Re: [gnugo-devel] worm amalgamation question

[Gunnar Farneback]

Paul wrote:
> could you please explain me why the two white worms are not
> amalgamated in this position? shouldn't they be amalgamated?
> 
> |XXX...
> |OOXXX.
> |.OOOX.
> |XX.OX.
> |.O.O..
> +------

This is tricky. A golden rule of the experimental connection code (now
default) is to not amalgamate tactically unstable worms because it
would lead to difficulties for the move valuation. What's really
important here is not to amalgamate a tactically critical worm to a
living dragon.

In this case both worms are tactically critical, which makes it more
reasonable to amalgamate them. I don't think that's safe generally (*)
but here both have the same defense point, which might be a sufficient
condition.

Curiously this position has the interesting property that the two O
strings cannot be solidly connected into a single string without
allowing a tactical capture. But they can live (in seki) if they
don't. That is somewhat anomalous to the assumptions made by the
connection reading code.

(*) In this position amalgamation through the bamboo joint in the
center would be inappropriate:

.......
.OO..O.
.OXXXO.
.OX.XO.
.XOOOX.
.X..XX.
.XOOOX.
.OX.XO.
.OXXXO.
.OO..O.
.......

/Gunnar