Author: Peter Fendrich
Date: 06:28:32 10/25/02
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On October 25, 2002 at 08:39:47, Nagendra Singh Tomar wrote: >Hi everybody! > >In my endeavour to understand the techniques of comp chess I am in the process >of understanding the benefits (what/why/how) of a zero window search. Told >superficially it can be said that since we have a very small window to search we >search less nodes, but I want to understand it to the minute details. > >What I observe and understand is follows. > >score = -alphabeta(board, -beta, -alpha, depth-1) <-- 1st call (full width) >. >. update alpha if score more than alpha >. >score = -alphabeta(board, -alpha-1, -alpha, depth-1) <-- subsequent calls > zero window > >Any saving whatsoever that can be achieved by reducing the window size, should >be because of beta cutoffs (fail-highs) increasing. >Here we are keeping beta as -alpha (same as we would have kept it for a full >window search, though we have kept alpha as -alpha-1 in contrast to -beta for a >full win search) >But in the subsequent call (child nodes) this alpha (-alpha-1) will be negated >and passed as beta, which will be very less. Is the saving in nodes searched, >due to this beta being passed as a very small value in the child->child nodes >searched. >I know I have made it complex, but I hope you understand me. > >IOW changing the value of alpha cannot reduce the number of nodes searched. But >the fact that this alpha will be passed as beta in the subsequent call to >alphabeta, results in lot of beta cuts and hence reduced nodes searched. > > >Am I thinking on the right lines ? > >tomar I think that you've mixed up the alpha and beta values from the calling nodes view. -alpha-1 is a temporary beta value and not a new alpha. I would explain it for myself like this: I assume that the first search (the full width) is with the best move in the node. Therfore I reduce the beta value to the most narrow window possible, namely alpha+1. So the the rest of the searches will use (alpha, alpha+1) until proved wrong (score >= alpha+1) and I have to re-search with a wider window. Now beta replaced by alpha+1 negated is -aplha-1. Maybe that's what you said? Peter
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