Author: Robert Hyatt
Date: 06:46:55 01/26/04
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On January 26, 2004 at 06:13:10, Tord Romstad wrote: >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? wrong ply. :) I want to fail high after playing a null-move. Normally the null-move will fail low at the next ply in most all cases, which means that at the _next_ ply all moves are searched, and mate in 1 will get backed up. Yes, an occasional null-move search at the next ply fails high, which means the normal null-move search value at the current ply does not produce a fail high and we keep on searching here normally, not using the null-move search. And not seeing the mate extension... > >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. That's my perspective on null-move mate threat detection. It will miss one but not that often... > >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. > You should read Donninger's null-move paper in the JICCA. He talks about a null-move threat detection algorithm just as you explain. I've used it in the past but I haven't in several years. >Tord
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