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.
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