Author: Vasik Rajlich
Date: 04:58:17 12/14/05
Go up one level in this thread
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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