Luhn test in Swift.

luhnTest

func lunhCheck(number : String) -> Bool
{
    let reversed = reverse(number).map { String($0).toInt()! }
    return reduce(enumerate(reversed), 0, {(sum, val) in
        let odd = val.index % 2 == 1
        return sum + (odd ? (val.element == 9 ? 9 : (val.element * 2) % 9) : val.element)
        }) % 10 == 0
}

lunhCheck("49927398716")
lunhCheck("49927398717")

This was helpful, thanks for posting. I however found the nested ternaries very difficult to read, I also didn't like the forced unwrapping. So, I came up with the following, hope this can help anyone who lands here.

func luhnCheck(number: String) -> Bool {
    var sum = 0
    let digitStrings = reverse(number).map { String($0) }

    for tuple in enumerate(digitStrings) {
        if let digit = tuple.element.toInt() {
            let odd = tuple.index % 2 == 1

            switch (odd, digit) {
            case (true, 9):
                sum += 9
            case (true, 0...8):
                sum += (digit * 2) % 9
            default:
                sum += digit
            }
        } else {
            return false
        }
    }

    return sum % 10 == 0
}