Author: Tord Romstad
Date: 03:13:10 01/26/04
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On January 25, 2004 at 17:55:13, Robert Hyatt wrote: >On January 25, 2004 at 13:58:24, Uri Blass wrote: > >>I see in Crafty's code that Crafty extends mate threats but how can it know >>usually about mate threats when it is using fail hard and not fail soft? >> >>I do not see in crafty's code a special search after null move(even to reduced >>depth) in order to find if there is a mate threat. > >This is done _in_ the null-move search. It simply says "If I do nothing (play a >null-move) and my opponent mates me instantly, then this position has a threat >and should be searched deeper... > >I detect the threat when doing the null-move score and getting a mate score >backed >up. Crafty is not really "fail hard". It just is not fail-soft... I think you didn't quite understand Uri's question. I also don't understand how you can use the null-move search to detect mate threats. Let me try to explain why: After making the null move and calling search(), I don't necessarily search all moves. I stop searching as soon as I find a move which fails high (just like in all other nodes of the tree, of course). It is obviously possible that there exists a mate-in-one in the position, but this mate might not be found because the mating move is not searched first and one of the earlier moves fails high. What am I missing? Instead of using the null-move search to find mate threats, I try to detect them in the evaluation function. This is very cheap and works reasonably well, but it's not perfect. It happens that a mate threat is not found, and it happens (not often) that the evaluation function detects a mate threat which isn't there. While writing this post, I got an idea which perhaps could work. Before the ordinary null-move search, do a null-move search with the depth reduced to zero (or one, if you don't search checks in the qsearch), and beta lowered to -MATE_VALUE+Ply+2. This search should terminate very quickly, and should also reliably detect all threats of mate in one. If a mate is found, you extend and disable the ordinary null move search. Tord
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