Author: Alessandro Damiani
Date: 14:27:46 01/13/04
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>You need to be _very_ careful doing what you describe. It leads to a >scoring discontinuity, or a "boundary condition" that can be very bad >with minimax/alpha-beta search. If you know what a discontinuous function >is, you just defined one (IE in Laplace transforms, you see "unit step >functions" that have this property.) The problem here is that when you >are searching positions where you _cross_ the boundary in the search, the >program gets to pick when and where it crosses, and since you are talking >about a positional score of +/- 3 pawns or more, what it might do is give >up two pawns rather than lose the 3 pawn advantage. Not a good plan, >usually. Your evaluation function _really_ should be a continuous function >that has no such "discontinuities". > >IE you might do just fine where you have lots of pieces and use that >trick, and once there are no pieces, you are also OK. But around the >"edge" where you go from "have to have not" the search can do some amazing >things to use that discontinuity to produce results wildly different from >what you would normally expect to see. IE early in Crafty I did that for >endgames, where material <= N. And at the transition point, I saw some >_ugly_ ideas pop out of the search. Now it is a smooth transition from one >phase to the next with no sudden large jumps.. > >I think Hans Berliner wrote a paper about this topic but I can't recall the >title for the life of me... SNAC (Smoothness, Non-Linearity, and Application Coefficients). I remember the content, but I don't remember where I put it. Maybe in one of the boxes that are in the cellar since I relocated. ;-) Alessandro
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