Author: Bas Hamstra
Date: 12:19:00 08/25/99
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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