# Luhn-Algorithmus

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 number with less 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 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.

``````#![allow(unused)]
fn main() {
pub fn luhn(cc_number: &str) -> bool {
let mut sum = 0;
let mut double = false;

for c in cc_number.chars().rev() {
if let Some(digit) = c.to_digit(10) {
if double {
let double_digit = digit * 2;
sum +=
if double_digit > 9 { double_digit - 9 } else { double_digit };
} else {
sum += digit;
}
double = !double;
} else {
continue;
}
}

sum % 10 == 0
}

#[cfg(test)]
mod test {
use super::*;

#[test]
fn test_valid_cc_number() {
assert!(luhn("4263 9826 4026 9299"));
assert!(luhn("4539 3195 0343 6467"));
assert!(luhn("7992 7398 713"));
}

#[test]
fn test_invalid_cc_number() {
assert!(!luhn("4223 9826 4026 9299"));
assert!(!luhn("4539 3195 0343 6476"));
assert!(!luhn("8273 1232 7352 0569"));
}
}
}``````