I recently came across some programming challenges online that I decided to try out. They are heavily math based and took me quite a bit of research. This particular challenge was to both calculate a check digit on a credit card and validate a credit card number. The algorithm I found that does this best is called Luhn Algorithm.

Luhn Algorithm is described as the following (via Wikipedia):

- From the right most digit, which is the
*check digit*, moving left, double the value of every second digit; if the product of this doubling operation is greater than 9 (e.g., 8 × 2 = 16), then sum the digits of the products (e.g., 16: 1 + 6 = 7, 18: 1 + 8 = 9) or alternatively subtract 9 from the product (e.g., 16: 16 - 9 = 7, 18: 18 - 9 = 9). - Take the sum of all the digits.
- If the total modulo 10 is equal to 0 (if the total ends in zero) then the number is valid according to the Luhn formula; else it is not valid.

Based on this description, I came up with the following code:

public static class LuhnAlgorithm { private static readonly int[] DoubleDigitCalculation = { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 }; public static int GetLuhnsCheckDigit(this string account) { int checkValue = LuhnsCalculation(account.Select(c => c - '0').ToArray(), false); return checkValue == 0 ? 0 : 10 - checkValue; } public static bool LuhnsPass(this string account) { return LuhnsCalculation(account.Select(c => c - '0').ToArray(), true) == 0; } private static int LuhnsCalculation(int[] digits, bool includesCheckDigit) { int index = 0; int modIndex = includesCheckDigit ? digits.Length % 2 : digits.Length % 2 == 1 ? 0 : 1; return digits.Sum(d => index++ % 2 == modIndex ? DoubleDigitCalculation[d] : d) % 10; } }

This bit of code passed all the tests and is very fast. Although it doesn't have a ton of use, it was fun to come up with.

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