Pierre Zakia wrote:
In the position here below, I am curious to understand what in Gnubg
engine yields such discrepancy between 4 ply and roll out.
GNU Backgammon ID de position: G27HAAiY2+ABAw
ID de match : MAHyAAAAAAAE
Thanks for posting this position---it's very interesting!
First let me mention that eXtreme Gammon (XG) finds this position tricky
as well. XG 3-ply plays 13/9(2) 13/5. (XG and GNU use slightly
definitions of "ply" I think, but that's not important here.) So I don't
think that 13/9(2) 13/5 is "obviously wrong." Two things to note:
1. 13/9(2) 13/5 wins more gammons than 20/16(2) 13/9(2), so it's probably
the better play at certain match scores.
2. Suppose you change the position slightly---instead of having the cube
centered, have X own the cube on 2. I think that if you roll it out then
you'll see that 20/16(2) 13/9(2) comes out ahead by a much smaller margin.
So part of the reason that 20/16(2) 13/9(2) comes out so far ahead in your
original position is the somewhat subtle fact that O is hoping to double
next turn---but if O gets hit then she won't be able to double. So with
the cube in the middle, O is much more cautious about leaving shots.
Anyway, I don't think that this is a fault of the software. It's just
that the position is more subtle than you may have thought at first
glance.
Tim
