Problem Statement

The Luhn test is used by some credit card companies to distinguish valid credit card numbers from what could be a random selection of digits. Those companies using credit card numbers that can be validated by the Luhn test have numbers that pass the following test:

For example, if the trial number is 49927398716:

Write a function/method/procedure/subroutine that will validate a number with the Luhn test, and use it to validate the following numbers:

Let's start with the solution:

let luhn s =
  let rec g r c = function
  | 0 -> r
  | i ->
      let d = c * ((int_of_char s.[i-1]) - 48) in 
      g (r + (d/10) + (d mod 10)) (3-c) (i-1)
  in
  (g 0 1 (String.length s)) mod 10 = 0
;;


# List.map luhn [ "49927398716"; "49927398717"; "1234567812345678"; "1234567812345670" ];;
- : bool list = [true; false; false; true]