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Subject: Zero-width Window Null Move Search

Author: Dezhi Zhao

Date: 01:52:16 06/15/98


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?



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