Luhn Algorithm in Swift

Luhn.swift

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
}

luhnCheck("49927398716")
luhnCheck("49927398717")