4 ms·

What occurred once is not what chess programmers would call a “move”, but rather what chess players would call “move”. His definition of a move is one ply of a

What occurred once is not what chess programmers would call a “move”, but rather what chess players would call “move”. His definition of a move is one ply of algebraic notation. From a chess programming perspective algebraic notation is just a data format and doesn’t have any greater significance. In programming terms a move is a data structure that allows you to derive one position from another according to the rules of chess. In Stockfish a move is a 14 bit number, the first 5 bits are the destination square, the next 5 bits are the origin square, the next 2 bits are the promotion piece, and the last 2 bits are the move type (normal, promotion, en passant or castle)

maverwa 2y ago
I was wondering on how Stockfish encodes the destination and origin square in 5 bits each. I think they don't, at least the code on github uses 6 bits each, which actually gives you 64 possible values, so works out fine: https://github.com/official-stockfish/Stockfish/blob/master/src/types.h#L351-L361 https://github.com/official-stockfish/Stockfish/blob/master/... Thats 16 bits. Thank you for sending me down that (small) rabbit hole!
- zarzavat 2y ago
  Yes you are correct, I had 2^5 = 64 in my head for some reason.
Retric 2y ago
Comparing games with that notation alone would make moving your queen e2-e3 in game A the same as moving a pawn e2-e3 in game 2. I don’t think anyone would actually agree those are the same move. What Stockfish considers a move includes the full board state, it simply doesn’t need to pass that information around internally. Thus removing that ambiguity but means there’s a great number of moves that have only occurred once. Also, it’s 6bit + 6bit + 2bit + 2bit = 16 bits which isn’t an arbitrary number. There’s no need to actually encode that something is a promotion because it can be inferred from the board state, but there’s zero cost to pass an extra bit around so it’s included anyway.
deleted 2y ago
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maverwa 2y ago

I was wondering on how Stockfish encodes the destination and origin square in 5 bits each. I think they don't, at least the code on github uses 6 bits each, which actually gives you 64 possible values, so works out fine: https://github.com/official-stockfish/Stockfish/blob/master/src/types.h#L351-L361 https://github.com/official-stockfish/Stockfish/blob/master/... Thats 16 bits. Thank you for sending me down that (small) rabbit hole!

zarzavat 2y ago
Yes you are correct, I had 2^5 = 64 in my head for some reason.

Retric 2y ago

Comparing games with that notation alone would make moving your queen e2-e3 in game A the same as moving a pawn e2-e3 in game 2. I don’t think anyone would actually agree those are the same move. What Stockfish considers a move includes the full board state, it simply doesn’t need to pass that information around internally. Thus removing that ambiguity but means there’s a great number of moves that have only occurred once. Also, it’s 6bit + 6bit + 2bit + 2bit = 16 bits which isn’t an arbitrary number. There’s no need to actually encode that something is a promotion because it can be inferred from the board state, but there’s zero cost to pass an extra bit around so it’s included anyway.

deleted 2y ago

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