Author: Tord Romstad
Date: 15:45:34 01/06/04
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On January 06, 2004 at 16:24:07, Robert Hyatt wrote: >Yes, but Ferret is not using Hsu's Singular Extension algorithm. not even >close. Bruce is using a "SE approximation" that works very well, but it is >not to be confused with what Hsu defined as singular extensions. > >I did the full DB implementation in Cray Blitz, and using non-recursive >null-move R=1, it seemed to work pretty well. I have tried it more than >once in Crafty, and it simply did not work reasonably whatever I tried. I've >not decided that it is hopeless, but I have not played with it further in at >least a couple of years now... Have you experimented with Bruce's approximation as well? What were the results? I am tempted to try something similar to singular extensions myself some day, but I'm afraid I would have to modify the idea a bit to make it work with my MTD(f) search. >I came to the same conclusion that somehow, null-move with bigger R values simply >doesn't work very well. You extend, but null-move reduces the depth and things get >lost in the middle. That's interesting -- I have precisely the opposite experience. Small R values have never worked for me, except in the endgame. In the middle game, I currently use R=1, 2, 3 or 4 depending on the evaluation function. I use R=1 or 2 when the remaining depth is small and the evaluation function decides that the risk of a horizon problem is big (for instance when the side to move has a trapped or pinned piece, there are serious weaknesses in the king shield, when the opponent has a very dangerous passed pawn, and so on), and R=3 or 4 at all other nodes. Except in a few very tactically complicated positions, more than 90% of the nodes are searched with R=3 or 4. Omid once guessed that this could be related to what we do in the qsearch. I have a big and complicated qsearch which includes checks and a few other forcing moves as well as captures. It is possible that lower values of R work better with minimalistic qsearch functions like yours; this is one of the many things I should probably experiment with when I have some time on my hands. By the way, have you ever tried using R=2 at nodes where one of the last 3 or 4 moves was extended, and R=3 in all other nodes? This could perhaps help you avoid the "lost in the middle" problem you describe. I don't do exactly this myself, but I do other similar tricks in my search. For instance, I am very careful about doing forward pruning or reductions when there are one or more extensions in the last few plies leading to the position. Like all path-dependent search decisions (including the recapture extension, which is one of the other ideas which have never worked for me), it causes search inconsistencies, but to me it is worth the cost. Tord
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