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Subject: Re: When to do a null move search - an experiment

Author: Tord Romstad

Date: 08:32:26 04/26/04

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On April 26, 2004 at 10:39:42, José Carlos wrote:

>  An interesting experiment, of course. But I think your conditions are rather
>different from 'most' programs. I mean:
>  - You allow any number of null moves in a row (most programs don't do even
>two)

This has no importance, I think.  My experience is that I almost always get the
same score and PV when I enable/disable several null moves in a row, and that
the difference in number of moves searched is *very* tiny.

>  - You do pruning and reductions that are not public domain
>  This is important because your results mean: 'in a non-standard null move
>implementation (where you try more null move searches than most others) and with
>a lot of heavy pruning and reductions (I assume they're heavy according to your
>posts here and the high depths Gothmog gets) and in a MTD(f) root search,
>limiting null move application seems to benefit". This conclusion is, apart from
>interesting of course, very intuitive.

This is a very good point.  During our discussion last week, several people
claimed that checking the static eval before doing a null move search wasn't
really necessary, because the cost of a null move search would be tiny
compared to the cost of searching the moves anyway.  This isn't really
true for Gothmog, because most of the moves are usually not searched to
full depth.

I am not sure it is fair to describe my pruning and reductions as "not
public domain", though.  It is true that I haven't published any paper
and that my engine is not open source (at least not yet), but I am happy
to describe the tricks I use to anybody who wants to listen.  :-)

Whether the use of MTD(f) is important is hard to say, but my guess is
that it isn't.

>  Thanks for the data. You sure inspired many others to do similar tests (I'll
>do when I have time).

Yes, it would be interesting to see the results of similar experiments for
other engines.  It seems like a sure bet that this is a YMMV issue.  :-)

Tord




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