Computer Chess Club Archives




Subject: Re: Double Nullmove

Author: James Swafford

Date: 09:27:15 03/30/01

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On March 30, 2001 at 12:04:36, Andrew Dados wrote:

I dug this up from my notes (Nov 99):

Almost forgot - Vincent Diepeveen (Author of Diep) posted an interesting idea he
calls "double null move." The idea is to avoid zug problems by not allowing two
null searches to occur within three ply of each other. I implemented the idea,
and decided that it needed some refinement. I lost a full ply of search on the
first 100 problems of the Win At Chess suite. Not that I don't think double null
move helps avoid zug problems, but I think it costs too much. To avoid the
drawback of less search depth, I only activate the double null move when in the
endgame, which is determined at in the iterator. Overall, a very good idea by


>How expensive can double nullmove be?
>For bf of 3 you get about 1/3^3=1/27 more nodes.
>For endgames and bf of 2 you'll see estimated 1/8 more nodes.
>Actually my program uses double null in endgames and it seems 10% more nodes is
>maximal overhead I saw (compared to 'normal nullmove').
>However in those low material situations when other programs disable nullmove
>totally (except for pawn endings) double nullmove is an obvious winner.
>>On one hand, you catch zugzwang positions.  On the other hand, you make the
>>tree larger.  Which is better?  It is just another compromise decision where
>>you win some and lose some because of it.

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