Square and multiply

binexp.c

#include <stdio.h>
#include <stdint.h>

uint64_t bin_exp(uint32_t x, uint32_t k) {
    uint64_t res = 1;
    uint32_t mask = 1 << 31;

    while (mask) {
        res *= res;
        if (k & mask)
            res *= x;
        mask >>= 1;
    }

    return res;
}

int main(void) {
    for (int i = 1; i <= 15; i++)
        for (int j = 2; j <= 15; j++)
            printf("%d^%d = %llu\n", i, j, bin_exp(i, j));
}