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Subject: Re: Checks in the Qsearch

Author: Robert Hyatt

Date: 20:31:11 06/28/02

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On June 28, 2002 at 16:33:10, Scott Gasch wrote:

>I've been experimenting with checks in the qsearch.  While implementing this I
>ran across a couple of old posts from Bob who said, in essence: If side to move
>is in check in the qsearch but has had a chance to stand pat at a previous ply
>in the qsearch then the check is not forced and generating all replies to check
>is wasted nodes.
>
>I am struggling with this idea.  If you are in check and have no good way out
>your opponent will fail high and you will just stand pat where you could at a
>previous ply.  I get that.  So is the idea to only generate responses to check
>that have a chance at not failing low?  Maybe only capturing responses and
>blocking or king flees?  Imagine you are in check and have no capture responses
>so you conclude there is no good way out of check -- is it sound to return -MATE
>to force your side to stand pat where it could have at a previous ply?  I
>suppose the assumption here is that if you are in check and way below alpha
>running away or blocking the check is not going to do the trick and you will end
>up standing pat at a previous ply anyway.

Here is the problem.  It is my move.  I can stand pat.  Or I can make a
capture.  If I make a capture, you check me and now I have to get out of
check and am mated.  You back up a mate score and I will refuse to make this
capture and just stand pat.  So you can't _prove_ that every move leads to
a mate, because whenever I get to stand pat, I stop the mate right there.

The only way to fix this is to follow checks if and only if every move for
the side has been a check since q-search started.  This means the side
getting checked _never_ has a chance to stand pat (I assume you can't stand
pat when evading a check) so that a mate is really forced and can't be
hidden by a stand pat...




>
>Another idea that I read from was that generating non-capturing checks in the
>qsearch against a side that has had a chance to stand pat already is a waste.  I
>really don't understand this idea and disagree with it.  Imagine black has had
>an oppertunity to stand pat but instead plays RxN (N appears hung).  Well this
>looks really good unless white then generates Qd4+ forking blacks R and K and
>winning the R.  If you neglect to generate checks on a side who has already had
>the chance to stand pat you let him get away with RxN and like it.  If the only
>reason to add checks to the qsearch is to find mates then I agree -- checking
>after a side could stand pat is wasted.  But if the goal is to improve tactical
>play then I think this idea is not sound.
>
>Scott


Try it and see what happens to the size of your tree.  _then_ make the
decision whether the slight gain in tactical accuracy of refuting a capture
that fails to a check is worth the loss of a ply or so of overall depth
because your q-search explodes...



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