Author: Álvaro Begué
Date: 08:52:03 12/13/05
Go up one level in this thread
On December 13, 2005 at 08:47:51, Chrilly Donninger wrote:
>Just for the curious, below is the code for the Rybka-Mate bug. I have seen
>another bug in the Beta-Code too. There are in every programm hundreds of bugs,
>so it would be more surprising to say: I have seen no other bug.
>
>.text:0040CD07 mov ebp, [esp+868h+var_854]
>.text:0040CD0B cmp ebp, 0FFFF810Ch
>.text:0040CD11 mov dword_667A14, edi
>.text:0040CD17 jnz loc_40CDCA
>.text:0040CD1D mov eax, [esp+868h+arg_C]
>.text:0040CD24 neg al
>.text:0040CD26 pop edi
>.text:0040CD27 pop ebx
>.text:0040CD28 pop esi
>.text:0040CD29 pop ebp
>.text:0040CD2A sbb eax, eax
>.text:0040CD2C and eax, 0FFFF8300h
>.text:0040CD31 add esp, 858h
>.text:0040CD37 retn
>
>In C this reads as:
>if(Bestscore == -32500( // No legal move found
> if(InCheck) {
> return -32000;
> }
> else { // Stalemate
> return 0;
> }
>}
>
>The bug is, that all mates are the same. In fact shorter mates are better for
>the mate-giving side (or worse for the mated side) than longer mates. One has to
>subtract the Plies/Distance from the starting position.
>The bug is in my opinion a consequence of a bad design decision. There are 2
>different "minus-infinite" values. In BestScore minus infinite is -32500, for
>mate its -32000. If one sets bestscore at the first place to -32000+Ply one gets
>cleaner and more efficient code and avoids the bug.
>I had the same bug in Nimzo 1.0. But I must admit, that Rybka 1.0 is stronger.
>
>Chrilly
I use the exact same solution that Chrilly proposes, combined with the "store
only bounds for mating scores in the hash table" idea that I picked from Bruce
Moreland's website. As far as I can tell, it works great.
Álvaro.
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