# LĂ¶sungen

// This is the buggy version that appears in the problem.
#[cfg(never)]
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
}

// This is the solution and passes all of the tests below.
pub fn luhn(cc_number: &str) -> bool {
let mut sum = 0;
let mut double = false;
let mut digits = 0;

for c in cc_number.chars().rev() {
if let Some(digit) = c.to_digit(10) {
digits += 1;
if double {
let double_digit = digit * 2;
sum +=
if double_digit > 9 { double_digit - 9 } else { double_digit };
} else {
sum += digit;
}
double = !double;
} else if c.is_whitespace() {
continue;
} else {
return false;
}
}

digits >= 2 && sum % 10 == 0
}

fn main() {
let cc_number = "1234 5678 1234 5670";
println!(
"Is {cc_number} a valid credit card number? {}",
if luhn(cc_number) { "yes" } else { "no" }
);
}

#[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"));
}

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

#[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 "));
}
}