/* HardCore SoftWare  -  Arbitrary Precision Factorial Finder

   (C) 2002, Fractal (Doug Hoyte) of HardCore SoftWare (www.hcsw.org)

   This software is protected by the GNU GPL, see www.fsf.org

   compilation: gcc fact.c -o fact
*/


#include <stdio.h>

#define MAXDIGITS 10000



int char2num(char tp) {
  if (tp == '\0') return 0;
  return (int) tp - 48;
}

int num2char(char tp) {
  return (int) tp + 48;
}


void alpha2mp(char *alpha, char *mp) {

  int i,len = strlen(alpha);

  memset(mp, '\0', MAXDIGITS);
  for (i=0; i < len; i++) mp[i] = alpha[len-i-1];

}


void mp2alpha(char *mp, char *alpha, int maxlen) {

  int i,len = strlen(mp);

  memset(alpha, '\0', maxlen);
  for (i=0; (i < len) && (i < maxlen); i++) alpha[i] = mp[len-i-1];

}


void add2mps(char *mp1, char *mp2) {

  int tp,i=0,carry=0;

  while((mp1[i] != '\0') || (mp2[i] != '\0')) {
    tp = char2num(mp1[i]) + char2num(mp2[i]) + carry;
    if (tp > 9) carry=1; 
      else carry = 0;
    tp %= 10;
    mp1[i] = num2char(tp);
    i++;
  }

  if (carry) mp1[i] = '1';

}



/* Doesn't work for numbers < 1 */
void multmp(char *mp1, int imp2) {

  char tp[MAXDIGITS];
  int i;

  memset(tp, '\0', MAXDIGITS);
  strcpy(tp, mp1);

  for(i=0; i<imp2-1; i++) add2mps(mp1, tp);


}


void factorial(char *mp) {

  int imp;
  char tp[MAXDIGITS];

  memset(tp, '\0', MAXDIGITS);
  mp2alpha(mp, tp, MAXDIGITS);
  imp = atoi(tp);

  for (imp--; imp>1; imp--) multmp(mp, imp);

}


usage() {

  fprintf(stderr, "HCSW Factorial Finder\n\nusage: fact <number>\n\n");
  exit(0);

}


main (int argc, char *argv[]) {

  char tp[MAXDIGITS];
  char tp2[MAXDIGITS];
  int i;

  if (argc != 2) usage();
  for (i=0; i<strlen(argv[1]); i++) if (!isdigit(argv[1][i])) usage(); 

  alpha2mp(argv[1], tp);
  factorial(tp);
  mp2alpha(tp, tp2, MAXDIGITS);

  if (argv[1][1] == '\0' && argv[1][0] == '0')
    printf("Factorial of \"0\" is \"1\"\n");
  else printf("Factorial of \"%s\" is \"%s\"\n", argv[1], tp2);

}