CS50 Credit

credit.c

/* CS50 solution. Feel free to check with any online compiler. To get credit from Harvard you may have to make a few changes.
hmu at s.velaga@uky.edu if you have any questions. Also I haven't learned about proper code style and formatting - 
if you wanna hmu to tell me this looks like shit and how to make it look better pls do */

#include <stdio.h>
#include <inttypes.h>

int main ()

{
  printf ("Please enter a valid credit card number (without spaces) to calculate credit:\n");

  unsigned long long c;
  scanf ("%llu", &c);

  printf ("You entered: %llu", c);

  /* defines the intengers we are gonna use.
     dig = "digit", eod = "every other digit" */

  int dig, eod, eosum, sum, prod, sumprod, checksum, count;
  sum = 0;
  sumprod = 0;
  eosum = 0;
  count = 0;

  /* sld is the second to last digit in the credit card # */

  unsigned long long sld;
  sld = c / 10;

  /* PART A: counts how many digits are in the credit card # */

  unsigned long long ccount;
  ccount = sld * 10;

  while (ccount > 0)
    {
      count++;
      ccount = ccount / 10;
    }

  printf ("\nThe count is %d", count);

  /* PART B: gets the first two digits and the first digit that are in the credit card # */

  unsigned long long ccnum;
  ccnum = sld * 10;

  int firsttwodigits;
  int firstdigit;

  while (ccnum > 100)
    {
      ccnum = ccnum / 10;
      firsttwodigits = ccnum % 100;
    }

  firstdigit = firsttwodigits / 10 % 10;

  printf ("\nfirst two digits are %d", firsttwodigits);
  printf ("\nfirst digit is %d", firstdigit);

  /* PART C : this part adds up every other digit that was NOT multiplied by 2 */

  while (c > 0)
    {
      eod = c % 10;
      eosum += eod;
      c = c / 100;
    }

  /* PART D : this part multiplies every other digit by 2 and adds them up */

  while (sld > 0)
    {
      dig = sld % 10;
      prod = dig * 2;
      while (prod > 9)
	{
	  prod -= 9;
	}
      sumprod += prod;
      sld = sld / 100;
    }

  /* PART E : this part adds the sums found in PART C and PART D together */

  checksum = eosum + sumprod;

  printf ("\n\nSum of every other digit %d", eosum);
  printf ("\nSum of every other digit multiplied by 2 is %d", sumprod);

  printf ("\n\nChecksum is %d", checksum);

  /* PART F: If the checksum ends with a 0 and the credit card # has either
     13, 15, or 16 total numbers then it is valid */

  if (checksum % 10 == 0 && (count == 13 || count == 15 || count == 16))
    {
      printf ("\nThe credit card number you entered is correct");


      if (count == 15 && (firsttwodigits == 34 || firsttwodigits == 37))

	{
	  printf ("\nYour credit card is AMEX");
	}

      else if (count == 16 && (firsttwodigits >= 51 && firsttwodigits <= 55))

	{
	  printf ("\nYour credit card is MasterCard");
	}

      else if (count == 13 || count == 16 && (firstdigit == 4))

	{
	  printf ("Your credit card is VISA");
	}

    }

  else
    {
      printf ("\nI am sorry. The number you entered in is invalid");
    }

  return 0;
}