Author: Robert Hyatt
Date: 09:55:54 07/31/99
Go up one level in this thread
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
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