How to multiply a 64 bit integer by a fraction in C++ while minimizing error? [duplicate]

Answer 1

More or less copied from here, this seems to make the most sense to me:

int64_t muldiv64(const int64_t x, const int64_t n, const int64_t d)
{
    /* find the integer and remainder portions of x/d */
    const int64_t div = x / d;
    const int64_t mod = x % d;

    return div * n + (mod * n) / d;
}