Author: Robert Hyatt
Date: 12:23:12 04/08/02
Go up one level in this thread
On April 08, 2002 at 13:35:44, Rafael Andrist wrote: >On April 08, 2002 at 13:11:56, Robert Hyatt wrote: > >>On April 07, 2002 at 10:41:27, Michael Fuhrmann wrote: >> >>>[D] 1b6/8/8/7p/6k1/6P1/8/6K1 w - - 0 1 >>> >>>Crafty, without tablebases, scores this as about -4.5 for white. Shouldn't the >>>score be 0.0? >> >> >>Here is what is wrong, after a lot of analysis. >> >>1. static eval here is 0.00 as it should be. >> >>However, the search reaches a position where there are two things that >>can happen. It is possible for black to zugzwang white into playing g4, >>which lets black capture hxg4 and leaves this in a won endgame for black. >>Unfortunately, if black plays hxg4 then white is stalemated, but the evaluation >>doesn't understand this. It therefore concludes, by pushing this to the end >>of any PV, that black can force white to play g4 and get out of the drawn >>rook pawn ending. It doesn't realize that for black to do this, white will >>be stalemated after black plays hxg4 and so the draw is still there... >> >>There isn't an easy way to fix this, and it simply shows the weakness of the >>current search methodology... >> >> >> >>Here is the problematic position: >> >> +---+---+---+---+---+---+---+---+ >> 8 | | | | | | | | | >> +---+---+---+---+---+---+---+---+ >> 7 | | | | | | | | | >> +---+---+---+---+---+---+---+---+ >> 6 | | | | | | | | | >> +---+---+---+---+---+---+---+---+ >> 5 | | | | | | | | *P| >> +---+---+---+---+---+---+---+---+ >> 4 | | | | | | | P | | >> +---+---+---+---+---+---+---+---+ >> 3 | | | | | *B| | | *K| >> +---+---+---+---+---+---+---+---+ >> 2 | | | | | | | | | >> +---+---+---+---+---+---+---+---+ >> 1 | | | | | | | | K | >> +---+---+---+---+---+---+---+---+ >> a b c d e f g h >> >> >> >>White was just forced to play g4 (the pawn was on g3) since white can't move >>his king. If black plays Kxg4 then Crafty knows this is a draw. But at the >>last move in the normal search, then black can play hxg4 and the search won't >>realize it is a stalemate since the q-search is called next. As the search >>goes deeper, black moves the bishop to let the white king go to g1, then you >>check and the king has to go back to h1 which burns one ply. You repeat until >>the king goes to h1 and hxg4 is done in the q-search which doesn't (in Crafty, >>anyway) detect stalemate... >> >>cute position... > >With bitboards as you use, a stalemate detection in eval for a lone king >wouldn't be difficult. > >regards >Rafael B. Andrist I'm working on it right now in fact... for the case where the side on move has a lone king on the edge of the board... It will at least fix this particular class of positions...
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