Author: Vasik Rajlich
Date: 16:14:56 12/13/05
Go up one level in this thread
On December 13, 2005 at 16:29:53, Robert Hyatt wrote:
>On December 13, 2005 at 11:52:03, Álvaro Begué wrote:
>
>>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.
>
>The question is "why store only bounds"? Makes no sense to me. I've always
>stored exact (corrected for distance to root) for mate scores. Allows me to
>store all the EGTB mates as well.
>
>Never understood the reasoning for not storing exact mate scores. Never had a
>single problem doing so in either of my programs, spanning _many_ years...
The problem is when the hash entry ages.
Of course there are a number of workarounds ..
Vas
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