Bug in math?

Line 59 in 3cb39e9

if (_m <= v) v += _m;

Should this be if (m_ <= v) v -= m_; instead of +=? Or is there some other trickery happening?

We use a bit tricky technique.

(1)        unsigned int v = (unsigned int)(z - x * _m);
(2)        if (_m <= v) v += _m;
(3)        return v;

x is equal to the floor(a * b / mod) or floor(a * b / mod) + 1 (If you want to know the proof, please refer the comments of the source code). Therefore, -mod <= (z - x * _m) < mod is satisfied. As a result, (_m <= v) holds iff -mod <= (z - x * _m) < 0 is satisfied, because v is unsigned int.

Actually I don't remember why we used unsigned int, not int, for v.

Ah, I see, thanks!