Luhn Algorithm for generating random valid credit card numbers with swift

luhn_algorithm.swift

var card_number = ""

func make_random_number(num:Int)->[Int]{
    var result = [Int]()
    for _ in 0...num {
        result.append(Int(arc4random_uniform(9)))
    }
    return result
}
func luhn_algorithm(){
    
    var random_master_int = make_random_number(num: 12)
    //Add random master card numbers here [51,52,53,54]
    random_master_int.insert(5, at: 0)
    random_master_int.insert(4, at: 1)
    print(random_master_int)
    
    for (index,element) in random_master_int.enumerated(){
        if index % 2 != 0 {
            var r = random_master_int[index]  * 2
            var characters = String(r).characters.flatMap{Int(String($0))}
            var sum = characters.reduce(0, +)
            print(sum)
        }
    }
    
    
    var sum = random_master_int.reduce(0, +) * 9
    random_master_int.append(sum % 10)

    var random_master_string = random_master_int.map {String($0)}
    random_master_string
    
    
    var first4 = "\(random_master_string[0...3].flatMap({$0}).joined())"
    var second4 = "\(random_master_string[4...7].flatMap({$0}).joined())"
    var third4 = "\(random_master_string[8...11].flatMap({$0}).joined())"
    var fourth4 = "\(random_master_string[12...15].flatMap({$0}).joined())"
    card_number.append(first4)
    card_number.append(second4)
    card_number.append(third4)
    card_number.append(fourth4)

    print(card_number)
}