Author: Uri Blass
Date: 08:30:56 07/25/03
Go up one level in this thread
On July 25, 2003 at 11:15:25, José Carlos wrote: >On July 25, 2003 at 10:52:41, Uri Blass wrote: > >>On July 25, 2003 at 10:31:20, José Carlos wrote: >> >>>On July 25, 2003 at 08:10:47, Uri Blass wrote: >>> >>>>On July 25, 2003 at 04:34:35, Amir Ban wrote: >>>> >>>>>On July 25, 2003 at 02:41:22, Dann Corbit wrote: >>>>> >>>>>>Now, a qsearch ending in checkmate may or may not really be a checkmate. After >>>>>>all, we only tried certain moves and it could very well be that the checkmate >>>>>>could be avoided. >>>>>> >>>>>>So, the burning question is... >>>>>>What should we do when the qsearch ends in a mate? >>>>>>There are lots of alternatives, from the primitive "return a mate" to "send a >>>>>>danger signal up the tree and let the regular search deal with it" to >>>>>>"extending" to... >>>>>> >>>>>>What is your favorite choice and why? >>>>> >>>>>I don't see where opinion comes in. In a node where all legal moves are not >>>>>considered static eval is the minimum. >>>>> >>>>>Amir >>>> >>>>I think that it is not so simple. >>>> >>>>Suppose you find in the qsearch that all captures are losing because of >>>>checkmate. >>> >>> You miss the point. It's not that all captures lead to checkmate, it's that >>>you don't detect checkmates. >> >>There are programs that detect checkmate in the qsearch. >> >> Particularly, Amir was talking about a position >>>with no captures out of check. If you don't try all legal moves, you don't know >>>if you're checkmated. >> >> >>I assume in this discussion that the program knows that it is checkmated in a >>leaf position. >> >>Movei knows for a leaf position if it is a checkmate or not a checkmate. >> >> >> You can assume it if you want, but I don't think that the >>>probability of capturing the checking piece, or capturing something to go out of >>>check, is bigger than 0.50 for all in-check positions, thus you're gonna make >>>more than 50% mistakes. >>> >>> José C. >> >>I was not talking about a situation when the king is in check and I think that >>Dann also was not talking about it because he talked about checkmate. >> >>I will explain it by a diagram >>suppose the following position is a position when qsearch is called >> >>[D]r3qrk1/5p1p/7Q/5B2/8/4P3/R4PPP/6K1 b - - 0 1 >> >> >>You analyze Rxa2 Qxh7# >> >>What is the value that you return from qsearch. >> >>You can return the evaluation of the root and you can be more passimistic >>because you detect checkmate in the search. >> >>I think that Dann meant to this in the original post because he said in the >>original post >> >>"Now, a qsearch ending in checkmate may or may not really be a checkmate." >> >>He did not say >>"Now, a qsearch ending in check may or may not really be a checkmate." >> >>Uri > > I don't think Dann meant that, because the answer would be obvious. If you >_know_ that the position is checkmate, what could be the reason for not >returning checkmate?. If the reason is the qsearch is selective, then you >couldn't return checkmate anywhere in the tree (nobody uses pure minimax). No I can return checkmate if the first move of the qsearch is leading to checkmate but I cannot do it for the root position if only the second move that is a reply to capture does it. practically I do not generate captures if the score is above beta but simply return beta and if the score is behind alpha then I return alpha so changing it to something smaller is not going to change much and the only case when there may be a difference is when the static score is between alpha and beta. Uri
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