Author: Georg v. Zimmermann
Date: 02:17:33 06/02/01
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On June 01, 2001 at 16:13:41, Scott Gasch wrote:
>Hi all,
>
>I have some questions about recapture extensions.
>
>First of all, what is the point? I assume to extend in a position where a
>recapture move is the only reasonable move so as to see further beyond the
>forced exchange. Somewhat like singular extensions or one legal reply
>extensions, I think a properly done recapture extension should not be too
>expensive. I'd also expect that a well done recapture extension would improve
>the tactical strength of the engine.
>
>Well, I've been trying to get a good recapture extension working on my engine
>for a while now. I've tried a bunch of different recapture definitions /
>conditions. Most recently I did:
>
> 1. last move was a capture and this move captures the last moved piece
> 2. this is the first capture considered (weighed by SEE)
> 3. do not allow two recaptures in a row
> 4. this move restores the material balance to what it was at the root.
>
>...in which case I extended 1/2 ply. Since I am expecting better tactical
>ability with the recapture turned on, I test it with the ECM suite. Every time
>I try, my results are worse with the recapture turned on than with no recapture.
>
>I know that a bunch of other engines are doing recapture... so there must be a
>good idea there somewhere. Can anyone address these questions or propose a
>solid recapture condition?
>
>Thanks,
>Scott
I do this :
/* Capture extensions. */
if ( CapturedPiece[ply-1] &&
(ply > 1) &&
(treemoves[ply-2].to() == treemoves[ply-1].to()) &&
(!AIBoard.getattacks(AIBoard.getColorOffMove(),
(treemoves[ply-1].to()) ) ) )
{
extensions += CAPTURE_EXTENSION;
/* [... some debug stuff ] */
}
In other words :
If I capture to the same square and no other capture to that square was
possible, extend by half a ply.
This helps a lot in my program, for Crafty the last time I tried it solved one
WAC position more.
Regards,
Georg v. Zimmermann
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