Calculation of factorial and sum of number digits with GMP. Timing implemented for OS X.

gmp_test.c

#include <stdio.h>
#include <string.h>
#include <time.h>
#include <mach/mach_time.h>
#include <gmp.h>

void factorial(mpz_t n, mpz_t res)
{
  mpz_t unit, zero, cnt;
	mpz_init_set_ui (unit, 1);
	mpz_init_set_ui (zero, 0);
	mpz_init_set_ui (cnt, 1);
	
	mpz_set (res, unit);
	while(mpz_cmp (cnt, n) <= 0) {
		mpz_mul (res, res, cnt);
		mpz_add (cnt, cnt, unit);
	}
}

unsigned long sum_digits(mpz_t n)
{
	unsigned long ret = 0, i_cnt;
	char *t = mpz_get_str(NULL, 10, n);

	for(i_cnt = 0; i_cnt < strlen(t); ++i_cnt) {
		ret += (unsigned long) (t[i_cnt] - '0');
	}

	return ret;
}

void do_calc(void)
{
	unsigned long nums[] = {1, 12, 123, 1234, 12345, 123456};
	unsigned int lim = sizeof(nums) / sizeof(unsigned long);
	unsigned long i_cnt, ret = 0;

	for(i_cnt = 0; i_cnt < lim; ++i_cnt) {
		mpz_t a, b;
		mpz_init_set_ui (a, nums[i_cnt]);
		mpz_init (b);

		factorial (a, b);
		ret += sum_digits(b);
	}

	printf("%lu\n", ret);
}

double time_it(void (*func)(void), int n) {
	double total_duration;

	mach_timebase_info_data_t info;
	mach_timebase_info(&info);

	uint64_t start;
	uint64_t duration;

	int i;

	for(i = 0; i < n; ++i) {
		start = mach_absolute_time();
		func();
		duration = mach_absolute_time() - start;
	
		duration *= info.numer;
		duration /= info.denom;

		total_duration += duration;
	}

	return total_duration / 1e9 / n;
}

int main(int argc, char *argv[])
{
	printf("Elapsed time: %f s\n", time_it(do_calc, 10));
}