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Subject: Re: Nullmove question

Author: Tord Romstad

Date: 08:33:49 11/06/01

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On November 05, 2001 at 14:32:50, Robert Hyatt wrote:

>On November 05, 2001 at 07:00:48, Tord Romstad wrote:
>
>>On November 04, 2001 at 17:41:06, Michel Langeveld wrote:
>>
>>>Don't forget to look at Bruce Mooreland's excellent homepage about
>>>computerchess: http://www.seanet.com/~brucemo/toc.htm
>>
>>This looks like a very good site --- thanks to you for mentioning it, and
>>to Bruce for writing it!
>>
>>I was a bit surprised to see the suggested implementation of the null move
>>heuristic on Bruce's homepage, the alpha-beta window is different from what
>>I have always used.  This is the code fragment:
>>
>>  MakeNullMove();
>>  val = -AlphaBeta(depth - 1 - R, -beta, -beta + 1);
>>  UnmakeNullMove();
>>  if (val >= beta)
>>    return beta;
>>
>>In my code, I have something like
>>
>>  val = -AlphaBeta(depth - 1 - R, -beta, mate_value);
>
>
>
>No reason.  All you want to do, is with minimal effort, prove that if you
>do _nothing_ the score will return >= beta.  Whether it is > beta, or = beta
>means nothing here.  using mate-value means you will not be able to prune
>many lines in the null-move search that would be discarded by alpha/beta
>with X, X+1 as the bounds.

I don't follow you here, I'm afraid.  My code will never return a value
bigger than beta.  It returns a number between -mate_value and beta.  The
point is that if the value returned is close to -mate_value, I know that
the side which tried the null move is threatened with a mate in a few moves,
and I can extend the search.

>>which I always thought was the normal way of doing things.  The advantage
>>of my method is that I can often detect serious tactical threats, while
>>Bruce's approach has the obvious advantage that it searches fewer nodes.
>>Without extensive testing, it is not clear to me which technique works
>>better.
>>
>>What alpha-beta window do the rest of you use?  If anybody has tried both,
>>which approach works better?
>>
>>Tord
>
>I've always used X,X+1 for null-move.  I've tried others, but none were as
>efficient.  I don't see how it would matter if the score is == beta, or way
>above beta.

I agree, of course, but I really don't understand how you think this relates
to the discussion.

> All that says is that you are winning, or you are _really_
>winning, without having to play a move.  Now if the null-move search fails
>_low_ that tells you something.  IE the difference between alpha and alpha-100
>would be interesting.  Alpha means doing nothing is not good.  alpha-100
>failing low means doing nothing loses a pawn.  Which may mean the opponent
>has a serious threat that you have to actively hold off by playing a move.

How do you determine how serious the opponent's threat is if you search with
a window smaller than (-mate_value, beta)?

Tord



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