Ways to do modulo multiplication with primitive types

Question

Is there a way to build e.g. (853467 * 21660421200929) % 100000000000007 without BigInteger libraries (note that each number fits into a 64 bit integer but the

Answer 1

You could try something that breaks the multiplication up into additions:

// compute (a * b) % m:

unsigned int multmod(unsigned int a, unsigned int b, unsigned int m)
{
    unsigned int result = 0;

    a %= m;
    b %= m;

    while (b)
    {
        if (b % 2 != 0)
        {
            result = (result + a) % m;
        }

        a = (a * 2) % m;
        b /= 2;
    }

    return result;
}