Author: Don Dailey
Date: 22:04:57 06/24/98
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>I have always used this method too, except that I keep a 3rd killer >which is for the best move at nodes where alpha < score < beta (this >only helps a little). I've never tried keeping a counter for killers. > > >John I have been experimenting with the counter idea and so far it has not proven to be of any value. My best implementation of the counter method saves about 1/2 percent of the nodes over the method you and me are using but using more memory and involves more bookeeping so their is a slight slowdown. I'm amazed that it works at all and yet the counter method seems completely broken to me and illogical. I can easily construct an example where 1 move is a killer a very tiny percentage of the time and yet will always be tried first. It's sort of like a race, but where one guy gets a head start and the coach at random intervals makes one of the other runners start at the beginning again! In several positions the counter method had much greater node counts and and in several others it was better. I tried 20 positions. I think the reason it does not do as badly as one would expect is that it is probably not too common to have more than 2 moves compete for killer status. This method would be quite sound if there were never more than 2 moves competing to be the cutoff king. I have always seen the counter method described in the literature and always wondered what the hell they were talking about. I never ever implemented it for the reasons I have stated above. The strange thing about this, is that the only way to make it work is to constantly reset the counters, an idea Bruce passed on to me. I think this cures it of the "unfair race" behavior by making everyone start at the beginning every once in a while! It also solves the "locality" problem, where the position changes fundamentally due to an ancestor node and the number one killer may not work for an entire subtree. Potentially there must be a big improvement we can make, because almost any move ordering experiment will drastically help at least SOME positions. That means, for at least those positions, that there is something much better than what you are doing! Maybe there is a way to consistantly get something very close to the best and bypass most of the noise? - Don
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