Swift Implementation of Luhn Algorithm

luhnAlgorithm.swift

//: Playground - noun: a place where people can play
/*
From wikipedia:
From the rightmost digit, which is the check digit, moving left, double the value of every second digit; if the product of this doubling operation is greater than 9 (e.g., 8 × 2 = 16), then sum the digits of the products (e.g., 16: 1 + 6 = 7, 18: 1 + 8 = 9).
Take the sum of all the digits.
If the total modulo 10 is equal to 0 (if the total ends in zero) then the number is valid according to the Luhn formula; else it is not valid.

*/
import UIKit


func luhnAlgorithm(cardNumber: String) -> Bool{
    var luhn_sum = 0
    var digit_count = 0
    //reverse the card
    for c in cardNumber.characters.reverse() {
        //count digits
        print(c.self)
        let this_digit = Int(String(c as Character))!
        print(this_digit)
        digit_count++
        //double every even digit
        if digit_count % 2 == 0{
            if this_digit * 2 > 9 {
                luhn_sum = luhn_sum + this_digit * 2 - 9
            }else{
                luhn_sum = luhn_sum + this_digit * 2
            }
        }else{
            luhn_sum = luhn_sum + this_digit
        }
        
    }
    if luhn_sum % 10 == 0{
        return true
    }
    return false
}

luhnAlgorithm("6011497870742170")

let this_digit = Int(String(c as Character))! - it might be crash