[Bug-gnubg] evaluation bug?

[Paul Selick]

Hi,

Concerning evaluation of positions which may be too good for a double,
I think there is an evaluation bug.

I have noticed some early game positions (where the probably of
backgammon is very small) receiving evaluations greater than 2
(which would be somewhere between gammon and backgammon)
For example, with no Jacoby rule, after
42: 8/4 6/4             36: 24/15
33: 13/10*(2) 8/5(2)    55:
a 2-ply search gives the Equity as being over 1.4 and as far as I can
tell, looking at the source code and outputting some of the values, many
of the Equities that were being averaged to produce this figure were
over 2.
For example, I printed the values in the last of these calculations:
They included at line 5498 of eval.c values of:
rW=1.819175
arOutput[0]=0.821732
rTG=0.652716
  working out to a value greater than 2 for rEq
  I don't understand exactly what this formula
       rEq = rW +
          ( rW - 1.0 ) * ( arOutput[ 0 ] - 1.0 ) / ( rTG - 1.0 ) ;
is supposed to be doing, but it looks at a glance it looks to me as
if the second term is supposed to be subtracted rather than added to rW.
That is, my guess was that the formula was trying to take the value rW
for a win and find the equity by deleting something for the losses, whose
probably must be represented by 1.0-arOutput[0]
  But since 2 of the 3 factors in the second term are negative, the
result gets added to rW, which looks wrong to me.

Yours sincerely,
Paul Selick