// Function: luhnCheck
// Description: Runs Luhn's algorithm on a given number.
// Uses Functions: get_check_num
// Parameters: Number being tested.
// Pre-Conditions: Positive integer input.
// Post-Conditions: Output must be 0 or 1.
// Return: 0 - passes algorithm, 1 - does not pass algorithm.
public static int luhn_check(ulong num)
{
int sum = 0;
int checkDigit = CardFunctions.get_check_num(num);
int[] digits = num.ToString().Select(Convert.ToInt32).ToArray(); // Convert num into array of digits.
int everyOtherNumber = 0; // For tracking every other number.
for (int i = digits.Length - 2; i >= 0; i--) // Run through digits in reverse order, skipping the ending digit.
{
digits[i] -= 48; // Convert char value to digit value.
if (i % 2 == 1)
{
digits[i] *= 2;
if (digits[i] > 9)
{
digits[i] -= 9;
}
}
sum += digits[i];
everyOtherNumber++;
}
digits[digits.Length - 1] -= 48; // Convert check digit char value to digit value.
// Number does not pass the test if the calculated check digit does not match the original last digit.
if ((sum * 9) % 10 != checkDigit)
{
return(1);
}
else
{
return(0);
}
}
static void Main(string[] args)
{
Console.Write("Enter card number: ");
ulong num = Convert.ToUInt64(Console.ReadLine());
Console.WriteLine("Result: " + CardFunctions.luhn_check(num));
Console.ReadKey();
}