luhn check sum

luhn_check_sum.swift

func luhnCheckSum(number stringWithoutSpaces: String) -> Bool {
    guard let _ = Int(stringWithoutSpaces) else { return false }
    var even = 0
    var sum = 0
    for char in stringWithoutSpaces.reversed() {
        guard let decimals = Int(String(char)) else { continue }
        let added: Int = decimals * ( 1 << (even & 1) )
        even = even + 1
        sum = sum + added % 10 + added / 10
    }
    return sum % 10 == 0
}