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Subject: Re: The Code for the Rybka-Mate-Bug

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.

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.


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