/*

Testing Big-Number library by calculating factorial(100) a.k.a. 100!

====================================================================

For the uninitiated:

factorial(N) := N * (N-1) * (N-2) * ... * 1

Example:

factorial(5) = 5 * 4 * 3 * 2 * 1 = 120

Validated by Python implementation of big-numbers:

--------------------------------------------------

In [1]: import math

In [2]: "%x" % math.factorial(100)

Out[]: '1b30964ec395dc24069528d54bbda40d16e966ef9a70eb21b5b2943a321cdf10391745570cca9420c6ecb3b72ed2ee8b02ea2735c61a000000000000000000000000'

... which should also be the result of this program's calculation

*/

#include <stdio.h>

#include "bn.h"

void factorial(struct bn* n, struct bn* res)

{

struct bn tmp;

/* Copy n -> tmp */

bignum_assign(&tmp, n);

/* Decrement n by one */

bignum_dec(n);

/* Begin summing products: */

while (!bignum_is_zero(n))

{

/* res = tmp * n */

bignum_mul(&tmp, n, res);

/* n -= 1 */

bignum_dec(n);

/* tmp = res */

bignum_assign(&tmp, res);

}

/* res = tmp */

bignum_assign(res, &tmp);

}

int main()

{

struct bn num;

struct bn result;

char buf[8192];

bignum_from_int(&num, 100);

factorial(&num, &result);

bignum_to_string(&result, buf, sizeof(buf));

printf("factorial(100) using bignum = %s\n", buf);

return 0;

}