Author: Omid David Tabibi
Date: 18:43:54 07/15/03
Go up one level in this thread
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:
[D]3rk3/2p5/8/3P4/8/8/3K4/8 b - - 0 1
Black(1): c7c5
White(2): d2d3
Black(2): d8d7
White(3): d3d2
Black(3): d7d8
White(4): d2d3
Black(4): d8d7
White(5): d3d2
Black(5): d7d8
White(6): d2d3
1/2-1/2 {Draw by threefold repetition}
Actually in the above game it was already draw after black's 5th move Rd8, but
since when black played c5 the ep square was wrongly set to c6, that position
was considered different from the two arising later.
>
>>
>>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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