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Subject: Re: Interesting mate test for hashing

Author: Ed Schröder

Date: 04:48:44 09/10/99

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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



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