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Subject: Using too-shallow mate scores from the hash table

Author: David Eppstein

Date: 00:18:22 07/05/98


I found a new idea to add to my program yesterday...one of those
shouldn't-ever-hurt, could make a minor improvement ones.  Maybe it's only new
to me, but I didn't see it in a quick look thru the Crafty sources.  If it helps
at all, it would only be in tactical positions where a lot of large subtrees
have mate scores.

My eval has the property that it doesn't ever return a mate score unless it's
really mate (doesn't everyone's? actually it plays a different game than chess
and winning isn't called "mate" but who cares.)  So mate scores are special,
they might not be exact due to search extensions but they're always lower bounds
on the true score.

The new idea is, when I find a mate, save the position in the hash table, and
look it up again later (say in the next round of iterated deepening), if the
hashed mate score is greater than beta, I use it even when the hashed draft is
less than the depth I'm currently searching.  The new deeper search would have
found at least as good a score anyway, so you can just cut it off earlier and
save time. Same when eval is negative mate and is less than alpha.

Stupid huh?



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