's store also its properties.
side to move is 1 bit , we are at 148 now.
we know we have at most 5 pawns we can capture en passant.
that's 2.32 bits.

That bad luck so far is that added up with the pieces that's 117.04 bits
so doesn't fit in 117 bits yet.

So we are at 151 now. Then for each side we must store 2 bits for castling rights of the rook at most.

So we end at 155 bits for a position.

Now Bob will probably post something about repetitions and stuff like that, but he isn't hashing that into crafty either and he is hashing en passant and castling rights, so 155 bits it is