[Bug-gnubg] Oddity/Bug in Rollouts?

[Michael Petch]

Not sure anyone has been following rec.games.backgammon, but I am (And so
are others) wondering how this position can be " Too good to redouble, take"

I can understand that its possible to be Too good to redouble, take if the
opponent can be gammoned, I'm just not sure how this can be possible with
the following position. This rollout was done as Grandmaster checker (3ply),
supremo cube(2ply) for both players. The result was:

The score (after 2 games) is: mpetch 1, Monty 4 (match to 15 points)
Match Information:

Date: April 13, 2008
Move number 67:  Monty on roll, cube decision? GNU Backgammon  Position ID:
BwAAmG1bCkEAAA
                 Match ID   : UQngARAAIAAA
 +13-14-15-16-17-18------19-20-21-22-23-24-+     O: mpetch
 |    X             | O | X              O | OO  1 point
 |                  | O |                O | OO
 |                  |   |                O | OO
 |                  |   |                  | OO
 |                  |   |                  | OO
v|                  |BAR|                  |     15 point match
 |                  |   |                  |
 |                  |   |                  |
 |                  |   |                  |
 |                  |   | X  X  X  X  X    |     On roll
 |       X  X     X |   | X  X  X  X  X    |     4 points
 +12-11-10--9--8--7-------6--5--4--3--2--1-+     X: Monty (Cube: 2)
Pip counts: O 53, X 99

* Monty doubles

Alert: wrong double ( -0.025)! [very bad]
Cube analysis
Rollout cubeless equity  +0.587 (Money:  +0.587)

Cubeful equities:
1. No double            +1.015
2. Double, pass         +1.000  ( -0.015)
3. Double, take         +0.990  ( -0.025)
Proper cube action: Too good to redouble, take
Rollout details:
Player Monty owns 2-cube:
  0.795 0.000 0.000 - 0.205 0.003 0.000 CL  +0.587 CF  +1.015
 [0.001 0.000 0.000 - 0.001 0.000 0.000 CL   0.003 CF   0.005]
Player mpetch owns 4-cube:
  0.800 0.000 0.000 - 0.200 0.004 0.001 CL  +1.178 CF  +0.990
 [0.001 0.000 0.000 - 0.001 0.000 0.000 CL   0.006 CF   0.007]
Full cubeful rollout with var.redn.
1296 games, Mersenne Twister dice gen. with seed 824585691 and quasi-random
dice
Play: grandmaster 3-ply cubeful prune [grandmaster]
keep the first 0 0-ply moves and up to 8 more moves within equity 0.16
Skip pruning for 1-ply moves.
keep the first 0 2-ply moves and up to 2 more moves within equity 0.04
Cube: 2-ply cubeful prune [world class]