Author: Vasik Rajlich
Date: 00:23:46 12/14/05
Go up one level in this thread
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
This page took 0 seconds to execute
Last modified: Thu, 15 Apr 21 08:11:13 -0700
Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.