Peter Drake wrote: > 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.
It's often a good idea to bias capturing moves in the playouts, regardless whether it's a ladder or not. This would result in those stones being captured in most simulations. > 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? Don't bother with ladder breakers in playouts. If the tree part of the search has started to run a ladder, the best thing the playout can do is to keep running it until the bitter end. This way the losing player in the ladder will be rightfully punished and discouraged from trying that line of play. /Gunnar _______________________________________________ computer-go mailing list computer-go@computer-go.org http://www.computer-go.org/mailman/listinfo/computer-go/
