On Aug 1, 2008, at 8:08 AM, Mark Boon wrote:
The neighbours of the last move come in the picture because usually
it's only the last stone played that can be escaping a ladder and
it's the neighbours of the last move that could have been put into
atari. Nothing to do with the additional complexities Don mentioned.
Let me give a specific example. Suppose that, during a playout, the
tree leads us to this position, with O to play:
.........
...OO....
..O##a...
...Ob....
....c....
.........
.........
.........
.........
Having reached the frontier of the tree, we now finish the game using
Monte Carlo with a ladder reader. The last stone played, to the left
of a, is trapped in a ladder, but can escape if not chased. Our ladder
reader therefore suggests O play at a.
For the next move in the playout, if # only reads ladders from the
last move played, it will see that the O stone at a is not in a
ladder, so move is suggested. The playout now turns completely random,
and it's a coin toss as to whether the group will escape.
If we also search stones next to the last stone played, things only
get slightly better. # sees that its stones are in a ladder from which
they cannot escape, so it doesn't suggest b. If we tell it to play a
ladder breaker in such situations, it might play c, which is fine.
However, on O's next turn, c is not in a ladder, nor is any stone next
to c, so no move is suggested. Specifically, O does not make the vital
capture at b.
It seems too expensive to search every point on the board for ladders.
What to do?
Peter Drake
http://www.lclark.edu/~drake/
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