Author: Tord Romstad
Date: 07:19:07 02/11/06
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On February 11, 2006 at 09:04:39, Joachim Rang wrote: >the above mate is indeed incorrect. However as Maarten has pointed out the >initial position which you posted is indeed a mate in 14 (checked with a couple >of engines in backward analysis). In the second position after Nf6+ gxf6 it is a >mate in 6, the question is, why Glaurung prunes away the better answer Kf8? Now I have found the answer. As usual, the bug turned out to be very stupid and elementary. I had a "<" sign at a place in my code where it should have been a "<=", and this had the consequence that at all nodes where the tree was split between the two threads, only the first N-1 of the N legal moves would be searched. In the position after 1. Nf6+, there are three legal moves. Glaurung searched the moves 1... gxf6 and 1... Kh8 first, and skipped the third and last move 1... Kf8, which is the only move which avoids mate. After fixing this bug, the incorrect mate announcement disappears. Tord
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