Author: Robert Hyatt
Date: 09:34:34 07/31/99
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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
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