Author: Robert Hyatt
Date: 10:11:56 04/08/02
Go up one level in this thread
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...
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