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Subject: Re: Double Null move?

Author: Robert Hyatt

Date: 19:40:39 07/13/01

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On July 13, 2001 at 16:50:26, Uri Blass wrote:

>On July 13, 2001 at 14:25:23, Robert Hyatt wrote:
>>On July 13, 2001 at 12:28:01, Steve Maughan wrote:
>>>I'm thinking of implementing double null move in my program.  Now as far as I
>>>know the most conventional way is to do the normal null move search and if there
>>>is a cutoff follow it with a normal search at reduced depth to confirm no
>>>zugwag.  However I do remember that someone here (Vincent?) outlined a different
>>>way of doing double null move.  Is there another way?  If there is, what are the
>>>pros and cons of each?
>>That's the gist of it.  If the position is a zugzwang position, the second
>>null-move search will fail high, which will cause the first to fail low and
>>you don't run into the zug problem.
>>The downside is the cost.  The second null will fail low most of the time and
>>just generate wasted nodes.
>>The other downside is that not all null-positions are zugzwang problems.  In
>>fact, most null-move problems are caused by the R-value which bring the horizon
>>too close to spot a tactical threat.  Double null won't find any of those...
>>So you expend quite a bit of effort, to eliminate one small part of the total
>If you search to clearly reduced depth(for example before normal search with
>null move pruning to depth d when d>=6 you search without null move to depth
>d/2-2) then you may be less than 1% slower.
>I believe that it is a good deal to be 1% slower in order to avoid not seeing
>simple zunzwangs.
>I guess that you may earn 3 elo from not falling in some zunzwangs and lose only
>1 elo from being slightly slower.

It is far worse than 1%, as you try a null move after _every_ null move, except
you never do 3 in a row.  If you are searching to 14 plies, and you try a null
move at ply=2, remaining depth would be 9 (12 - R=2 - 1).  If you do a null-move
search at ply=3, remaining depth would be 6 (9 - R=2 - 1).  A 6 ply search is

Do a bunch of them and it is _really_ non-trivial...

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