Re: [Bug-gnubg] probably bug

[iandshaw]

Gnubg does finish it's turn. The move is actually 4/2/off - bearing off a
single man.

It's a play no human would make, but 4/off makes perfect sense to gnubg.
There are two reasons, and I'm not sure which one takes priority.

Firstly, Gnubg sees that after either play, the correct cube action is
Double/Pass. Therefore it makes no practical difference which play is
made.

I think that in cases like this, gnubg picks the play with the highest
cubeless equity. As you can see below, it thinks 4/off wins 0.000001 more
games!

Secondly, there is really no difference between O's positions; he bears
off in five rolls even with 5 consecutive 21s. I suspect that the
difference is due to a rounding effect in the floating point maths.

    1. Cubeful 3-ply    4/off                        Eq.:  +1.000000
       0.999496 0.000028 0.000000 - 0.000504 0.000000 0.000000
        3-ply cubeful prune [grandmaster]
    2. Cubeful 3-ply    4/off 2/off                  Eq.:  +1.000000 (
+0.000000)
       0.999495 0.000028 0.000000 - 0.000505 0.000000 0.000000
        3-ply cubeful prune [grandmaster]
-- Ian

> So gnubg should to move from 4 to off and from 2 to off (or looking on the
> board from 21,23 to the board)
>

>> (bert) set board fycAAPC9EwwAAA
>>     GNU Backgammon  Position ID: fycAAPC9EwwAAA
>>                     Match ID   : cAkAAAAAAAAA
>>     +24-23-22-21-20-19------18-17-16-15-14-13-+  O: gnubg
>>   O | O  O     O       |   |                  |  0 points
>>   O | O  O             |   |                  |
>>   O | O  O             |   |                  |
>>   O | O                |   |                  |
>>     | 7                |   |                  |
>>     |                  |BAR|                  |v (Cube: 1)
>>     | X                |   |                  |
>>     | X  X             |   |                  |
>>     | X  X  X          |   |                  |
>>     | X  X  X          |   |          X       |  On roll
>>     | X  X  X     X    |   |          X       |  0 points
>>     +-1--2--3--4--5--6-------7--8--9-10-11-12-+  X: bert
>>
>> (bert) set turn gnubg
>> 'gnubg' is now on roll.
>> (gnubg) set player gnubg chequerplay evaluation plies 3
>> gnubg will use 3 ply evaluation.
>> (gnubg) set dice 2 4
>> The dice have been set to 2 and 4.
>> (gnubg) play
>> Considering move...\
>> gnubg moves 4/off.