fast exponentiation

pow.c

// main.c
// Mark Roberts
// Thanks:
//     http://stackoverflow.com/questions/101439

#include <stdlib.h>
#include <stdio.h>
#include <stdint.h> // uint64_t => [0, ULONG_LONG_MAX]

// Same algorithm referenced on StackOverflow:
//     http://en.wikipedia.org/wiki/Modular_exponentiation
uint64_t ipow(uint64_t base, uint64_t exp) {
	uint64_t result=1;
	while (exp) {
		if (exp&1) {
			result *= base;
		}
		exp >>= 1;
		base *= base;
	}
	return result;
}

int main(int argc, char **argv[]) {
	if (argc != 3) {
		fprintf(stderr, "Usage: exp BASE POW\n");
		exit(2);
	}
	uint64_t a, b;
	if (sscanf((char *)argv[1], "%llu", &a) < 1) { // should match a uint64_t
		fprintf(stderr, "ERROR: `BASE' was not uint64_t\n"); // else error
		exit(1);
	}
	if (sscanf((char *)argv[2], "%llu", &b) < 1) { // should match a uint64_t
		fprintf(stderr, "ERROR: `POW' was not uint64_t\n"); // else error
		exit(1);
	}
	printf("%llu\n", ipow(a, b)); // print a uint64_t
	exit(0);
}