[Bug-gnubg] Possible cube evaluation problem

[Terje Pedersen]

Hi!

One of my users was upset that a position which XG evaluated as
Double/Take was evaluated to a No Double/Take by Gnu. The score 2A-5A
may make the cube action a bit tricky.

XGID=-AAbB-EBBB-----a--acbbbb--:0:0:1:00:3:0:0:5:10

Analyzed in 4-ply
Player Winning Chances:   74.79% (G:2.12% B:0.01%)
Opponent Winning Chances: 25.21% (G:1.40% B:0.01%)

Cubeless Equities: No Double=+0.5233, Double=+1.6234

Cubeful Equities:
       No double:     +0.8108 (-0.0040)
       Double/Take:   +0.8149
       Double/Pass:   +1.0000 (+0.1851)

Best Cube action: Double / Take

eXtreme Gammon Version: 2.19.211.pre-release, MET: Kazaross XG2

Gnu:

tu0iAAxlvm0AAA:cAmgAAAAGAAE

Cube analysis
2-ply cubeless equity  +0.506 (Money:  +0.487)
  0.739 0.021 0.000 - 0.261 0.012 0.000
Cubeful equities:
1. No double            +0.912
2. Double, pass         +1.000  ( +0.088)
3. Double, take         +0.742  ( -0.169)
Proper cube action: No double, take (65.7%)

The difference is fairly big in this case. I checked a few more
positions in the database and it seems that Gnu and XG agrees for the
most part but not always. Some other examples where they disagree:

XGID=--ADBBBbB---cB----bbb-b-b-:0:0:1:00:7:4:0:9:8
XGID=-ABbBBCC----bB-----bbbbba-:0:0:1:00:5:2:0:7:8
XGID=--ACBcC-BB--bB-----b-cbab-:0:0:1:00:11:8:0:13:8
XGID=-B-aCCC-C---aA------bdbcb-:0:0:1:00:7:4:0:9:8
XGID=-C-CBCB-c----B-a-abb-bb-b-:0:0:1:00:23:20:0:25:8
XGID=-a-BB-C-BBA-bCa--c-db-b---:0:0:1:00:9:6:0:11:8

Is it a bug or a feature? Hard to tell! :-)

Best regards,
TP