Re[2]: computer-go: Eyes

[rbrown@xxxxxxxxxxxxxxx]

Jean-Pierre Vesinet wrote:

[Jeff had asked:]
>> In a typical game of Go, is it overkill to try and make groups with more 
>> than 2 eyes?
>> 
>       Not at all. In Japanese rules, territory is defined by eyes.
> 
>       Quoting from http://www.cs.cmu.edu/~wjh/go/rules/Japanese.html :
> Empty points surrounded by the live stones of just one player are called
> "eye points." Other empty points are called "dame." Stones which are alive
> but possess dame are said to be in "seki." Eye points surrounded by stones
> that are alive but not in seki are called "territory," each eye point
> counting as one point of territory.
>        Jean-Pierre Vesinet

Jean-Pierre is right of course, but my sense of the original question is that
Jeff was again thinking of single-point eyes, and asking about something like
this:

  x x x x x x x x x
  x . x . . . x . x
  x x x x x x x x x

Here we have a group with two solid, single-point eyes at either end, and a
three-space eye in the middle.  For Black to play at the central point of
the three-space eye is indeed overkill, as the group is already permanently
alive, provided that Black does not fill all (or all but one) of his own eyes.

Perhaps "overlife" would be a better term than "overkill" -- once the group
is alive, it's alive, and making more single-space eyes is not helpful.
It is actually detrimental, as Black's group is not only already permanently
alive, but he will also lose one point of his territory (under territory rules)
for each stone he puts down.  Under area rules, Black could play there without
penalty, but only _after_ all meaningful moves had been played, and every last
dame had been filled; to do so any earlier would certainly incur a loss too,
just as under territory rules:  White would play the good moves while Black
filled up his area with stones.

The key thing to note from the web site that Jean-Pierre quoted is that any
surrounded space -- not just single points -- consists of what the rule
above calls "eye points".  So if you surround five contiguous points with
your stones, you have five of what the rule above calls "eye points", or
what I would call a five-space eye.

Even if the opponent throws a stone into my five-space eye, I would still
call it an eye.  It's an eye that happens also to have an enemy stone inside
it, but that doesn't stop it from being an eye.  Even dead groups can have
one eye; note that the rule above does distinguish between eyes and
"territory" -- by use of the qualifier "alive".  I'll have to visit the
site to see how they defined that, "alive".  Five bucks says it's circular.

Rich
-- 
Richard L. Brown              Office of Information Services
rbrown@xxxxxxxxxxxxxxxxx        University of Wisconsin System Administration
rlbrown6@xxxxxxxxxxxxxxxxx    780 Regent St., Rm. 246  /  Madison, WI  53715