Author: Scott Gasch
Date: 23:21:18 09/12/04
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On September 13, 2004 at 00:16:55, Dan Honeycutt wrote: >On September 12, 2004 at 14:22:08, Scott Gasch wrote: > >[snip] >>I tried this with mixed results. Overall my ECM/20 score dropped. Now I am >>thinking about what is wrong with this idea. For one, I use PVS so doing >>anything based on alpha in the search could cause problems. Imagine searching >>some line with a=b-1, deciding to extend based on that alpha, finding something >>great, researching with a real a..b window, deciding not to extend based on the >>new alpha, and missing the "greatness". >> > >I don't understand what "new alpha" you are talking about. I do the scout >search with a window of alpha, alpha + 1. If the search fails I research with >alpha, beta. If it's a reduction (which I expected to fail low but didn't) I >research with normal depth. If it's an extension I leave depth alone - hoping >to find that "greatness". > >Dan H. Ok so what I'm saying is this: you do a scout search where alpha = beta - 1. Minimal window. Now, your search does something, anything, based on the current value of alpha. Maybe it decides to extend or not. Maybe it does extended futility pruning. Whatever. Based on this decision you search some tree and find something that causes a fail high. Now your minimal window search has failed high, so you re-search the same tree. This time say alpha = beta - 100. So again you search the same move with a different window. And because your search is doing things based on the current value of alpha... and because the current value of alpha is not the same as when you were searching with a minimal window, this time you decide not to do something. Like extend a critical position. Or prune a critical move. You search a much different tree. Now your search returns failing low! So from the caller's standpoint you had a non-first move... you did a MWS which failed high. You researched it and it failed low. What do you do? My point is that doing things in search based on the current value of alpha is dangerous and can hurt search stability. Scott
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