Author: Robert Hyatt
Date: 10:50:49 10/01/03
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On October 01, 2003 at 09:08:28, Sergei S. Markoff wrote: >Hello! > >>What is the new quasy-Botvinnik extension if > >Well. I can inform chess community about one of my quasy-Botvinnik methods. >I will be glad to receive your own experiment results and/or other >considerations about this method. > >One of Botvinnik ideas is to determine attack/defence targets for further >analysis of attack/defend trajectories. One of the way to find this target is >null-move search. If the result of null-move search < beta we can see the move >which fail low search bound. This move is a current threat. For example it's a >threat of losing some piece. > >During the further search in this node we can found the defending move. The main >idea is to extend the search if the threat moves on ply and ply+2 has the same >target (capturing the same piece, promotes the same pawn). I can give you >example if you want. > >It's one of simpliest way to use target info. But I think we can receive more >improvement exploiting this idea. > >Best wishes, >Sergei It seems to me that Chrilly's paper on null-move search included this idea as the "threat extension". I have code that uses this, but I haven't made it an official part of Crafty since the 9.x versions. It did some good things, but it is not free. And it doesn't particularly work well with a pure PVS implementation. The basic idea was that if a move "fails high" (a real move) but the null-move search fails low at the same position, then this "fail high" move is holding off some threat. IE I have a piece hanging and I can't save it, but a check will push the loss beyond the search horizon. By noticing that the null-move search fails low (I lose the hung piece) but that playing move X fails high (saves the hung piece) the search becomes suspicious that moves X is a horizon move and extends the search enough to bring the loss back within the search horizon. The problem is that the null-move search has to fail "significantly low" and the base idea within PVS would be to search with a lowered alpha/beta window to see if it fails low _badly_. You only do this test on a fail-high or PV node, so it isn't horribly inefficient, but it is not free and after a lot of testing, using Crafty, I eventually got rid of it. I could probably supply the code if someone wants to see it, and, as always, YMMV since I only tested it in Crafty.
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