/*
 * fact.c
 *
 * Compute factorial of input integer
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "mpi.h"

mp_err mp_fact(mp_int *a, mp_int *b);

int main(int argc, char *argv[])
{
  mp_int  a;
  mp_err  res;

  if(argc < 2) {
    fprintf(stderr, "Usage: %s <number>\n", argv[0]);
    return 1;
  }

  mp_init(&a);
  mp_read_radix(&a, argv[1], 10);

  if((res = mp_fact(&a, &a)) != MP_OKAY) {
    fprintf(stderr, "%s: error: %s\n", argv[0],
	    mp_strerror(res));
    mp_clear(&a);
    return 1;
  }

  {
    char  *buf;
    int    len;

    len = mp_radix_size(&a, 10);
    buf = malloc(len);
    mp_todecimal(&a, buf);

    puts(buf);

    free(buf);
  }

  mp_clear(&a);
  return 0;
}

mp_err mp_fact(mp_int *a, mp_int *b)
{
  mp_int    ix, s;
  mp_err    res = MP_OKAY;

  if(mp_cmp_z(a) < 0)
    return MP_UNDEF;

  mp_init(&s);
  mp_add_d(&s, 1, &s);   /* s = 1  */
  mp_init(&ix);
  mp_add_d(&ix, 1, &ix); /* ix = 1 */

  for(/*  */; mp_cmp(&ix, a) <= 0; mp_add_d(&ix, 1, &ix)) {
    if((res = mp_mul(&s, &ix, &s)) != MP_OKAY)
      break;
  }

  mp_clear(&ix);

  /* Copy out results if we got them */
  if(res == MP_OKAY)
    mp_copy(&s, b);

  mp_clear(&s);

  return res;
}