The Math Trick Hidden in Your Credit Card Number

3 comments

• tromp 2 days ago

  Expressed in Haskell:

      import Data.Char
  
      card = "4024 6072 3695 0748"
  
      cardDigits = map digitToInt . filter (not . isSpace)
  
      doubleDigitSum d = 2 * d - if d<5 then 0 else 9
  
      doubleEveryOther = reverse . zipWith ($) (cycle [doubleDigitSum,id]) . reverse
     
      main = let
          cd = cardDigits $ card
          sd = sum . doubleEveryOther . init $ cd
          checkDigit = (- (sd + last cd)) `mod` 10
        in print checkDigit

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  • bediger4000 2 days ago

    I'm not conversant in Haskell, but this probably won't work on American Express card numbers - they have 15 digits, the final digit is still the Luhn checksum.

    I did credit card processing 20 years ago - AmEx was very often the fly in the ointment.

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    • tromp 2 days ago

      The above code should work for any number of digits. It follows the spec given in the article, specifically:

          1. Write out all but the last digit of the card number.
      
          2. Write out all but the last digit of the card number.
      
          3. Starting from the right, double every other number.