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Subject: Re: Interesting EGTB bug

Author: Tony Werten

Date: 00:09:12 04/17/02

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On April 16, 2002 at 23:15:47, Tom Likens wrote:

>
>Hello Everyone,
>
>I recently had what I considered an interesting endgame tablebase bug which
>cost my program 1/2 a point on ICS.  The problem occurred when a KR vs. KR
>endgame was reached and the opponent captured the program's rook.  Instead of
>grabbing the rook with its king it moved the king away and immediately lost the
>game.  Looking at the log file revealed that it had been in the endgame
>tablebases for a while.  In fact, the way I structure this is that if I'm not
>in swindle mode and the root position is a tablebase draw then it simply prints
>out the PV to the maximum search depth and returns (skipping the search
>entirely).
>
>The problem was that the EGTB probe code did not recognize the K vs. K
>situation as a draw (essentially a case of missing the 2-man tablebase file).
>So instead of leaving the tablebases it played a losing move that kept it in
>the tablebases, aarrgghh!!  Simple to fix, but it begs the larger question of
>what happens in general if a program has access to an N piece tablebase but
>not all the N-1 piece tablebases that the position could become if a piece or
>pieces were traded off.  Since the tablebases represent "perfect" knowledge I
>give them preference over values returned by the evaluation function.  Could
>this phenomenon be at work in subtle ways during the regular search,
>influencing the program into staying in the tablebase no matter what the cost?
>
>How do other programmers handle this?  Or is it even a real problem?
>
>regards,
>--tom

Search for the current position in the tablebase. Then try all moves and search
them in egtb as well. If the best move is not following the current position (ie
also draw, checkmate or checkmated in rootposition-1) then disable egtb's and
don't retry until a piece is captured or promoted.

Tony



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