Author: Robert Hyatt
Date: 09:05:41 10/05/99
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On October 05, 1999 at 05:14:23, Bas Hamstra wrote: >I have been thinking about "Delta pruning". The way I saw it (a while ago) in >Crafty is like this: > > Delta = Alpha - Material - Margin; > >Then for each capture: > > if(CaptureValue < Delta) skip this capture. > >Now I would like a raction to the following statements: > >(Material - Margin) is a cheap estimation of the current eval. If positional >bonusses get frequently more than a pawn (or even more) "Margin" should be set >to near the maximum the positional part of the score can be, or else this >estimation can become very inaccurate and introduces much errors. Reaction? > >If positional bonusses get frequently much more than a pawn, maybe it is more >accurate to use this: > > Delta = Alpha - Eval; > >Because with one move the Eval won't change *that* much. Reaction? > > > >Regards, >Bas Hamstra. You are overlooking something _important_ in crafty. the "alpha" value _is_ the real evaluation for the current position, and includes both the material score _and_ the positional evaluation. So my test is safe in that I am betting that this single capture can't add more than 100 positional points. It can be wrong, such as when the last piece is removed, because maybe a pawn can run then. But overall it is very safe and I haven't found cases where it screws the search up at all....
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