Author: Eugenio Castillo
Date: 01:35:13 11/22/99
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On November 22, 1999 at 03:11:18, Peter McKenzie wrote: >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. > >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. > >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. > >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. > >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. Eugen 7.7 needs 9ply for first position (about 6 million nodes; draw by checks are not nice improved) and 4ply for second position (76000 nodes). Cheers, Eugenio.
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