[Bug-gnubg] Possible evaluation bug

[Terje Pedersen]

Hi!

I just ran into what looks like a gnu evaluation bug where gnu seems
to suggest that I should try to avoid backgammon when a gammon will
lose the match anyway. command file:

set player 0 human
new match 0
set output rawboard off
set xgid XGID=----BBCA-----A----B-B--eB-:0:0:1:51:0:3:0:5:10
move 24/19 18/17
next
previous
analyze move
show board

$ ./gnubg.exe -t -c commands.txt
GNU Backgammon 1.05.000  Nov 27 2016
Copyright (C) 1999, 2000, 2001, 2002, 2003, 2004 by Gary Wong.
Copyright (C) 2015 by Gary Wong and the AUTHORS; for details type
`show version'.
This program comes with ABSOLUTELY NO WARRANTY; for details type `show
warranty'.
This is free software, and you are welcome to redistribute it under
certain conditions; type `show copying' for details.
Moves for gnubg must now be entered manually.
A new session has been started.
TTY boards will be given in ASCII.
The dice have been set to 5 and 1.
 GNU Backgammon  Position ID: PgAAALYLBDMMAA
                 Match ID   : cImmADAAAAAE
 +13-14-15-16-17-18------19-20-21-22-23-24-+     O: gnubg
 | X              X |   |    X        O  X | OO  3 points
 |                X |   |    X        O  X | OO
 |                  |   |             O    | OO
 |                  |   |             O    | OO
 |                  |   |             O    | OO
v|                  |BAR|                  |     5 point match (Cube: 1)
 |                  |   |                  |
 |                  |   |                  |
 |                  |   | X                |
 |                  |   | X  X  X          |     Rolled 51
 |                X |   | X  X  X          |     0 points
 +12-11-10--9--8--7-------6--5--4--3--2--1-+     X: terje


Cube analysis
2-ply cubeless equity  -2.238 (Money:  -2.039)
  0.086 0.000 0.000 - 0.914 0.771 0.440
Cubeful equities:
1. No double            -2.058
2. Double, pass         +1.000  ( +3.058)
3. Double, take         -2.430  ( -0.372)
Proper cube action: No double, take (10.9%)

Rolled 51 (-0.066):
     1. Cubeful 2-ply    24/18                        Eq.:  -1.850
       0.144 0.000 0.000 - 0.856 0.716 0.430
        2-ply cubeful prune [world class]
     2. Cubeful 2-ply    24/19 6/5                    Eq.:  -1.994 ( -0.143)
       0.106 0.000 0.000 - 0.894 0.735 0.627
        2-ply cubeful prune [world class]
     3. Cubeful 2-ply    24/19 7/6                    Eq.:  -2.015 ( -0.164)
       0.102 0.000 0.000 - 0.898 0.739 0.626
        2-ply cubeful prune [world class]
*    4. Cubeful 2-ply    24/19 18/17                  Eq.:  -2.065 ( -0.214)
       0.089 0.000 0.000 - 0.911 0.743 0.647
        2-ply cubeful prune [world class]
     5. Cubeful 2-ply    13/7                         Eq.:  -2.165 ( -0.314)
       0.071 0.000 0.000 - 0.929 0.780 0.641
        2-ply cubeful prune [world class]
     6. Cubeful 2-ply    13/8 6/5                     Eq.:  -2.180 ( -0.329)
       0.070 0.000 0.000 - 0.930 0.787 0.663
        2-ply cubeful prune [world class]
     7. Cubeful 2-ply    18/13 6/5                    Eq.:  -2.200 ( -0.349)
       0.060 0.000 0.000 - 0.940 0.783 0.692
        2-ply cubeful prune [world class]
     8. Cubeful 2-ply    7/2 6/5                      Eq.:  -2.206 ( -0.355)
       0.068 0.000 0.000 - 0.932 0.799 0.669
        2-ply cubeful prune [world class]
     9. Cubeful 2-ply    18/17 18/13                  Eq.:  -2.212 ( -0.361)
       0.057 0.000 0.000 - 0.943 0.781 0.700
        2-ply cubeful prune [world class]
    10. Cubeful 2-ply    18/13 7/6                    Eq.:  -2.229 ( -0.378)
       0.056 0.000 0.000 - 0.944 0.789 0.702
        2-ply cubeful prune [world class]

Any idea what is going on here? I am using gnu bg for evaluating moves
on Backgammon Studio when playing online matches and was hit in the
face with this double blunder which I was confident couldn't possibly
be correct. XG doesn't think it is much of an error:

    2. XG Roller++ 24/19 18/17                  eq:-2.2250 (-0.0057)
      Player:   7.58% (G:0.00% B:0.00%)
      Opponent: 92.42% (G:76.45% B:64.53%)


Best regards,
TP