public/subjects/luhn_algorithm

luhn_algorithm

Instructions

Create a function which checks if a number is valid per the Luhn formula.

The function will receive a string and return a boolean.

Invalid inputs: An empty string or a number with only one digit are considered invalid. Handling spaces: Spaces are accepted but have to be ignored during the calculation (in other words they won't affect the result).

The Luhn formula is used to check if a number is a valid credit card number and in some other scenarios where you need to check a number fast and without accessing a database.

We can summarize the formula as follow:

• We want to check the number 4539 3195 0343 6467
• We take every second digit starting by the right
• We multiply those digits by 2
• If the result is more than 9 we subtract 9 from it
• We sum all the digits
• If sum is evenly divisible by 10 then this number is valid

So we will get:

• 4539 3195 0343 6467
• 4_3_ 3_9_ 0_4_ 6_6_: numbers to modify
• 8_6_ 6_9_ 0_8_ 3_3_: modified numbers (for numbers over 9 we subtracted 9 already)
• 8569 6195 0383 3437: the new sequence of digits
• 80: the sum of all digits
• 80 is evenly divisible by 10 so the result is true

Expected Function

pub fn is_luhn_formula(code: &str) -> bool {
}

Usage

Here is a possible program to test your function,

fn main() {
    println!("{}", is_luhn_formula(""));
    println!("{}", is_luhn_formula("1"));
    println!("{}", is_luhn_formula("79927398713"));
    println!("{}", is_luhn_formula("7992 7398 713"));
    println!("{}", is_luhn_formula("1234567890123456"));
}

And its output:

$ cargo run
false
false
true
true
false
$