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Subject: Re: About False Fail Highs, professionals, and MTD searches

Author: Vincent Diepeveen

Date: 11:36:11 04/13/02

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On April 12, 2002 at 16:17:22, Dieter Buerssner wrote:

>On April 12, 2002 at 15:08:51, Gian-Carlo Pascutto wrote:
>>Do they have a trick which they use to determine whether a
>>fail high is real or not? I don't see how this could be
>One explaination could be, that they just totally trust the hash table. If you
>fail high at root, one ply deeper in the line, you will have an upperbound. If
>you research, adjust beta with this upperbound (trust, that it is a real correct
>upperbound), and just don't allow any better scores, so at the root, you will
>never get back a score worse than before in the research.
>BTW. I think, I have seen fail highs in Shredder 5, that seemed wrong (and
>switched away again in the next depth). But, as you note, I also allways have
>seen it keeping at the same ply.
>I believe, this "total trusting in previous scores with the same search depth"
>makes the branching factor smaller, but for me, it too often needed more depth
>to find the same correct solution. This has IMO to do, with pruning decisions,

I don't doubt that for you it is impossible to make the same assumption,
obviously because you use lazy evaluation and i'm not sure, perhaps you
can lighten up our brain again whether you also use futility in qsearch.

>that are wrong (for example null move eats too many plies, to see the threat).
>Researching with a different window, and null move may not fail high again in
>the same position. And a search with a real move would show, that the lower
>bound from the null move in the previous search was just wrong. But when you
>trust the lower bound from the previous null move fail high and adjust alpha
>accordingly ...

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