Author: James Robertson
Date: 07:58:25 11/22/99
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On November 22, 1999 at 06:43:08, Ed Schröder wrote: >>Posted by Peter McKenzie on November 22, 1999 at 03:11:18: > >Hi Peter, > >>Greetings, >>Here are a couple of interesting positions to try on your favourite programs. >> >>Positon 1 >>The first position comes from the game DarkThought-LambChop at the 1999 >WCCC in >>Paderborn. Lambchop had an advantage but went astray by allowing a perpetual >>check. Here is the position: >> >>5k1r/1R2bp1p/2p1pp2/7B/8/8/P1q3PP/6QK w - - >> >>DarkThought, which must have seen the draw several ply earlier, played Rxe7 >>which forces the draw (after Kxe7 Qa7+ etc). I tried this position on Chop, >>which needed a whole 11ply to see that Rxe7 draws. I mentioned this >>position in channel 64 on ICC, and Bob said that Crafty only needs 6ply. > >Same here: > >00:00:01 6.00 -0.17 1.Qe3 Rg8 2.Rb8+ Kg7 3.Qg3+ Kh6 > 4.Qe3+ Rg5 5.Bg4 Qxa2 6.Qh3+ Kg7 (1) > >00:00:02 6.23 -0.17 1.Rxe7 >00:00:02 6.23 0.00 1.Rxe7 Kxe7 2.Qa7+ Kd8 3.Qb8+ Kd7 > 4.Qa7+ Kd6 5.Qd4+ Kc7 (2) > >00:00:02 7.00 0.00 1.Rxe7 Kxe7 2.Qa7+ Kd8 3.Qb8+ Kd7 > 4.Qa7+ Kd6 5.Qd4+ Kc7 (2) > >>I asked Bob if he was doing any fancy stuff in search to see perpetuals >>faster, but he said he wasn't. Oh well, maybe I have a bug I thought. But looking at >>the position, its obvious the drawing line is longer than 6ply. The black >>king can go to d6,c7,c8,d7,d8: quite a number of squares to walk around. > >Solved in Q-search. > >>Then it struck me why Crafty saw it so fast. Anytime the black king got to >>the back rant, Crafty would see Qa8+ followed by Qxh8 winning a rook for white. >>Then it would hit the qsearch (or nullmove and hit the qsearch) where black >>has no good capture. The difference being that LambChop (which looks at >>checks in the qsearch) would hit the qsearch, and see Qb1+ forces checkmate. > >Just extend Q-search one ply more in such cases. > >>So here is a position where Crafty very quickly gets the right move and >>evaluation, but for the 'wrong reasons'. Bob informs me that this is >>called the 'inverse horizon effect' :-) >> >>So how do other programs do on this position? > > >>Position 2 >>This one happened in a blitz game Ferret-LambChop on ICC: >> >>2r1r2k/1p1q1ppp/pn1p1b2/nNp2b2/2PP4/PP2BN2/Q3BPPP/2R1R1K1 w - - 0 18 >> >>Ferret played the crushing Nxd6 which works because white has not one but two >>pawn forks coming up. The main line being: >>Nxd6 Qxd6 dxc5 Rxc5 Bxc5 Qxc5 b4, a 7ply line. But of course, the normal >>horizon effect kicks in so LambChop will then think Rxe2 Qxe2 helps before >>'realising' the pawn fork is still there. > >Rebel Century: > >00:00:00 4.00 0.63 1.dxc5 axb5 2.cxb6 bxc4 3.bxc4 >00:00:00 4.09 0.94 1.Nxd6 Qxd6 2.dxc5 Rxc5 3.Bxc5 Qxc5 > 4.b4 Rxe2 5.Rxe2 (0) > >00:00:00 5.00 1.32 1.Nxd6 Nxb3 2.Qxb3 Qxd6 3.dxc5 Rxc5 > 4.Bxc5 Qxc5 (0) > >Some hints: extend (almost) every recapture. On the horizon: extend one >ply if the side to move has an interesting double attack such as a fork. > >Ed How do you detect such double attacks without killing speed? James > > >>So Lambchop needs 9ply, and around 2min and 1.2M nodes on my P133 to see >this little combination. A bit slow for my liking! How does your favourite >>program do?? > > >>In days gone by, I might have seen it faster due to the recapture extension >>but >>these days I try to limit that extension much more so it doesn't help here. >>Other programs might do some static analysis of the pawn fork to see this one >>faster. I guess you could include that sort of thing in your evaluation, or >>perhaps base some sort of extension on it. I think GNUChess had some hung >>piece >>code to deal with this sort of thing. I'd be interested in hearing any >>thoughts >>from fellow programmers on this issue. >> >>cheers, >>Peter
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