Solution

  1. // This is the buggy version that appears in the problem.
  2. #[cfg(never)]
  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. // This is the solution and passes all of the tests below.
  26. pub fn luhn(cc_number: &str) -> bool {
  27.     let mut sum = 0;
  28.     let mut double = false;
  29.     let mut digits = 0;
  31.     for c in cc_number.chars().rev() {
  32.         if let Some(digit) = c.to_digit(10) {
  33.             digits += 1;
  34.             if double {
  35.                 let double_digit = digit * 2;
  36.                 sum +=
  37.                     if double_digit > 9 { double_digit - 9 } else { double_digit };
  38.             } else {
  39.                 sum += digit;
  40.             }
  41.             double = !double;
  42.         } else if c.is_whitespace() {
  43.             // New: accept whitespace.
  44.             continue;
  45.         } else {
  46.             // New: reject all other characters.
  47.             return false;
  48.         }
  49.     }
  51.     // New: check that we have at least two digits
  52.     digits >= 2 && sum % 10 == 0
  53. }
  55. fn main() {
  56.     let cc_number = "1234 5678 1234 5670";
  57.     println!(
  58.         "Is {cc_number} a valid credit card number? {}",
  59.         if luhn(cc_number) { "yes" } else { "no" }
  60.     );
  61. }
  63. #[cfg(test)]
  64. mod test {
  65.     use super::*;
  67.     #[test]
  68.     fn test_valid_cc_number() {
  69.         assert!(luhn("4263 9826 4026 9299"));
  70.         assert!(luhn("4539 3195 0343 6467"));
  71.         assert!(luhn("7992 7398 713"));
  72.     }
  74.     #[test]
  75.     fn test_invalid_cc_number() {
  76.         assert!(!luhn("4223 9826 4026 9299"));
  77.         assert!(!luhn("4539 3195 0343 6476"));
  78.         assert!(!luhn("8273 1232 7352 0569"));
  79.     }
  81.     #[test]
  82.     fn test_non_digit_cc_number() {
  83.         assert!(!luhn("foo"));
  84.         assert!(!luhn("foo 0 0"));
  85.     }
  87.     #[test]
  88.     fn test_empty_cc_number() {
  89.         assert!(!luhn(""));
  90.         assert!(!luhn(" "));
  91.         assert!(!luhn("  "));
  92.         assert!(!luhn("    "));
  93.     }
  95.     #[test]
  96.     fn test_single_digit_cc_number() {
  97.         assert!(!luhn("0"));
  98.     }
  100.     #[test]
  101.     fn test_two_digit_cc_number() {
  102.         assert!(luhn(" 0 0 "));
  103.     }
  104. }