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Rollout: Double from the bar?

Posted By: Timothy Chow
Date: Tuesday, 16 March 2010, at 11:24 p.m.

In Response To: Double from the bar? (Timothy Chow)

Some background: A few weeks ago, a newbie on rec.games.backgammon posted a slight variant on the ancient "Jellyfish cheats" complaint: He said that Jellyfish would double in middle-game positions where it had less than 60% game-winning chances, and then would get a lucky roll and gammon him. Clearly, he reasoned, Jellyfish must be looking ahead to see what rolls it's about to get. Attempts to educate him were, of course, fruitless.

It so happened that shortly after that discussion, I encountered the present position while playing a training game against GNU 2-ply. GNU doubled, which surprised me slightly but didn't shock me. It then got a lucky roll and proceeded to gammon me. As the rollout below shows, GNU's CPW is less than 60% here, but the double is clear. It is important that the cube is in the center; if White owned the cube it would not be a redouble. Also the Jacoby rule plays a role; White needs to double in order to activate gammons. Without Jacoby, GNU thinks it's a marginal initial double.

I'll give three rollouts. First, here's the rollout of the given position.

gnubg	101
tchow	134
Position ID: a7ZAwAk7swUDQA Match ID: MAEAADAAAAAA

• gnubg doubles

Cube decision
Rollout cubeless equity		+0.306
Cubeful equities:
1.	Double, take	+0.368
2.	Double, pass	+1.000	+0.632
3.	No double	+0.227	-0.141
Proper cube action:		Double, take
Rollout details
Win W g W bg Lose L g L bg Cubeless Cubeful Centered 1-cube 0.576 0.321 0.014 - 0.424 0.178 0.004 +0.306 +0.227 Standard error 0.001 0.002 0.000 - 0.001 0.001 0.000 0.004 0.006 Player tchow owns 2-cube 0.583 0.333 0.019 - 0.417 0.177 0.005 +0.672 +0.368 Standard error 0.001 0.002 0.001 - 0.001 0.002 0.001 0.008 0.013
Full cubeful rollout with var.redn.
1296 games, Mersenne Twister dice gen. with seed 850942287 and quasi-random dice
Play: supremo 2-ply cubeful prune [world class]
keep the first 0 0-ply moves and up to 16 more moves within equity 0.32
Skip pruning for 1-ply moves.
Cube: 2-ply cubeful prune [world class]

Next is a rollout with White owning the cube.

gnubg	101
tchow	134
Position ID: a7ZAwAk7swUDQA Match ID: AQEAADAAAAAA

• gnubg doubles

Alert: wrong double ( -0.084)! [bad]

Cube decision
Rollout cubeless equity		+0.304
Cubeful equities:
1.	No double	+0.448
2.	Double, pass	+1.000	+0.552
3.	Double, take	+0.365	-0.084
Proper cube action:		No redouble, take (13.2%)
Rollout details
Win W g W bg Lose L g L bg Cubeless Cubeful Player gnubg owns 2-cube 0.573 0.325 0.020 - 0.427 0.184 0.003 +0.304 +0.448 Standard error 0.001 0.002 0.001 - 0.001 0.002 0.001 0.004 0.006 Player tchow owns 4-cube 0.581 0.335 0.019 - 0.419 0.178 0.004 +0.670 +0.365 Standard error 0.001 0.002 0.001 - 0.001 0.001 0.001 0.009 0.013
Full cubeful rollout with var.redn.
1296 games, Mersenne Twister dice gen. with seed 850848504 and quasi-random dice
Play: supremo 2-ply cubeful prune [world class]
keep the first 0 0-ply moves and up to 16 more moves within equity 0.32
Skip pruning for 1-ply moves.
Cube: 2-ply cubeful prune [world class]

Finally, here's a rollout with the cube in the center but no Jacoby rule.

gnubg	101
tchow	134
Position ID: a7ZAwAk7swUDQA Match ID: AQEAADAAAAAA

• gnubg doubles

Alert: wrong double ( -0.084)! [bad]

Cube decision
Rollout cubeless equity		+0.304
Cubeful equities:
1.	No double	+0.448
2.	Double, pass	+1.000	+0.552
3.	Double, take	+0.365	-0.084
Proper cube action:		No redouble, take (13.2%)
Rollout details
Win W g W bg Lose L g L bg Cubeless Cubeful Player gnubg owns 2-cube 0.573 0.325 0.020 - 0.427 0.184 0.003 +0.304 +0.448 Standard error 0.001 0.002 0.001 - 0.001 0.002 0.001 0.004 0.006 Player tchow owns 4-cube 0.581 0.335 0.019 - 0.419 0.178 0.004 +0.670 +0.365 Standard error 0.001 0.002 0.001 - 0.001 0.001 0.001 0.009 0.013
Full cubeful rollout with var.redn.
1296 games, Mersenne Twister dice gen. with seed 850848504 and quasi-random dice
Play: supremo 2-ply cubeful prune [world class]
keep the first 0 0-ply moves and up to 16 more moves within equity 0.32
Skip pruning for 1-ply moves.
Cube: 2-ply cubeful prune [world class]

Messages In This Thread

• Double from the bar?
  Timothy Chow -- Monday, 15 March 2010, at 8:12 p.m.
  • Double from the bar?
    Chase -- Monday, 15 March 2010, at 11:54 p.m.
  • Rollout: Double from the bar?
    Timothy Chow -- Tuesday, 16 March 2010, at 11:24 p.m.
    • Sorry, editing screwup
      Timothy Chow -- Tuesday, 16 March 2010, at 11:27 p.m.