Computer Chess Club Archives


Search

Terms

Messages

Subject: Re: Cutting down my qsearch

Author: Robert Hyatt

Date: 08:04:32 04/23/01

Go up one level in this thread


On April 23, 2001 at 10:43:02, Uri Blass wrote:

>On April 23, 2001 at 09:45:45, Ulrich Tuerke wrote:
>
>>On April 23, 2001 at 09:34:20, Robert Hyatt wrote:
>>
>>>On April 23, 2001 at 05:52:00, JW de Kort wrote:
>>>
>>>>Dear friends,
>>>>
>>>>I have added a qsearch to my program and as a result the number of nodes in my
>>>>search has exploded. Sometimes the program searches ten times as much nodes in
>>>>the qsearch than in the regular search. Can anybody give me some advice on
>>>>methodes i can use to limit the number of nodes in the qsearch?
>>>>
>>>>Thanks in advance.
>>>>
>>>>Jan Willem
>>>
>>>
>>>A simple idea works well:
>>>
>>>When you enter a q-search node, if the current score is (say) a queen below
>>>alpha, then capturing a pawn or piece is not going to bring the score back up
>>>to alpha.  Those captures are useless to examine..
>>
>>I think that this is just what I had suggested in my reply, but you have
>>explained far better why this should work.
>>
>>You say "useless to examine" . After all, this algo involves some kind of
>>forward pruning and there is a certain risk in missing some tactics.
>>
>>However, I admit that this trick appears to be reasonably safe. I guess,
>>everybody is doing it.
>>
>>Nevertheless I had observed that in some rare cases, a test position will be
>>solved later (i.e. in a higher iteration) as a consequence of this trick.
>>
>>Do you agree ?
>
>I do not understand what is the reason that a test position will be solved at
>later ply.
>
>
>If you search only captures in the qsearch and if the score is more than a queen
>below alpha then one capture that is not a capture of the king is not going to
>change the fact that the score is below alpha.
>
>Common sense tells me that if you capture in the qsearch more than your first
>capture then the line is illogical because it is possible that the opponet could
>avoid the line by not capturing
>
>Example:Searching a line like 1.PxQ PxP 2.PxR is illogical in the qsearch
>because black may save the rook by a different move.
>
>
>Uri

Suppose you search to 8 plies.  And at ply=9 the right move is QxR which
appears to lose the Q for a R.  But after NxQ you play RxR and now you are
a rook ahead.  Futility pruning in the q-search will see QxR as a move that
appears to lose material.  And leave the score way below alpha.

One ply deeper, QxR is no longer a q-search move, it is a normal search move
and it doesn't get pruned away here.  Then the queen for two rooks trade is
seen to be good.



This page took 0 seconds to execute

Last modified: Thu, 15 Apr 21 08:11:13 -0700

Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.