Author: Robert Hyatt
Date: 13:13:52 04/26/04
Go up one level in this thread
On April 26, 2004 at 15:56:13, Sune Fischer wrote: >On April 26, 2004 at 15:02:23, Robert Hyatt wrote: > >>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? > >Yes. > >>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. > >Yes > >>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. > >Exactly, you want the nullmove to FL when you are in zugzwang, so >I don't see how this is an argument in _favor_ of (5)? Maybe we are semantically dancing around the head of the pin? IE I don't do 2 nulls in a row as in most middlegame positions zugzwang is not an issue and the second (of two consecutive nulls) is wasted effort. I have plans to experiment with turning this double-null on in endgames however, as there zugzwang is _very_ common and the detection would seem to be a more-than-break-even deal. IE I am not saying double null is always bad. Just that it is bad in the middlegame and as a result I don't do double-null anywhere at the moment... > >>>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. > >In this case his nullmove will FL so you waste a cheap nullmove to depth X-2R. >On the other hand, suppose his nullmove fails high, then you have saved a full >search to depth X-R! Yes, but in the middlegame you almost _always_ waste that X-2R search. In today's 14-16 ply (plus extensions to make it even worse) that is a non-trivial wasted effort. In endgames it will likely pay off... > >It seems logical that if a nullmove is worth while (statisticly) at depth X, it >should also be worth while (statisticly) at depth X-R. No, for one reason. If the depth=X search is done, in a reasonable place, the X-R search will fail low and be wasted effort. If you can't guarantee that the X-R search is going to pay off fairly frequently, then doing it just burns search space. In the endgame this is different since zugzwang is a big part of such positions... > >Assuming of course we have no game specific knowledge of en prised or pinnes >pieces or anything like that. > >It's like you're hoping so much that your first nullmove will FH that you don't >even want to try a quick check to see if it might actually FL :) All I really care about is fail highs on zugzwang positions. Fail lows are not an issue... > >>>So I get the same results as Tord here, it's weaker (slightly, but measurably) >>>with this restriction on. >>> >> >>As I said, YMMV... > >Oh of course, as always! :) > >-S.
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