Author: Robert Hyatt
Date: 17:13:13 07/15/03
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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. > >More concretely: > >8/8/8/8/1R4pk/8/5P2/5K2 w - - 0 1 > >1. f4 Kh3 2. Ra4 Kh4 3. Rb4 Kh3 4. Ra4 Kh4 5. Rb4 > >Is this "5. Rb4" drawing by third repetition of the position >occured on the board after "1. f4" or not ? > >Andrea
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