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Up Topic The Rybka Lounge / Computer Chess / Why would Zappa Mexico II 64 bit Play Rc8 in this position??
- - By dragon49 (****) Date 2010-02-09 02:07
I'll attach the fen after the text.

Can somebody more knowledgeable about Chess Engines than I am explain how Zappa Gives up a pawn, then takes 3 moves until it understands it is lost??

I was playing a Long Game Between Rybka 3 64bit mp and Zappa Mexico.  1 Gig of Hash Each. 3-4-5 Men Tablebases.  Rybka has R +2 p vs R +p.  It looks like it will end up in a draw.  Zappa has plenty of time left, and in the position, goes to d24 and plays Rc8, giving up a pawn??  The Game continues RXh4 +2.86 Rc2+ +0.00 (How does Zappa think this is equal??)  Kg3 +2.86 Rc3+ +0.00 Kh2 +3.66 Kg6 +4.25 - Finally Zappa sees it is lost and eventually gets mated.

Why would Zappa Mexico II 64 bit Play Rc8 in this position??

7r/8/5k2/8/6Rp/7P/5KP1/8 b - - 0 113
Parent - - By Uly (Gold) Date 2010-02-09 02:32
I haven't looked at it but I deduce that Zappa thinks Rybka has to play some forced move that leads to a position Zappa has stored in its knowledge as a draw. Only later on it extends some of the other moves that it was reducing (or pruning) and notices they lead to a loss.

This is common to all engines and a common theme (engines that wrongly think a position is drawn and go for it) Zappa's search was just unlucky.
Parent - - By dragon49 (****) Date 2010-02-09 02:58
It is clearly a bug with hash table usage, as with a Fresh GUI Instance with Zappa's Hash cleared, it does not produce Rc8?? after going through d24 - It likes Kf5.  Could this be a weird situation where Zappa is harmed by too large of a hash table?  Are their endgame situations, like this one, where it is better to have a very small hash table, or no hash table at all?

In any event, this shows that engines have a lot of things to improve, with a top engine producing such a "howler."
Parent - By Mark (****) Date 2010-02-09 04:18
Clearly some kind of bug. With 6-man egtbs white mates in 40 after Rc8, but even without egtbs, all engines should see that Rc8 is bad...
Up Topic The Rybka Lounge / Computer Chess / Why would Zappa Mexico II 64 bit Play Rc8 in this position??

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