Author: Uri Blass
Date: 03:09:53 01/20/01
Go up one level in this thread
On January 20, 2001 at 05:29:23, Ricardo Gibert wrote: >On January 20, 2001 at 00:17:26, Robert Hyatt wrote: > >>On January 19, 2001 at 21:51:09, Uri Blass wrote: >> >>>On January 19, 2001 at 21:33:08, John Merlino wrote: >>> >>>>On January 19, 2001 at 17:23:03, Scott Gasch wrote: >>>> >>>>>Hi all, >>>>> >>>>>Here's a pretty simple test position. This comes from the WAC suite... my >>>>>engine does fairly well on this suite as a whole but seems blind to this >>>>>solution. >>>>> >>>>>[D]8/7p/5k2/5p2/p1p2P2/Pr1pPK2/1P1R3P/8 b - - >>>>> >>>>>The solution is Rxb2 -- black's connected passers are unstoppable after the >>>>>recapture. >>>>> >>>>>My program refuses to find this solution... even at 9 ply it misses it. The >>>>>strange thing is that from the other side after Rxb2 it sees that white is toast >>>>>very quickly... score dropping to -500 or so after about 1 second. >>>>> >>>>>My question is, of course, how this move is missed. I've tried kicking up the >>>>>value of connected passers and passed pawns in general. I've tried adding a >>>>>special rule to eval about connected passers on the 7th, on move, with control >>>>>of a queening square. I've tried cutting back my futility margin in qsearch and >>>>>always extending a full ply for checks (It usually extends only 3/4 ply for >>>>>checks after the iteration depth). And still it does not find Rxb2. >>>>> >>>>>Even stranger is if I run a static eval with the two connected passers rolling >>>>>towards the queening square after the rook exchange the eval puts black ahead! >>>>>I can't seem to figure this out... either my pruning is too aggressive or there >>>>>is some other bug in the engine...? >>>>> >>>>>I hope someone out there can give me a little advice. Thanks! >>>>> >>>>>Scott >>>> >>>>Just to make things difficult, Chessmaster 8000 seems to have this one's number, >>>>AND it reports the correct move at ply 9, despite other postings here that have >>>>stated that it should take somewhere between 12 and 14 plies to find the result. >>> >>>You are right and it is also clear from the main line that it can see the queen >>>on the board and does not solve it for wrong positional reasons that vincent >>>suggests. >>> >>>Uri >> >> >>I would disagree with Vincent's reasoning. Crafty solves it for the _right_ >>positional reasons before it sees the tactical win. It notes that the king >>can't stop the pawns, and it _knows_ that a rook can't stop them. Hence this >>is the _right_ reason here... even without seeing deep enough to see the >>actual promotion. It only takes crafty a few seconds to see the actual >>promotion so it is really moot... but to say that at 8 plies it solves it for >>the wrong reasons is a stretch at least. ie if you have a rook, I have two >>connected passers on the 7th, and your king is too far away to help, I am going > >But Whites King is not too far away! It is obstructed by the e-pawn. Without the >e-pawn, the King is just in time. White loses due to the connected passed pawns >*plus* obstruction of its King. Unless your eval handles connected passed *and* >obstruction correctly, Crafty must solve it by searching a bit deeper. If it is >not doing that, it is guessing. I did not check Crafty18 but the previous Crafties versions are only guessing. Here is a simple example to prove this point: [D]8/1R2k2p/8/5p2/p4P2/P1pp1K2/7P/8 b - - 0 1 Crafty17.14 evaluates this position as an advantage for black at very small depthes(1.71 at depth 4) inspite of the fact that it can see the line Kd6 Ke3 d2 Ke2 when the white king stops both pawns. The rule should usually be that if the king is in the square of both pawns promotion square then the king stop the pawns. If it is white to move and white is not in the square of both pawns the question should be if the king has a legal move to get there. In the original position it has not a legal move to get there and the reason is the white pawn at e3. Uri
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