Author: Tord Romstad
Date: 13:54:31 01/17/04
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On January 17, 2004 at 16:28:15, Klaus Friedel wrote: >Now the question: >What are characteristic attributes of posiitions in which the null search is >more likely to fail with error 2 (besides zugzwang positions). Dieter's example is a good one. In my experience, there are three main problems with null move pruning. The first one is, of course, zugzwang positions. The second is the horizon effect, as illustrated by Dieter's example. The third is that it tends to prune away maneuvers where a piece is temporarily moved to a bad square on its way to a very good square. In order to solve the first two problems, I use the evaluation function to decide whether or not to use null move pruning, and also to choose the null move reduction factor. When the number of safe legal moves (that is, moves which do not lose material) is low, or there is very little material left, the risk of zugzwang is big, and it is best to avoid the null move. When the king safety for the side to move is bad, when the opponent has a one or more dangerous-looking advanced pawns, or if the side to move has a pinned or trapped piece, I reduce the reduction factor by 1. I don't yet do anything to cope with the third problem. Sergei Markoff (SmarThink) makes use of one of Botvinnik's ideas, called "trajectories". I am not sure precisely what this means, but as far as I have understood he does some kind of horizontal search within the evaluation function to identify paths a piece can use to safely reach some advantageous square, and avoids null move pruning at nodes directly after a move which is part of such a path. The idea certainly seems interesting, and I'll probably give it a try sooner or later. Tord
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