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Subject: Re: Can nullmoves behave like this?

Author: Severi Salminen

Date: 02:20:01 01/12/01

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>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.

It truly is logical but looks odd when you first see it: fail low, and then
score doesn't chance at all...so stupid.

>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.

I don't have hashtables yet, but turning nullmove off makes the problem
disappear. Do you think that this nullmove problem might occur even without
hashtables (for me it does occur - only in this position though)? The reason I'm
asking this many times is that I _really_ don't want to try to spend hours of
finding a bug which doesn't actually exist.

Thanks for all the help,
Severi



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