Author: Paul Byrne
Date: 00:19:31 10/28/01
I've been looking at tablebases lately and was wondering if anyone
has ever played with this idea (before I spend too much time on it!)
Basically, take the normal (Nalimov or whatever) tablebases in their
uncompressed form and divide the mate-in-x or loss-in-x values by some
number N. So mate-in-1 through N become (+1) mate-in-(N+1) through 2*N
become (+2), etc. Then compress the resulting files as usual.
The idea being to reduce the number of different values to make the file
more compressible.
This would, of course, make life a little more difficult for the engine. :)
When the position is not yet into a tablebase, there would be little effect --
other than any mate/loss scores being a little inaccurate. Once the position
on the board is actually a tablebase position, a short search would have to
be done to determine the correct move. For example, if the position OTB is
scored as (+5), then one would search for the move that forces a (+4) position
the quickest.
I did a little test to see if the space savings is worthwhile...
Using the kbbkn.nbb/nbw tablebases (something with long mates) and the
kqnkn.nbb/nbw tablebases (mostly short mates) with various N's -- the
numbers are the percentage of the regular _compressed_ tablebases:
N=2 N=3 N=4 N=5 N=10 N=100
kbbkn.nbb 84.67 75.00 68.26 63.74 52.08 23.11
kbbkn.nbw 82.96 73.21 66.98 62.13 47.80 9.10
kqnkn.nbb 64.40 48.51 40.58 31.93 19.21 5.86
kqnkn.nbw 63.51 46.43 37.20 28.70 6.36 2.57
The last 2 columns were just out of curiousity. :)
The question is, how large can N be made while still allowing tablebase
positions to be played out in a reasonable amount of time? Don't know (yet).
None of this would help for generating tablebases, of course. You still need
the normal tablebases to generate the reduced ones. And I'd imagine most
engine authors would prefer the full tablebases, but for the average
player I don't know they'd notice much difference and could save a few GB
of disk space... less stuff to distribute or download too.
I suppose a lot of folks have enough disk space for 3/4/5 man tables
nowadays, but when the 6 man tables become more complete/common, this may
help some.
-paul
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