Author: Robert Hyatt
Date: 20:59:10 05/24/00
Go up one level in this thread
On May 24, 2000 at 22:56:50, Albert Silver wrote: >On May 24, 2000 at 22:26:31, Roberto Waldteufel wrote: > >>On May 24, 2000 at 20:00:53, Robert Hyatt wrote: >> >>>On May 24, 2000 at 19:51:35, Roberto Waldteufel wrote: >>> >>>>I have noticed a weakness in my program in situations where there are advanced >>>>passed pawns, but often the promotion is not seen before it is already too late >>>>to stop, or else can only be stopped by sacrifice of material. Usually the >>>>program has missed a chance to stop the pawn much more cheaply, being distracted >>>>by some small material gain on the other side of the board, or else pushing the >>>>promotion beyond the search horizon by means of a series of checks from which >>>>the oponent can ultimately escape safely. Does anyone extend their searches for >>>>passed pawn pushes? If so, how is the extension triggered - does it depend on >>>>the rank of the pawn, the king position, and/or the currant remaining search >>>>depth? At present I consider pawn promotions in the qsearch (depth<=0) and I >>>>extend 1 ply for pawn pushes to the seventh rank at frontier nodes (depth=1), >>>>so the oponent cannot stand pat without giving the pawn a chance to promote, but >>>>I do no other pawn-triggered extensions at all. I think that pushes to the >>>>sixth rank probably should be extended as well, but to ensure that promotion >>>>occurs before standing pat this would need a lot of extra full-width plies. Any >>>>suggestions? >>>> >>>>Best wishes, >>>> >>>>Roberto >>> >>> >>>Make the value of a passed pawn larger. And make this value proportional to >>>the number of squares it has advanced. Then you will try to restrain/blockade >>>the pawn based on positional grounds. >> >>I already apply a bonus in the evaluation for passed pawns which increases as >>the pawns advance, and is smaller if the pawn is blockaded. Most of the time >>this works quite satisfactorily; the trouble arises when this bonus is exceeded >>by a material gain, say of a pawn or two, which causes the program to gobble the >>pawns and then push the promotion beyond the horizon with forcing moves, >>typically checks or long swap sequences. I could make the passed pawn bonus >>bigger, but this would cause problems with lazy evaluations, as my total >>positional score is never worth more than 2 pawns, so I make use of lazy >>evaluation to speed things up when the material score is more than 2 pawns below >>alpha or above beta. This means that if the program reaches the qsearch and is 2 >>pawns or more outside the search window on material it does not even bother to >>test for passed pawns - maybe it should test anyway, but that increases the >>qsearch workload quite a lot. How big is your passed pawn bonus relative to to >>material value? I might have to increase my tolerance to 3 pawns instead of two >>and increase my bonus, but it would be nice to catch the "dangerous" cases in >>the main search and extend the pawn push moves if only I could find a way to >>identify dangerous from silly pawn pushes. >> >>Best wishes >> >>Roberto > >Don't forget some of those magic combinations: 2 connected pawns on the 6th >defeat a rook, or other similar ones. I find Hiarcs very good at these types of >positions. > > Albert Silver Yes... and once most pieces are off the board, two connected passers are worse than two split passers. A king easily stops two connected passers while it is much harder to stop pawns that are separated by one or more files.
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