Author: Christophe Theron
Date: 09:40:37 01/17/04
Go up one level in this thread
On January 17, 2004 at 04:51:30, Russell Reagan wrote:
>On January 16, 2004 at 19:06:21, Harald Faber wrote:
>
>>
>>[D]2r1k1b1/pp4Q1/3p4/q5p1/4P3/1PN2P2/PP6/1K5R b - - 0 1
>>
>>Analysis by Shredder 8:
>>
>>1. +- (#153): 33...Lf7 34.Th8+ Ke7 35.Txc8 d5 36.Sxd5+ Kd6 37.Dxf7 De1+ 38.Tc1
>>Dc3 39.bxc3 b5 40.Dxa7 Kc6 41.Dc7#
>>2. +- (#151): 33...Tc7 34.Dxg8+ Kd7 35.Sd5 Tc2 36.Th7+ Kc6 37.Dc8+ Dc7 38.Txc7+
>>Kb5 39.Kxc2 a5 40.Dxb7#
>>3. +- (#7): 33...De5 34.Dxg8+ Kd7 35.Th7+ De7 36.Txe7+ Kxe7 37.Dxc8
>
>Oops...this is easily explainable. It took a minute for me to remember this. It
>is probably a fail high in a root aspiration search. You know how sometimes you
>will see a program search like this?
>
>1. +0.25 e4 e5 Nf3 Nf6
>2. +0.27 e4 e5 Nf3 Nf6 Bb5
>3. ----- d4! <--- This is the line of interest
>3. +0.45 d4 d5 c4 e6
>
>The line where there is no score and no PV usually means that a root aspiration
>search was done and the search failed high. Most programs don't give a PV, or
>don't give a score, or don't give either. Shredder probably just gives both.
>
>The reason is that the fail high in the aspiration search just means that this
>new move should be better, but you don't have an exact score yet, and you have
>to re-search with a full alpha-beta window to get the exact score. It is
>possible that #153 and #151 are the inaccurate scores returned by a fail high
>aspiration search at the root, and they are resolved to exact scores later.
I think you have found the correct explanation.
Chess Tiger can do that at times. It finds a forced mate with an inaccurate
score because the real score is outside the window. But that inaccurate score is
interpreted by the machine to human best line converter. A value close to +30000
or -30000 is converted to a mate score (distance to mate = +30000 - score for
example). This gives an incorrect distance to mate because the score is not
correct, it's just a bound (the program knows that the real score is above
that).
When the program resolves the fail-high, the correct score is found. In turn it
converts to a correct distance to mate.
I think this problem is even worse with programs using MTD(f).
Christophe
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