Author: Uri Blass
Date: 02:41:20 12/14/05
Go up one level in this thread
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
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