alt Hacker NewsEither we have to say that the position does not dictate the possible moves, or that this does not fully capture the position. The problem here is that drawing can become an option or a requirement based on information that this representation doesn't capture.
First the simpler version of this problem. After 50 full moves without a capture or pawn move, a draw MAY be claimed. After 75 moves, a draw MUST be claimed. This requires a count to be kept that may require up to 7 more bits.
The bigger problem is draw by repetition. If a position repeats exactly (same castling and en passant options) for a third time, then a draw MAY be claimed. If it repeats exactly for a fifth time, then a draw MUST be claimed. (Usually it is claimed on the third time, but you don't have to.) Applying this rule correctly requires not just knowing the current position, but what positions have occurred previously, and how often. Back to the last pawn move, capture, or change in potential castling status. This may require (per the first rule) knowing what up to 75 different past positions were.
The best way to store this history is almost certainly not as a list of positions, but as a history of moves. But, even if done efficiently, we will need more bytes for that history than we needed for the position.
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The question is, are we storing the state of a chess game, or the state of a chess board?
If a game, you might also include timers or other state as well, including full position history.
Problem need to include the year in the encoding then
> third time, then a draw MAY be claimed. If it repeats exactly for a fifth time, then a draw MUST be claimed
Wow I don't know any online or offline platform which draws on five fold repetition. Didn't know that was a thing at all!
Both examples you have provided are not exactly pertaining to chess POSITION, but rather technicalities to put an upper bound on the time a game may take. Yes, there are rules like 75-move rules, or three-fold repetition, but they have no material bearing on the pieces. On the other hand, FEN does capture information like whether you're eligible for castling, which does make a difference in terms of chess position.
> Either we have to say that the position does not dictate the possible moves, or that this does not fully capture the position.
It does not fully capture the history needed for determining future claims of draw by repetition. But by definition, the position fully captures the position.
The notion of position used by the FEN notation [1] includes the board diagram, side to move, castling rights, en-passant options, as well as the number of halfmoves since the last capture or pawn advance, and the total number of moves. The last one or last two are often ignored in everyday notions of position.
[1] https://en.wikipedia.org/wiki/Forsyth%E2%80%93Edwards_Notati...