Author: Nagendra Singh Tomar
Date: 09:27:49 10/25/02
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On October 25, 2002 at 11:16:48, Robert Hyatt wrote: >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. > > >Not quite. Because you are passing that null-window to the next ply, _all_ of >this sub-tree >will be searched with a null-window. This means that every search will either >fail high or >fail low, which is about as efficient as you can get. But for fail-low we anyway have to search all the nodes. fail-high is the actual saver. Is;nt it ?? > >>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. >> > >Correct. > > >> >>Am I thinking on the right lines ? >> >>tomar
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