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Subject: playing style from search (was Re: Junior's long lines)

Author: Jay Scott

Date: 16:19:35 12/29/97

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On December 27, 1997 at 14:04:12, Don Dailey wrote:
>I felt the same way when I read this.  I don't believe you can change
>a programs playing "style" very much with only search changes.

I'm not so sure. I think drastic search changes can have big effects
on a program's style. If you've only tried mostly-full-width search
with the usual kinds of pruning and extensions, then you've never
tried a drastically different search algorithm.

Several years ago I wrote a chess program named Kon to look into
automated learning of search control. I hacked up a crude fixed
evaluation function by hand and gave the program a method to learn
from experience which moves to search. Kon used a breadth-first
search algorithm which kept the entire tree in memory,
so it was quite different from the usual alpha-beta stuff.

Kon played on what was then the Internet Chess Server. Before
learning, I think the ICS rating of the final version was
something like 1300. Search control learning was able to boost
this to 1700 or so, which was respectable if you figure that
the search was slow and the evaluator was extremely simpleminded.

I was struck by the stylistic difference between Kon and the
typical alpha-beta program. Kon was much less consistent: sometimes
it would find a clever, deep tactic that was beyond the skill
of a human B-player or a B-strength alpha-beta program, and
sometimes it would fall for two-movers. The search control
learning had picked up how to spot some tactical moves, but it
wasn't able to figure them all out, even some shallow ones. In a
losing position Kon would usually search broadly and shallowly,
struggling to find a move that stopped all the threats. In a
tactical position it would typically search deeply and narrowly,
trying to fathom the tricks, and sometimes completely ignore
a whole set of what it considered boring moves at the first ply.
A full-width program would have noticed that some of those
"boring" moves were actually tricky a few plies down the line.

Kon was popular with lower-rated players on ICS. I think that
was partly because of its interesting style (and partly because
there weren't any other programs weak enough to be fun for them
to play).

The evaluation function sets the program's goals, but the search
controls how well the program can meet them. Changing the search
can reposition a program on the defensive<->aggressive spectrum
(a la Genius, if I can believe what I'm reading) and on the
solid<->speculative spectrum (which you could also call
consistent<->inconsistent). Kon was pretty much equally defensive
and aggressive, but definitely on the speculative side of solid.

  Jay



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