Author: Nagendra Singh Tomar
Date: 05:39:47 10/25/02
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
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