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Subject: Re: Zero-width Window Null Move Search

Author: Vincent Diepeveen

Date: 10:06:18 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?

It all depends on how much do you extend when you have a nullmove
search, and do you do checks in q-search?

If you don't extend too much and don't do checks in q-search, then
it will not make much different i guess.

I use a higher reduction factor to limit the number of nodes needed for
q-search. Diep extends so many stupid lines that R=2 versus R=3 saves
me a ply sometimes, but average many tens of %

Now that i use R=3 instead of R=2 i still  see the difference
between zero window of nullmove anymore, but less than with R=2.

The difference however is still there in favor for the zero window nullmove,
because you only need that >= beta bound.

Greetings,
Vincent



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