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")

Influenced by: https://gist.github.com/jonelf/bccf58698adc12e53fd6

Swift 2:

private static func luhnCheck(cardNumber: String) -> Bool {
        var sum = 0
        let reversedCharacters = cardNumber.characters.reverse().map { String($0) }
        for (idx, element) in reversedCharacters.enumerate() {
            guard let digit = Int(element) else { return false }
            switch ((idx % 2 == 1), digit) {
            case (true, 9): sum += 9
            case (true, 0...8): sum += (digit * 2) % 9
            default: sum += digit
            }
        }
        return sum % 10 == 0
    }

Swift 3

private static func luhnCheck(cardNumber: String) -> Bool {
        var sum = 0
        let reversedCharacters = cardNumber.characters.reversed().map { String($0) }
        for (idx, element) in reversedCharacters.enumerated() {
            guard let digit = Int(element) else { return false }
            switch ((idx % 2 == 1), digit) {
            case (true, 9): sum += 9
            case (true, 0...8): sum += (digit * 2) % 9
            default: sum += digit
            }
        }
        return sum % 10 == 0
    }

Swift 4

extension String {
    func luhnCheck() -> Bool {
        var sum = 0
        let reversedCharacters = self.reversed().map { String($0) }
        for (idx, element) in reversedCharacters.enumerated() {
            guard let digit = Int(element) else { return false }
            switch ((idx % 2 == 1), digit) {
            case (true, 9): sum += 9
            case (true, 0...8): sum += (digit * 2) % 9
            default: sum += digit
            }
        }
        return sum % 10 == 0
    }
}

Swift 5

private static func luhnCheck(cardNumber: String) -> Bool {
    var sum = 0
    let reversedCharacters = cardNumber.reversed().map { String($0) }
    for (idx, element) in reversedCharacters.enumerated() {
        guard let digit = Int(element) else { return false }
        switch ((idx % 2 == 1), digit) {
        case (true, 9): sum += 9
        case (true, 0...8): sum += (digit * 2) % 9
        default: sum += digit
        }
    }
    return sum % 10 == 0
}