/* 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.");
    }
}