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Subject: Selective search and transposition tables

Author: Tord Romstad

Date: 07:55:21 07/15/03


Like many others, I use static pruning in addition to null-move pruning in the
last few plies of my search.  Currently, the last 3 plies of my search are
selective.  I am not sure that this simple approach is the best.  At slow time
controls, it seems that a higher number of selective plies are better (although
I still do not have enough data to make any certain conclusions about this).
However, high selectivity causes too many tactical oversights when there is
little time available.

I have been tempted to try a Rebel-like selective search, and let the number
of selective plies vary with the iteration number.  But this seems to cause
problems with the transposition table.  To be able to check whether a
transposition table entry has a sufficiently high search depth to be used for
cutoff decisions, I will have to store both the brute force depth and the
selective depth in the hash table entry.  But what is the best way to compare
two different depth/selective depth pairs?  Is 1 ply full-width + 4 plies
selective better than 2 plies full-width + 2 plies selective?

I would be interested to learn how those among you who use variable
selectivity based on iteration solves this problem.

Tord




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