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Subject: The Code for the Rybka-Mate-Bug

Author: Chrilly Donninger

Date: 05:47:51 12/13/05


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







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