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

Author: Vasik Rajlich

Date: 04:58:17 12/14/05

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On December 14, 2005 at 05:41:20, Uri Blass wrote:

>On December 14, 2005 at 03:23:46, Vasik Rajlich 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
>>
>>Thanks for the comments. I finally thought about this issue in more detail and
>>came up with an even better scheme - at least it's better as far as I can see.
>>
>>Rather than:
>>
>>mate score = BIGINT - plies from root
>>
>>it should be
>>
>>mate score = BIGINT - 50 move counter value
>>
>>This gets around the issue of aging mate-value hash entries and allows you to
>>put your search return value directly into the hash table.
>>
>>An engine might still prefer capturing something to mating, but whoever isn't
>>resigning under that circumstance deserves all the punishment he gets there :)
>>
>>Vas
>
>You can also simply have mate score=BIGINT-plies from the beginning of the game.
>
>Uri

Yes indeed this seems the simplest. Also makes mate reporting trivial.

Vas



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