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Subject: Re: NULL MOVE

Author: Don Dailey

Date: 22:17:35 02/24/99

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On February 24, 1999 at 21:52:21, Will Singleton wrote:

>On February 24, 1999 at 15:53:49, Don Dailey wrote:
>>Your statement about not using two null moves in row shouldn't matter.
>>I had this rule in my program and Don Beal asked me, "why?"  I took
>>it out and the program worked fine.   I think a few years ago
>>my implementation of null move needed this rule to prevent infinite
>>recursion, but I cannot remember for the life of me why this was so.
>>The worst that can happen is that you do more  depth reduced
>>searches which is such a tiny fraction of the whole you will not be
>>able to measure the difference in time.   But even this won't happen
>>if you do not do a null move search when the "stand pat" score is
>>already below beta.  Some programs do the null move selectivity anyway,
>>or they do it if the score is CLOSE to beta.  However I decided to
>>ignore any minor speedups this gave because it also introduces
>>some risk.  I really doubt you can prove one is better than the
>>other and my current program doesn't even register a speedup for
>>- Don
>Two nulls in a row??  I don't understand that.  How could either side move at
>all if you have consecutive null moves?  Or maybe you mean multiple nulls along
>the same branch?

I mean consecutive null moves.  Player A makes a null move and the first
thing Player B tries is also a null move.  If player A thinks he is winning,
then player B's null move search will fail and he will be forced to resort
to the full width search. This full width search is what will be returned
to player A, NOT the result of a series of consecutive null moves.

Remember, a null move result will only be returned for one side and
only if that side is happy (happy means >=beta)  It is true that if you
are doing a very deep search you will get some, even several consecutive
null moves, but at least one side (and possibly both) will reject this.

Perhaps another way to think about it is this:  Pretend that your
move generator returns all the legal moves plus a NULL or PASS move.
Your search may generate lots of lines where players make several
consecutive null moves.  But unless these actually produce a beta
cutoff (or zugzwang) they will get ignored.

This clearly will not break the chess program and in fact you have
just implemented a null move search, although a bad one since there
is no depth reduction.   So just add depth reduction and you have
a real null move implementation!

- Don

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