using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("AMEX VALID: " + LuhnAlgorithm.ValidateCheckDigit("371449635398431"));

        Console.WriteLine("AMEX INVALID: " + LuhnAlgorithm.ValidateCheckDigit("371449635398439"));

        Console.WriteLine("MC VALID: " + LuhnAlgorithm.ValidateCheckDigit("5454545454545454"));

        Console.WriteLine("MC INVALID: " + LuhnAlgorithm.ValidateCheckDigit("5454545454545456"));

        Console.WriteLine("MC Corporate VALID: " + LuhnAlgorithm.ValidateCheckDigit("5132850000000008"));

        Console.WriteLine("MC Corporate INVALID: " + LuhnAlgorithm.ValidateCheckDigit("5132850000000009"));

        Console.WriteLine("MC Fleet VALID: " + LuhnAlgorithm.ValidateCheckDigit("5567356006664742"));

        Console.WriteLine("MC Fleet INVALID: " + LuhnAlgorithm.ValidateCheckDigit("5567356006664744"));

        Console.WriteLine("VISA VALID: " + LuhnAlgorithm.ValidateCheckDigit("4055011111111111"));

        Console.WriteLine("VISA INVALID: " + LuhnAlgorithm.ValidateCheckDigit("4055011111111114"));

        Console.WriteLine("DISCOVER VALID: " + LuhnAlgorithm.ValidateCheckDigit("6011000995500000"));

        Console.WriteLine("DISCOVER INVALID: " + LuhnAlgorithm.ValidateCheckDigit("6011000995500009"));

        Console.WriteLine("US DEBIT EBT VALID: " + LuhnAlgorithm.ValidateCheckDigit("9999999800002773"));

        Console.WriteLine("US DEBIT EBT INVALID: " + LuhnAlgorithm.ValidateCheckDigit("9999999800002775"));

    }

}

​

public static class LuhnAlgorithm

{

   private static readonly Int32[] _doubledValues = new Int32[] { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };

​

   public static Boolean ValidateCheckDigit(String str)

   {

      if (String.IsNullOrEmpty(str) || str.Length < 2)

      {

         return false;

      }

​

      var sum = 0;

      var shouldApplyDouble = true;

      for (var index = str.Length - 2; index >= 0; index--)

      {

         var currentDigit = str[index] - '0';

         if (currentDigit < 0 || currentDigit > 9)

         {

            return false;

         }

         sum += shouldApplyDouble ? _doubledValues[currentDigit] : currentDigit;

         shouldApplyDouble = !shouldApplyDouble;

      }

      var checkDigit = (10 - (sum % 10)) % 10;

​

      return str[^1] - '0' == checkDigit;

   }

}