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Subject: Re: Can nullmoves behave like this?

Author: Bas Hamstra

Date: 20:42:28 01/11/01

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On January 11, 2001 at 06:16:47, Severi Salminen wrote:

>On January 10, 2001 at 20:45:40, Bas Hamstra wrote:
>
>>The fail high and then low problem is normal if you use nullmove and aspiration
>>window. Vincent (and me too BTW) doesn't use aspiration window and therefore
>>doesn't see it. Nullmove causes search inconsistencies.
>>
>>Bas.
>
>Thank you very much! If this is the same in reversed order (first the search
>fails low and then after infinite window search the true score is within the
>original window) I can sleep well again :) And focus on improving other areas.
>
>Severi

It used to bother me a lot too. Then Bruce Moreland explained that search
inconsistencies (which this is about) are unavoidable if you use hashtables and
pruning based on alpha/beta values. To quote him: "If you use a hashtable, you
have dirt in you hair. If you do nullmove, you are buried in mud". Pretty clear
:-) If you think about it, it's logical. Nullmove makes errors. With different
windows different branches will be pruned resulting in different errors. Enough
to affect the rootscore. I saw it all the time. Bruce too. When I hacked the
ExChess code to see if it had the same behaviour a year ago the answer was: yes.

If you turn nullmove and hash off the problem should not occur at all, or else
you have a bug. I don't use aspiration anymore, as soon as I can measure it
matters a lot I will put it back in. At this point I am not convinced, though
most do use it.

Ciao!
Bas.




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