Author: Ed Schröder
Date: 08:40:40 09/10/99
Go up one level in this thread
On September 10, 1999 at 08:01:35, Alessandro Damiani wrote:
>On September 10, 1999 at 07:48:44, Ed Schröder wrote:
>
>>On September 10, 1999 at 00:19:37, Robert Hyatt wrote:
>>
>>>Here is an interesting position given to me by Steffen Jakob:
>>>
>>> /p/P5p/7p/7P/4kpK/// w
>>>
>>> +---+---+---+---+---+---+---+---+
>>> 8 | | | | | | | | |
>>> +---+---+---+---+---+---+---+---+
>>> 7 | *P| | | | | | | |
>>> +---+---+---+---+---+---+---+---+
>>> 6 | P | | | | | | *P| |
>>> +---+---+---+---+---+---+---+---+
>>> 5 | | | | | | | | *P|
>>> +---+---+---+---+---+---+---+---+
>>> 4 | | | | | | | | P |
>>> +---+---+---+---+---+---+---+---+
>>> 3 | | | | | *K| *P| K | |
>>> +---+---+---+---+---+---+---+---+
>>> 2 | | | | | | | | |
>>> +---+---+---+---+---+---+---+---+
>>> 1 | | | | | | | | |
>>> +---+---+---+---+---+---+---+---+
>>> a b c d e f g h
>>>
>>>
>>>Obviously black is getting crushed. He has one move, Kh3, which leads to a
>>>mate in 6. Steffen asked me to try this and Crafty found a mate in 4, which
>>>doesn't exist. I spent the entire day debugging this thing and here is what
>>>I found:
>>>
>>>If you recall the discussion here a couple of weeks ago, I reported that I store
>>>absolute mate scores (EXACT scores) in the hash table, and that I adjust them
>>>so that they are always stored as "mate in N from the current position". This
>>>has always worked flawlessly for me, and still does.
>>>
>>>For bounds, I once tried adjusting the bounds as well, but found quirks, and
>>>left them alone. Wrong answer. To fix this mate in 4 problem, I decided to
>>>adjust the bounds as well, but I now set any bound value that is larger than
>>>MATE-300, by reducing it to exactly MATE-300, but still using the "LOWER"
>>>flag to say that this is the lowest value this position could have. For bound
>>>values < -MATE+300, I set them to exactly -MATE+300 and leave the flag as is.
>>>
>>>This position is cute. Because not only is it a mate in 6, but there are
>>>transpositions that lead to mate in 7, mate in 8, and there are shorter (but
>>>non-forced) mates in 4 and 5. And there are stalemates, and positions with
>>>1 legal move, and so forth.
>>>
>>>You ought to find the following variation as one mate in 6:
>>>
>>>Kh3, f2, Kg2, Ke2, Kg3, f1=Q, Kh2, g5, hg, Kf3, g6, Qg2#
>>>
>>>If you find a shorter mate, it is wrong. If you find a longer mate, you
>>>are probably just extending like mad on checks (crafty finds a mate in 8 at
>>>shallow depths (9 plies, 2 secs on my PII/300 notebook), and doesn't find the
>>>mate in 6 until depth 10, 3 seconds.
>>>
>>>It is a good test as the transpositions are real cute with white's king caught
>>>in a tiny box, but with several different moves that triangulate and transpose
>>>into other variations...
>>>
>>>If you get it right, you have either handled the bounds right, or else you are
>>>very lucky. IE Crafty 16.17 gets this dead right. But if I disable the eval,
>>>it goes bananas, yet the eval is not important when mate is possible.
>>>
>>>Have fun...
>>>
>>>I did... :)
>>
>>A simple solution: do not store a position in the hash table if there is
>>no best-move. It solves the mate-cases and also repetition cases. Also
>>there is no speed loss of the search.
>>
>>Ed
>
>Do you mean by "no best-move"
> bestmove == 0
>or
> best<=alpha, after having searched all moves (best: minimax score)?
>
>What I do:
> if bestmove == 0 then don't store anything, just return the score (mate or
> stalemate).
>
>Alessandro
- For each new ply in the search: bound=-INF; bestmove=0;
- For each move on that ply: if (score > bound) { bound=score; bestmove=move; }
- For hashing: if (bestmove==0) do_not_update_hash_table
Ed
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