unsigned int gcd(unsigned int u, unsigned int v)
{
    unsigned int shift = 0;

    /* GCD(0,v) == v; GCD(u,0) == u, GCD(0,0) == 0 */
    if (u == 0) return v;
    if (v == 0) return u;
 
    /* Let shift := lg K, where K is the greatest power of 2
        dividing both u and v. */
    while (((u | v) & 1) == 0) {
        shift++;
        u >>= 1;
        v >>= 1;
    }
 
    while ((u & 1) == 0)
        u >>= 1;
 
    /* From here on, u is always odd. */
    do {
        /* remove all factors of 2 in v -- they are not common */
        /*   note: v is not zero, so while will terminate */
        while ((v & 1) == 0)
            v >>= 1;

        /* Now u and v are both odd. Swap if necessary so u <= v,
            then set v = v - u (which is even). For bignums, the
             swapping is just pointer movement, and the subtraction
              can be done in-place. */
        if (u > v) {
            unsigned int t = v; v = u; u = t; // Swap u and v.
        }
       
        v -= u; // Here v >= u.
    } while (v != 0);

    /* restore common factors of 2 */
    return u << shift;
}

unsigned int gcd(unsigned int u, unsigned int v)
{
    while (v != 0)
    {
        a %= b;
        swap(a, b);
    }
    return a;
}