// Function: luhn_check
// Description: Runs Luhn's algorithm on a given number.
// Uses Functions: get_last_digit
// Parameters: Number being tested.
// Pre-Conditions: Positive integer input.
// Post-Conditions: Bool output.
// Return: false - passes algorithm, true - does not pass algorithm.
public static bool luhn_check(ulong num)
{
int sum = 0;
int checkDigit = CardFunctions.get_last_digit(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 (everyOtherNumber % 2 == 0)
{
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(true);
}
else
{
return(false);
}
}
// Function: get_corrected_number
// Description: Runs Luhn's algorithm on a given number, and then returns the number with the proper check digit.
// Uses Functions: get_last_digit
// Parameters: Number being corrected.
// Pre-Conditions: Positive integer input.
// Post-Conditions: Number passes Luhn's algorithm.
// Return:
public static ulong get_corrected_number(ulong num)
{
int sum = 0;
int checkDigit = CardFunctions.get_last_digit(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 (everyOtherNumber % 2 == 0)
{
digits[i] *= 2;
if (digits[i] > 9)
{
digits[i] -= 9;
}
}
sum += digits[i];
everyOtherNumber++;
}
return(num - Convert.ToUInt32(checkDigit) + Convert.ToUInt32(sum * 9) % 10); // Replace the last digit with the correct checkDigit.
}
// Event function: textEntryBox_TextChanged
// Activates when the text entry box text changes.
private void textEntryBox_TextChanged(object sender, EventArgs e)
{
ulong numberEntry = 0;
string displayMessage = "Enter a card number!";
bool exceptionThrown = false;
correctedNumber.Text = "";
try
{
numberEntry = Convert.ToUInt64(textEntryBox.Text);
}
catch (System.FormatException)
{
exceptionThrown = true;
}
if (exceptionThrown) // If user did not enter a proper number.
{
pictureBox.Image = Properties.Resources.failure;
displayMessage = "Not a valid card number!";
}
else if (!CardFunctions.is_correct_length(numberEntry)) // If card number is not correct length.
{
pictureBox.Image = Properties.Resources.failure;
displayMessage = "Card number is not the correct length!";
}
else if (CardFunctions.luhn_check(numberEntry)) // If card number does not pass luhn's algorithm.
{
pictureBox.Image = Properties.Resources.failure;
displayMessage = "Card number does not pass Luhn's Algorithm!";
correctedNumber.Text = "*Corrected number: " + CardFunctions.get_corrected_number(numberEntry);
}
else // Card number is valid.
{
pictureBox.Image = Properties.Resources.success;
displayMessage = "This is a valid card number!";
}
titleLabel.Text = displayMessage;
}