Computer Chess Club Archives




Subject: Re: When to do a null move search - an experiment

Author: Robert Hyatt

Date: 12:02:23 04/26/04

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On April 26, 2004 at 14:25:01, Sune Fischer wrote:

>>(1) you are in some sort of zugzwang position where a null-move will fail high
>>for the wrong reason and wreck the search.  Classic examples here are positions
>>with very few pieces.  IE pawns vs a knight where the knight can be zugged.
>>Most require some minimal amount of material on the board to avoid this problem.
>>(2) there is a tactical issue that is hidden with the R reduction.  IE the
>>classic position with white pawns at f2, g3 and h2, black queen at h3 and black
>>pawn or bishop at f3, threatening mate on the move.  If the R reduction prevents
>>you from seeing the mate, you can have problems.
>>(3) The hash table proves that the null-move search will not fail high, meaning
>>that the search will be wasted effort.
>>(4) Obvious positions such as when the side on move is in check.  Not moving
>>can't fail high here as the king is lost.
>>(5) I don't allow two consecutive nulls.  It is a potentially cute way of
>>eliminating zugzwang problems, but it is only good for that, and it is not free
>>in positions where no zugzwang is possible.  I choose to not deal with it
>>although I have this on my "to do" list to test with (say) pawn-only endings.
>The first 4 I agree with, unfortunately 1 and 2 are not so easy to detect. :)
>I don't get the point of (5) though, how does it avoid zugzwangs?

Think about it this way.

You do a null move search.  If you are in zugzwang, it will fail high for the
wrong reason, since doing nothing in a zugzwang position is a good thing.  Agree
so far?

What we are hoping to show is that our position is so good, even if we do
nothing our opponent is busted.  IE We are a queen up, and the most valuable
piece our opponent attacks is a knight.  If we don't try to save the knight we
are _still_ winning.

Now, take the bad null-move case where we are in zugzwang.  The null search
fails high for the wrong reason.  But if, at the next ply, you try a null it
will _also_ fail high, causing that side to return beta, which makes _our_ null
search fail low and not kill us in the zugzwang position.

>I don't see any logical reason to do (5), because after you have nullmoved you
>want to see if the other side can FH so we may FL on the nullmove. The fastest
>way to do that is to do another nullmove.

In normal positions, if I am a queen up, and I try a null, if you also try a
null you are _still_ a queen down, the null-search fails low, and you continue
to search normally making that second consecutive null-search wasted effort.

>So I get the same results as Tord here, it's weaker (slightly, but measurably)
>with this restriction on.

As I said, YMMV...


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