integer pow() using Montgomery's ladder

ipow.c

/* Montgomery's Ladder Technique
 * http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.5.4956&rep=rep1&type=pdf
 */
#include <stdio.h>
#include <limits.h>

static unsigned long long ipow(unsigned g, unsigned k)
{
        unsigned long long r0, r1;
        unsigned j;
        r0 = 1;
        r1 = g;
        /* you can also set j to the position of the top most 1 bit */
        for (j = sizeof(k) * CHAR_BIT; j--; ) {
                /*
                fprintf(stderr, "\t[%d] r0 = %llu; r1 = %llu\n", j, r0, r1);
                */
                if ((k >> j) & 1) {
                        r0 = r0 * r1;
                        r1 = r1 * r1;
                } else {
                        r1 = r0 * r1;
                        r0 = r0 * r0;
                }
        }
        return r0;
}

static void ladder_test(unsigned g, unsigned k)
{
        unsigned long long r;
        r = ipow(g, k);
        fprintf(stdout, "%u^%u = %llu\n", g, k, r);
}

int main()
{
        ladder_test(3, 30);
        ladder_test(64, 7);
        return 0;
}