Quote:
Originally Posted by bleep69
What exactly does a roll-out do? Makes sure?
Right!
For a given play, the neural net produces estimates of wins, gammon wins, backgammon wins, losses, gammon losses, and backgammon losses. In GnuBg, a 0-ply "analysis" uses those estimates to choose a play. These estimates are not perfect.
To improve the result, a bot can use more than one ply, where each ply is a turn. In GnuBg, for instance, a 1-ply analysis calculates the win rates listed above by making a candidate play, and then letting the opponent take a turn. For the opponent, GnuBg checks all 21 possible rolls, feeding each one into the neural net to determine the best play. GnuBg then averages the values that are returned by the net. The average is weighted, so that non-doublets count twice as much as doublets.
Further plies repeat the process (in a fashion), allowing you to look ahead further and further.
A "rollout" plays a game to its conclusion many times. The same statististics for wins, gammon wins, and so on, are easy to determine if you play all the way to the end. By doing this a large number of times, eventually the numbers will stabilize, giving you fairly accurate estimates. When the number of trials is large, rollouts are considered to be the most accurate way to assess a position.
A 1-ply rollout chooses the moves in a game using a 1-ply analysis. Similarly, a 2-ply rollout uses 2-ply analysis. Here you can get confused because of the way plies are counted. GnuBg is the odd man out. Its ply counts are always one less than XG or Snowie. Thus, 2-ply in GnuBg is the same as 3-ply in XG and Snowie.
There is a general consensus that the 3-ply rollouts of Snowie and XG are the minimum you should use for serious work. Neil Kazaross, a backgammon Giant, frequently uses 4-ply in XG. For GnuBg users the 3-ply minimum of XG translates into a 2-ply minimum for GnuBg, the setting that is used when you select "World Class" or "Supremo."
Each game in a "truncated rollout" is stopped after a certain number of moves. At that point, the neural net is called to estimate the various win percentatges. Again, there is some confusion in terminology, because the word ply is used to describe the "depth" of a truncated rollout. If you stop after five turns, for instance, the rollout is said to be truncated at five plies.
The most powerful analysis level in XG is a truncated rollout called XG Roller++ or XGR++, for short. These are the settings is uses:
XGRoller++ for Checker Plays
360 games with VR
Truncated after 5 moves using 3-ply analysis
First 2 decisions in 3-ply
Then 2-ply checker and 3-ply cube
XGRoller++ for Cube Decisions
360 games with VR
Truncated after 7 moves using 3-ply analysis
Stop when reaching 0.010 Confidence (minimum 180 games)
First 2 decisions in 3-ply
Then 2-ply checker and 3-ply cube
Hope this helps.
Mike