Exercise: Luhn Algorithm

The Luhn algorithm is used to validate credit card numbers. The algorithm takes a string as input and does the following to validate the credit card number:

  • Ignore all spaces. Reject numbers with fewer than two digits.

  • Moving from right to left, double every second digit: for the number 1234, we double 3 and 1. For the number 98765, we double 6 and 8.

  • After doubling a digit, sum the digits if the result is greater than 9. So doubling 7 becomes 14 which becomes 1 + 4 = 5.

  • Sum all the undoubled and doubled digits.

  • The credit card number is valid if the sum ends with 0.

The provided code provides a buggy implementation of the luhn algorithm, along with two basic unit tests that confirm that most of the algorithm is implemented correctly.

Copy the code below to https://play.rust-lang.org/ and write additional tests to uncover bugs in the provided implementation, fixing any bugs you find.

  1. #![allow(unused)]
  2. fn main() {
  3. pub fn luhn(cc_number: &str) -> bool {
  4.     let mut sum = 0;
  5.     let mut double = false;
  7.     for c in cc_number.chars().rev() {
  8.         if let Some(digit) = c.to_digit(10) {
  9.             if double {
  10.                 let double_digit = digit * 2;
  11.                 sum +=
  12.                     if double_digit > 9 { double_digit - 9 } else { double_digit };
  13.             } else {
  14.                 sum += digit;
  15.             }
  16.             double = !double;
  17.         } else {
  18.             continue;
  19.         }
  20.     }
  22.     sum % 10 == 0
  23. }
  25. #[cfg(test)]
  26. mod test {
  27.     use super::*;
  29.     #[test]
  30.     fn test_valid_cc_number() {
  31.         assert!(luhn("4263 9826 4026 9299"));
  32.         assert!(luhn("4539 3195 0343 6467"));
  33.         assert!(luhn("7992 7398 713"));
  34.     }
  36.     #[test]
  37.     fn test_invalid_cc_number() {
  38.         assert!(!luhn("4223 9826 4026 9299"));
  39.         assert!(!luhn("4539 3195 0343 6476"));
  40.         assert!(!luhn("8273 1232 7352 0569"));
  41.     }
  42. }
  43. }