Author: Dave Gomboc
Date: 23:11:12 08/03/99
Go up one level in this thread
On July 31, 1999 at 12:55:54, Robert Hyatt wrote: >On July 31, 1999 at 12:34:34, Robert Hyatt wrote: > >>On July 31, 1999 at 03:08:42, Ed Schröder wrote: >> >>>>This is easy to test. >>>> >>>>My hypothesis: simple search is not good enough to discover that all moves >>>>but one lead to mate, in any positions except for those near the point where a >>>>game is already over (one side is mating the other). >>>> >>>>Ed's: A simple search is good enough to discern forcing moves. >>>> >>>>How about someone looking for positions where all moves but one lead to a >>>>forced mate... IE one move must _not_ get mated, while all the rest do. >>>>Then we decide whether the short search of Rebel can see this or not. >>>> >>>>Then we decide how often this kind of position occurs, and how often (when it >>>>does) is a shallow search enough to recognize the forced nature. >>>> >>>>I don't think (a) it will work very well; (b) that it is worth the effort to >>>>search with alpha=-inf, beta=+inf for every root move; (c) that by the time >>>>this might have a chance of identifying a forcing move, the game is already >>>>over and saving time is pointless... >>>> >>>>My opinion, of course... >>> >>>How about going one step further. Some years ago I did an experiment. >>>Search the first iteration without A/B, then: >>> >>>if (best_score - second_best_score > margin_one) limit time control. >>>if (best_score - second_best_score > margin_two) limit time control even more. >>> >>>etc. >>> >>>Also I tried this for the second iteration as well. Results were not bad at all >>>as it also catches forced moves that aren't recaptures and escapes from >>>checks. Moves sequences like 1..g5 2.Bg3 were also discovered and >>>2.Bg3 was played very fast. I also remember a case 1.a7 Ra8 preventing >>>the pawn to promote. Since 1..Ra8 was the only move 1..Ra8 was played >>>instantly. >>> >> >> >>that's an easy one to break. Take the position Cray Blitz vs Belle (I will >>try to find the FEN but it is in one of the test suites (Bxh6 is a draw, Qxb6 >>loses). >> >>I'll bet you that you discover that Qxb6 is +3 better than any other move with >>a 1 ply search. And a 2 ply search... and a 3, 4, 5 and 6 ply search... and >>beyond... until you finally see that it loses badly. >> >>Using your approach will get you killed there. Care to guess how I know? I >>was there. I used a scheme almost exactly like yours in 1980 or so, and it >>made that very same mistake in that very same game, and lost quickly... A 2 >>minute search would have shown Bxh6 drew and Qxb6 lost. But CB assumed that >>"Qxb6 was 'easy'" >> >> >> >> >>>Note that Q-search in Rebel's first and second iteration were limited to 6 and >>>8 plies to prevent the search to explode when A/B is not active. I also do >>>check extensions in Q-search to discover mates which catches the most >>>important ones but not all of course. >>> >>>Ed Schroder > > >Here is the position: > >5r1k/6p/1n2Q2p/4p//7P/PP4PK/R1B1q/ w > >the position in ascii: > > +---+---+---+---+---+---+---+---+ > 8 | | | | | | *R| | *K| > +---+---+---+---+---+---+---+---+ > 7 | | | | | | | *P| | > +---+---+---+---+---+---+---+---+ > 6 | | *N| | | Q | | | *P| > +---+---+---+---+---+---+---+---+ > 5 | | | | | *P| | | | > +---+---+---+---+---+---+---+---+ > 4 | | | | | | | | | > +---+---+---+---+---+---+---+---+ > 3 | | | | | | | | P | > +---+---+---+---+---+---+---+---+ > 2 | P | P | | | | | P | K | > +---+---+---+---+---+---+---+---+ > 1 | R | | B | | *Q| | | | > +---+---+---+---+---+---+---+---+ > a b c d e f g h > > >and my search results (PII/300 notebook): > >(early iterations like Qxb6 a lot, scores over +4.) > 8 8.27 0.26 1. Qxb6 Rf1 2. Qd8+ Kh7 3. Qd3+ e4 > 4. Qxf1 Qxf1 5. b4 e3 > 8-> 9.56 0.26 1. Qxb6 Rf1 2. Qd8+ Kh7 3. Qd3+ e4 > 4. Qxf1 Qxf1 5. b4 e3 > 9 10.54 0.57 1. Qxb6 Rf1 2. Qd8+ Kh7 3. Qd3+ e4 > 4. Qxf1 Qxf1 5. b4 e3 6. b5 > 9-> 12.16 0.57 1. Qxb6 Rf1 2. Qd8+ Kh7 3. Qd3+ e4 > 4. Qxf1 Qxf1 5. b4 e3 6. b5 > 10 13.17 -- 1. Qxb6 > 10 43.47 0.00 1. Qxb6 Rf1 2. Qd8+ Kh7 3. Qd3+ e4 > 4. Qxf1 Qxf1 5. Kg3 Qe1+ 6. Kh2 Qf1 > 10-> 51.88 0.00 1. Qxb6 Rf1 2. Qd8+ Kh7 3. Qd3+ e4 > 4. Qxf1 Qxf1 5. Kg3 Qe1+ 6. Kh2 Qf1 > 11 54.47 -- 1. Qxb6 > 11 1:47 -2.58 1. Qxb6 Rf1 2. Qd8+ Kh7 3. Qd3+ e4 > 4. Qxf1 Qxf1 5. a4 e3 6. Ra2 Qxc1 7. > b4 Kg6 > 11 2:12 0.00 1. Bxh6 Qxa1 2. Qxe5 Rf6 3. Bg5 Rf2 > 4. Qe8+ Kh7 5. Qh5+ Kg8 6. Qe8+ Kh7 > >As I mentioned, this is from Cray Blitz vs Belle, 1981 ACM NACC tournament. > >Bob Conveniently, this position can be found on page 314 of Chess Skill in Man and Machine, Second Edition, accompanied with annotation by Mike Valvo and David Slate. Cray Blitz vs Belle, Rd. 4, 1981 NACC November 10, 1981, Los Angeles, California, USA Dave
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