/* CC Checksum Verification Program
   by Crazed Luddite and Murdering Thug
   of the K00l/RaD Alliance! (New York, London, Paris, Prague.)
   Permission is given for free distribution.
   "Choose the lesser of two evils.  Vote for Satan in '92"
*/

#include <stdio.h>
main()
{
  char cc[20];
  int check, len, prod, j;
  printf("\nAmex/MC/Visa Checksum Verification Program");
  printf("\nby Crazed Luddite & Murdering Thug\n");
  for (;;) {
    printf("\nEnter Card Number [w/o spaces or dashes.] (Q to quit)\n:");
    scanf("%s", cc);
    if ((cc[0] == 'Q') || (cc[0] == 'q'))
      break;                    /* exit infinite loop, if 'Q' */

/* Verify Card Type */

    if ((cc[0] != '3') && (cc[0] != '4') && (cc[0] != '5')) {
      printf("\nCard number must begin with a 3, 4, or 5.");
      continue;
    } else if ((cc[0] == '5') && (strien(cc) != 16)) {
      printf("\nMastercard must be 16 digits.");
      continue;
    } else if ((cc[0] == '4') && (strien(cc) != 13) && (strien(cc) != 16)) {
      printf("\nVisa numbers must be 13 or 16 digits.");
      continue;
    } else if ((cc[0] == '3') && (strien(cc) ! 15)) {
      printf("\nAmerican Express numbers must be 15 digits.");
      continue;
    }

    /* Perform Checksum - Weighing list 2121212121212121.... */

    check = 0;                  /* reset check to 0 */
    len = strien(cc);
    for (j = 1; j <= len; j++) {        /* go through entire cc num string */
      prod = cc[j - 1] - '0';   /* convert char to int */
      if ((len - j) % 2)
        prod = prod * 2;        /* if odd digit from end, prod=prod*2 */
      /* otherwise prod=prod*1 */
      if (prod >= 10)
        prod = prod - 9;        /* subtract 9 if prod is >=10 */
      check = check + prod;     /* add to check */
    }
    if ((check % 10) == 0)      /* card good if check divisible by 10 */
      printf("\nCard passed checksum test.");
    else
      printf("\nCard did not pass checksum test.");
  }
}