[Bug-gnubg] Bug in PositionBearoff() and PositionIndex()

[Philippe Michel]

I think there is something wrong (but harmless with the compilers we've 
used) in these functions from positionid.c.

In PositionBearoff() for instance :

    for( j = nPoints - 1, i = 0; i < nPoints; i++ )
        j += anBoard[ i ];

    fBits = 1u << j;

    for( i = 0; i < nPoints; i++ ) {
        j -= anBoard[ i ] + 1;
        fBits |= ( 1u << j );
    }

the last "1u << j" is always 1u << (-1) which is undefined (and apparently 
happens to end as 1<<31) : start with nPoints - 1, count checkers on 
the lower nPoints then substract (checkers+1) for each of nPoints.

I don't really understand what the index thus calculated means, but 
setting the bit at the other end of fBits is probably not what is intended 
and the way it is done is unsound anyway.

The code in PositionIndex() is almost identical.

Doing one less loop with
    for( i = 0; i < nPoints - 1; i++ )
gives the same result in gnubg since apparently fBits is only used in some 
masking with values smaller than 2^31. I didn't look at what happens with 
the bearoff database building commands.

Could someone who understand this part of the code confirm that this is 
the right thing to do or suggest a proper fix ?