using System;

​

public class Program

{

    // generates a random number

    static Random rng = new Random();

    public static void Main()

    {

        BeginCCValidation();

        ProgramIntroduction();

        // for random number generator

         int[] digits;

         digits[i] = rng.Next(0, 10);

    }

    static string MenuLoop(string userInput)

    {

        userInput = Console.ReadLine();

        if (userInput.ToLower() = "validate")

        {

            BeginCCValidation();

            ProgramIntroduction();

        }

    }

    static int GenerateCCNumber(int[] digits)

    {

            for (int i = 0; i < digits.Length; i++)

        {

        }

    }

​

​

​

    static void ProgramIntroduction()

    {

        Console.WriteLine("program is a credit card number validator and generator, enter your credit card number");

    }

​

    static string GetCreditCardNumber()

    {

        Console.WriteLine("Credit Card Validator and Generator");

        return Console.ReadLine();

    }

​

    // validate an inputted number using the Luhn Sum to verify if it is a valid credit card number

    static void ValidateCCNumber(string number)

    {

        // create an array that is the same size as the strings length

        int[] digits = new int[number.Length];

        // go through each character in the number string one at a time

        for (int i = 0; i < digits.Length; i++)

        {

            // loop for conversion of one number at a time

            if (int.TryParse(number.Substring(i, 1), out digits[i]) == false)

            {

                return;

            }

        }

        // Call a function with parameters -- save the return value and pass in a parameter through the brackets

        int sum = CalculateLuhnSum(digits);

        // Call a function with parameters again

        if (ValidateLuhnSum(sum) == true)

        {

            Console.WriteLine("Valid");

        }

        else

        {

            Console.WriteLine("InValid");

        }

    }

​

    // calculate the luhn sum which is described b the Luhn algorithm

    static int CalculateLuhnSum(int[] digits)

    {

        // double every other digit starting from the 2nd last

        for (int i = digits.Length - 2; i >= 0; i = i - 2)

        {

            // double the current digit

            digits[i] = digits[i] * 2;

            // 5 * 2 = 10 --> 1 + 0 = 1

            // 6 * 2 = 12 --> 1 + 2 = 3

            // 14 --> 1 + 4 = 5

            // 16 --> 1 + 6 = 7

            // this is to split the double digit numbers to be the sum of the digits

            if (digits[i] >= 10)

            {

                digits[i] = digits[i] - 9;

            }

        }

​

        // calculate the sum of the new digits

        int sum = 0;

        for (int i = 0; i < digits.Length; i++)

        {

            sum = sum + digits[i];

        }

​

        return sum;

    }

​

    // returns true if luhn sum is divisible by ten

    static bool ValidateLuhnSum(int luhnSum)

    {       

        // repeatedly subtract 10 while the sum is not a single digit

        while (luhnSum >= 10)   

        {

            luhnSum -= 10;

        }

        // if all the subtraction leads to 0, then it was divisible by 10

        if (luhnSum == 0)

        {

            return true;

        }

        // if the subraction doesnt lead to 0, it was not divisble  by 10

        else 

        {

            return false;

        }

    }

    // Starts the Credit Card Validitation

    static void BeginCCValidation()

    {

        // Call a procedure with no parameters

        ProgramIntroduction();

        // Call a function with no parametrs -- save its return value

        string ccNumber = GetCreditCardNumber();

        // Call a procedure with a parameter -- need to pass the parameter through the brackers!

        ValidateCCNumber(ccNumber);

    }

}