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

Author: Fabien Letouzey

Date: 03:43:34 04/26/04

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On April 26, 2004 at 06:31:58, Tord Romstad wrote:

>In my tournament, I all four versions face each other 50 times.  The
>results:

>           Gothmog A   Gothmog B   Gothmog C   Gothmog D       Sum
>Gothmog A    XXXX        26.5        31.0        29.5         87.0/150
>Gothmog B    23.5        XXXX        30.5        26.0         80.0/150
>Gothmog C    19.0        19.5        XXXX        31.5         70.0/150
>Gothmog D    20.5        24.0        18.5        XXXX         63.0/150

>It is interesting that the version with the most restrictive criterion
>achieves the highest score.  This confirms my growing suspicion that
>recursive null move pruning doesn't work nearly as well as most people
>believe, and that it might be a good idea to make an effort to reduce
>its use as much as possible.  I will try to experiment with even more
>restrictive criterions (perhaps static_eval-biggest_hanging >= beta+margin)
>and see whether that improves the strength further.

>The number of games is of course still not sufficiently big to make any
>definitive conclusions.  If I find the time, I will add more games and
>more versions to the tournament.

>Tord

Could you add one with the even more restrictive condition that I use: eval() >=
beta (aka fail-high nodes)?  I expect it not to do as well, but a comparison is
always interesting.

One of the justifications for this choice is as follows.  I consider null move
as a "sit down" (my position is good enough, I challenge you), and the search
stops if both players pass (eval() is then returned).  The fact that I play Go
is not unrelated :)  Note that this has nothing to do with double null move, in
which a search is done after two null moves.

So the philosophy is different from normal null move (evaluate threats) and a
small optimisation leads to the fail-high precondition.

Fabien.




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