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Subject: Re: Zero-width Window Null Move Search

Author: Robert Hyatt

Date: 06:41:02 06/15/98

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On June 15, 1998 at 04:52:16, Dezhi Zhao wrote:

>In some earlier posts by Don Dailey and others, they mentioned
>about zero-width window null move search and said it's an efficent
>way to implement null move search. So I compared the zero-width null
>move to the original full-width with my Xiangqi (Chinese chess) PVS
>engine.
>Here is the results of playing 20 moves (same path for both method,
>without opening book) from the initial position.
>
>The m/c usually searches 2 to 4M nodes for each posoition.
>If the transposition table is cleared between searches, the savings
>of zero-width are generally around serveral hundreds of nodes,
>and the max is 9K for one position.
>If the transposition table is partially cleared (keep only last
>iteration
>entries) between searches, the savings become hard to interpret. You
>save several hundreds of nodes in a postion, lose that in the
>following position, and ocassionally lose much more than than previous
>saving (save 8.4K and lose 140K in the next for example).
>
>I think that the savings are negligible, which are caused by the fact
>that in PVS most of the nodes are of zero-width window already.
>
>These results also remind me of the word "vapor-ware" that Dr. Hyatt
>called NegaScout over PVS.  So I checked Crafty 14.13 again,  and found
>that Crafty uses full-width window null move search. Why? My best guess
>is that Dr. Hyatt has done extensive tests over null move search
>abnormaly
>as he mentioned several times in CCC, and found null move search window
>is
>related to the abnormaly. Am I right?


no...  I do it that way simply because 99.999% of the nodes searched are
already using alpha,alpha+1...  There is nothing wrong with doing every
null-move search using beta-1,beta, for example, and it probably makes
sense to do it that way, I just did not because of PVS already taking
care of that for the most part.

I will run some tests to see if it makes any difference at all, but
suspect that it will have virtually no effect, because it will only
affect a very few searches, total...



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