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:

func luhn(_ number: String) -> Bool {
    return number.reversed().enumerated().map({
        let digit = Int(String($0.element))!
        let even = $0.offset % 2 == 0
        return even ? digit : digit == 9 ? 9 : digit * 2 % 9
    }).reduce(0, +) % 10 == 0
}

luhn("49927398716") // true
luhn("49927398717") // false