Author: Robert Hyatt
Date: 13:46:04 08/25/99
Go up one level in this thread
On August 25, 1999 at 15:27:43, Bas Hamstra wrote: >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. the trick with eval > beta is not alpha/beta. That is a programmer's trick. However, if you are at a leaf/tip position, that is perfectly correct... because with traditional alpha/beta scores can't get > beta or less than alpha. If you turn off null move, and turn off hashing, you won't see _any_ of this nonsense. >> >>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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