Author: Robert Hyatt
Date: 12:02:23 04/26/04
Go up one level in this thread
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... >-S.
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