[jade-cat]

There are two more things that should be considered. Both of them unlikely to influence a position, but they can matter - just like the en passant target.

The fifty move rule[0] is quite simple, just store a number, fits in five bits. But the threefold repetition rule[1] is quite a pickle - it basically means that to know everything about a position you need to know every position that occurred before it.

[0] https://en.wikipedia.org/wiki/Fifty-move_rule [1] https://en.wikipedia.org/wiki/Threefold_repetition

[thdc]

Yeah, it seems like the author is storing more information than just the position but not enough to figure out all of the possible next states of the game.

Another straightforward thing missing is the player's turn; this could determine whether the position is a stalemate or not.

[Scarblac]

Not all previous positions, only the ones with the same pieces, pawn positions, castling rights and en passant rights.

But the standard, FEN, doesn't store that either. It's used more in the context of a full game than with individual positions.

[jade-cat]

Fair enough. Still up to 50 board states that can influence the current one (the 50 move rule is coming to help here)

FEN doesn't store previous states, but EPD can. It just goes to show how meanings and requirements change depending on context, which is super interesting in and of itself :P

[hinkley]

I prefer the Ko rule of Go.

You can’t repeat the last position. But repeating a pattern of part of the board every two turns can force progress to resolution. The entire board never repeats, but it also stops the loop earlier.

[drivers99]

They'd also need to store whose turn it is, so I'm guessing the article is strictly about "positions" and not game state.

[notfed]

I don't see how it can be considered a position without whose turn it is.