Author: Uri Blass
Date: 19:35:20 07/15/03
Go up one level in this thread
On July 15, 2003 at 21:43:54, Omid David Tabibi wrote: >On July 15, 2003 at 20:13:13, Robert Hyatt wrote: > >>On July 15, 2003 at 17:13:47, Andrea Griffini wrote: >> >>>A question about repeated positions... is the en-passant square >>>part of the "position" even when no en-passant pawn capture is >>>possible (because there is no capturing pawn, or because the >>>theoretically capturing pawn is pinned on the file or because >>>just the en-passant capture is "pinned") ? >> >>I don't believe so. At least not by the rules of FIDE. If a move is >>illegal, it is not possible to play it. For EP, if the pawn can't >>be taken because (a) it wasn't moved two squares, (b) no pawn is on >>an adjacent file to take it; (c) a pawn is on the adjacent file but >>can't legally make the capture due to being pinned; then the move >>simply is not possible and a position with every piece on the same >>square, where one was reached by two pawn pushes and the other by >>one two-square pawn push, would be considered identical. >> >>> >>>If this is not the case then checking for repeated positions >>>gets way more annoying as my position hash code doesn't tell >>>the whole story ... :( >> >>I don't hash the ep status if (a) the pawn didn't advance two squares; >>(b) there is no pawn on either of the adjacent files that _can_ make a >>pseudo-legal EP capture. I don't screen out the pinned business, however, >>and haven't had it cause a problem. >> > >I also do the hasing exactly like this, but thinking about it, a problem can >arise: 1)I think that even if the price of fixing it is only being 0.01% slower then it is still better not to fix it if you care about playing strength. 2)Movei knows about pins but I cannot decide that a pinned pawn cannot do the en passent capture because the pawn can be pinned by a bishop and in part of these cases it can do the enpassent capture. Another point is that there are cases when enpassent capture is illegal inspite of the fact that the pawn is not pinned because there is an enemy rook or enemy queen in the same rank of the king. Uri
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