Author: Bas Hamstra
Date: 12:27:43 08/25/99
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Oops: forgot to tell recursive nullmove was ON. Will test without too. On August 25, 1999 at 15:19:00, Bas Hamstra wrote: >Well at least the popular ab approaches have a bug. > >Suppose you had no tricks, no extensions, no nothing. Just ab and a qsearch on >top of it. You set an aspiration window at the root, say [200, 300]. > >Now *if* the true ab value lies within this window, it should find it, right? > >WRONG! > >The fact that everyone has a > > if(Eval >= Beta) return Beta; > >somewhere in the qsearch, makes that this no longer must be true. Reason: >suppose you sacrifice lots of material, but can win more material back, due to >severe mating threats, or whatever. With the window [240, 260] the qsearch >concludes at some point to return Beta because of the material advantage (3 >pieces sacced), resulting in a fail-low on the [240, 260] window. However whith >a wider window it can NOT do this, and finds the true value of 246. It now sees >the material can be won back with rent. > >My conclusion is that a fail low on [240, 260] followed by a result of 246 >on [-inf, inf] is completely normal and unavoidable in many cases. It also >explains this: > > fail low on [240, 260] > fail high on [-inf, 241] > value = 246 on [-inf, inf] > >I don't like it. Suppose you start ab with [-inf, inf] and after a while >alphabeta itsself has established a window of [x, y], halfway the search. >Shouldn't the same phenomenon be possible? > > > >Regards, >Bas Hamstra.
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