Luhn algorithm

Luhn.swift

// https://en.wikipedia.org/wiki/Luhn_algorithm

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