[Bug-gnubg] Resignation bug reported at bgonline.org

[Philippe Michel]

A bug where gnubg offers to resign a single game from :

    GNU Backgammon  Position ID: OwAAwL13gAAAAA
                    Match ID   : AQEAABADgAAE
    +-1--2--3--4--5--6-------7--8--9-10-11-12-+  O: GNUbg (Cube: 2)
    | O  O  O  O       |   |                O |  49 points
    | O  O  O  O       |   |                  |  On roll
    | O  O  O  O       |   |                  |
    |    O  O          |   |                  |
    |                  |   |                  |
    |                  |BAR|                  |^
 XX |                  |   |                  |
 XX |                  |   |                  |
 XX |    X             |   |                  |
 XX | X  X             |   |                  |
 XX | X  X             |   |                  |  16 points
    +24-23-22-21-20-19------18-17-16-15-14-13-+  X: You
                    Pip counts : O 47, X 8

was reported at bgoline.org.


The problem is a combination of the neural net underestimating gammons 
lost by O :

Position ID:    OwAAwL13gAAAAA
Match ID:       AQEAABADgAAE

Evaluator:      Race

        Win     W(g)    W(bg)   L(g)    L(bg)   Equity    Cubeful
static: 0.000000 0.000000 0.000000 0.000050 0.000000  -1.000050  -1.000050
 1 ply: 0.000000 0.000000 0.000000 0.001895 0.000000  -1.001895  -1.001895
 2 ply: 0.000000 0.000000 0.000000 0.001417 0.000000  -1.001418  -1.001418

and the following code in the SanityCheck() function :

        float noise = 1 / 10000.0f;

        for (i = OUTPUT_WINGAMMON; i < NUM_OUTPUTS; ++i) {
            if (unlikely(arOutput[i] < noise)) {
                arOutput[i] = 0.0f;
            }
        }

I don't know the rationale for it. It seems it is from quite early in 
gnubg development, maybe it predates the other checks that are in this 
function. The latter seem to be enough to handle the certain outcomes in 
non-contact positions.

Is it really useful to keep these truncations for contact positions ?