# 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 number with less than two digits.

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

• After doubling a digit, sum the digits. So doubling `7` becomes `14` which becomes `5`.

• Sum all the undoubled and doubled digits.

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

Copy the following code to https://play.rust-lang.org/ and implement the function:

``````// TODO: remove this when you're done with your implementation.

pub fn luhn(cc_number: &str) -> bool {
unimplemented!()
}

#[test]
fn test_non_digit_cc_number() {
assert!(!luhn("foo"));
}

#[test]
fn test_empty_cc_number() {
assert!(!luhn(""));
assert!(!luhn(" "));
assert!(!luhn("  "));
assert!(!luhn("    "));
}

#[test]
fn test_single_digit_cc_number() {
assert!(!luhn("0"));
}

#[test]
fn test_two_digit_cc_number() {
assert!(luhn(" 0 0 "));
}

#[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"));
}