Author: Dann Corbit
Date: 14:18:23 04/23/02
Go up one level in this thread
On April 23, 2002 at 17:08:59, Slater Wold wrote: >On April 23, 2002 at 12:40:09, Robert Hyatt wrote: > >>Here is an endgame position posted a week or so: >> >>[D]1b6/8/8/7p/6k1/6P1/8/6K1 w - - 0 1 >> >>This endgame causes some problems to Crafty because it doesn't recognize >>stalemate in the q-search. >> >>The problem here is that black can't win, obviously. Bishop + wrong rook >>pawn. But if black could somehow zugzwang white into moving his g-pawn so >>that black could play hxg4, then this becomes an easy win for black. >> >>Crafty's endgame evaluation understands that with the wrong rook pawn, this >>is drawn, and here it concludes "black can't win" which is correct. However, >>as the search progresses, the trick to break my eval term is to get the white >>king on h1, with the black king at h3, and the black bishop at a7. Now the >>white king can't move and white is forced to play g4 where black follows up >>with hxg4 and the eval now says black wins. The problem with this position >>is that white is _still_ stalemated (white king at h1, black king at h3, >>black bishop attacking g1 so that the king can't move. It becomes easy for >>a full-width search to shuffle pieces around, then force the zugzwang (almost- >>zugzwang) to happen so that hxg4 happens at the last ply of full-search, or >>at the first ply of the q-search. Either case results in a stalemate, but >>since I don't try all legal moves in the q-search, I don't notice this. > >The difference between me and Bob: > >I'd just download the 5 man tables and call the problem "solved". :D > >Nice find Bob. It might not be much, but with 1B possible positions in chess, >perhaps this will be that little piece of knowledge that wins 1 important game. >And then it's all worth it. Or perhaps the generic algorith adds 150 ELO because the endgame is frequently evaluated wrong at the leaves. The tablebase files mask a bug[1]. The same algorithm problems will occur with 6,7,8,9... pieces left on the board. [1] 'Bug' is a little harsh. It's an inaccurate evaluation function (and all evaluation functions that do not resolve via win/loss/draw are innacurate). Every chess playing engine has an imperfect eval, but some are a lot more imperfect than the others.
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