Computer Chess Club Archives




Subject: Re: One mate to solve...

Author: Heiner Marxen

Date: 16:22:43 04/22/01

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On April 22, 2001 at 18:30:53, leonid wrote:

>What surpised me very much (just few days ago) is to find that in my mate solver
>selective demonstrate much higher NPS that brute force search. It is true that
>it is not that relevant for the quality of the search but is very curious. I
>never expected this way! In other part of my program opposit is true. Selective
>search have always lower NPS that brute force search. Specific to my program
>logic or normal for every program? I really don't know.


I have never measured it, but yes, that is a surprise at first.
The logical rasoning is like this:

The two versions of the program (selective and brute force) do nearly the same:
spanning/searching a tree by move generator and move execution, and evaluating
the leaves for mate.  The selective version also does some selection decision.

Therefore the selective version would have a somewhat smaller NPS, if the
execution of common components (generator, execution and leave evaluation)
would have the same average execution time for both versions.
Since the NPS is greater, it follows that the common components execute
faster on average in the selective tree.

How is that possible?

Move execution should have nearly constant execution time.
But move generation and leave evaluation (also tests for move legality,
which are contained somewhere in there) are faster for less crowded boards.

Bingo!  Those selected moves will be check moves and captures more frequently
than in the brute force version.  (NPS should also be higher for endgames)

Surprise solved.  Those search trees are sometimes hard to understand ;-)


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