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# n0mn0m's solution

## to Luhn in the Rust Track

Published at Nov 04 2020 · 0 comments
Instructions
Test suite
Solution

Given a number determine whether or not it is valid per the Luhn formula.

The Luhn algorithm is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.

The task is to check if a given string is valid.

## Validating a Number

Strings of length 1 or less are not valid. Spaces are allowed in the input, but they should be stripped before checking. All other non-digit characters are disallowed.

## Example 1: valid credit card number

``````4539 3195 0343 6467
``````

The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling

``````4_3_ 3_9_ 0_4_ 6_6_
``````

If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling:

``````8569 6195 0383 3437
``````

Then sum all of the digits:

``````8+5+6+9+6+1+9+5+0+3+8+3+3+4+3+7 = 80
``````

If the sum is evenly divisible by 10, then the number is valid. This number is valid!

## Example 2: invalid credit card number

``````8273 1232 7352 0569
``````

Double the second digits, starting from the right

``````7253 2262 5312 0539
``````

Sum the digits

``````7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57
``````

57 is not evenly divisible by 10, so this number is not valid.

## Rust Installation

Refer to the exercism help page for Rust installation and learning resources.

## Writing the Code

Execute the tests with:

``````\$ cargo test
``````

All but the first test have been ignored. After you get the first test to pass, open the tests source file which is located in the `tests` directory and remove the `#[ignore]` flag from the next test and get the tests to pass again. Each separate test is a function with `#[test]` flag above it. Continue, until you pass every test.

If you wish to run all ignored tests without editing the tests source file, use:

``````\$ cargo test -- --ignored
``````

To run a specific test, for example `some_test`, you can use:

``````\$ cargo test some_test
``````

If the specific test is ignored use:

``````\$ cargo test some_test -- --ignored
``````

To learn more about Rust tests refer to the online test documentation

Make sure to read the Modules chapter if you haven't already, it will help you with organizing your files.

## Further improvements

After you have solved the exercise, please consider using the additional utilities, described in the installation guide, to further refine your final solution.

To format your solution, inside the solution directory use

``````cargo fmt
``````

To see, if your solution contains some common ineffective use cases, inside the solution directory use

``````cargo clippy --all-targets
``````

## Submitting the solution

Generally you should submit all files in which you implemented your solution (`src/lib.rs` in most cases). If you are using any external crates, please consider submitting the `Cargo.toml` file. This will make the review process faster and clearer.

## Feedback, Issues, Pull Requests

The exercism/rust repository on GitHub is the home for all of the Rust exercises. If you have feedback about an exercise, or want to help implement new exercises, head over there and create an issue. Members of the rust track team are happy to help!

If you want to know more about Exercism, take a look at the contribution guide.

## Source

The Luhn Algorithm on Wikipedia http://en.wikipedia.org/wiki/Luhn_algorithm

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### luhn.rs

``````use luhn::*;

fn process_valid_case(number: &str, is_luhn_expected: bool) {
assert_eq!(is_valid(number), is_luhn_expected);
}

#[test]
fn test_single_digit_strings_can_not_be_valid() {
process_valid_case("1", false);
}

#[test]
#[ignore]
fn test_a_single_zero_is_invalid() {
process_valid_case("0", false);
}

#[test]
#[ignore]
fn test_a_simple_valid_sin_that_remains_valid_if_reversed() {
process_valid_case("059", true);
}

#[test]
#[ignore]
fn test_a_simple_valid_sin_that_becomes_invalid_if_reversed() {
process_valid_case("59", true);
}

#[test]
#[ignore]
process_valid_case("055 444 285", true);
}

#[test]
#[ignore]
process_valid_case("055 444 286", false);
}

#[test]
#[ignore]
fn test_invalid_credit_card() {
process_valid_case("8273 1232 7352 0569", false);
}

#[test]
#[ignore]
fn test_valid_number_with_an_even_number_of_digits() {
process_valid_case("095 245 88", true);
}

#[test]
#[ignore]
fn strings_that_contain_non_digits_are_invalid() {
process_valid_case("055a 444 285", false);
}

#[test]
#[ignore]
fn test_valid_strings_with_punctuation_included_become_invalid() {
process_valid_case("055-444-285", false);
}

#[test]
#[ignore]
fn symbols_are_invalid() {
process_valid_case("055Â£ 444\$ 285", false);
}

#[test]
#[ignore]
fn test_single_zero_with_space_is_invalid() {
process_valid_case(" 0", false);
}

#[test]
#[ignore]
fn test_more_than_a_single_zero_is_valid() {
process_valid_case("0000 0", true);
}

#[test]
#[ignore]
fn test_input_digit_9_is_correctly_converted_to_output_digit_9() {
process_valid_case("091", true);
}

#[test]
#[ignore]
/// using ascii value for doubled non-digit isn't allowed
/// Convert non-digits to their ascii values and then offset them by 48 sometimes accidentally declare an invalid string to be valid.
/// This test is designed to avoid that solution.
fn test_using_ascii_value_for_doubled_nondigit_isnt_allowed() {
process_valid_case(":9", false);
}

#[test]
#[ignore]
/// valid strings with a non-digit added at the end become invalid
process_valid_case("059a", false);
}

#[test]
#[ignore]
/// valid strings with symbols included become invalid
fn test_valid_strings_with_symbols_included_become_invalid() {
process_valid_case("055# 444\$ 285", false);
}

#[test]
#[ignore]
/// using ascii value for non-doubled non-digit isn't allowed
/// Convert non-digits to their ascii values and then offset them by 48 sometimes accidentally declare an invalid string to be valid.
/// This test is designed to avoid that solution.
fn test_using_ascii_value_for_nondoubled_nondigit_isnt_allowed() {
process_valid_case("055b 444 285", false);
}

#[test]
#[ignore]
/// valid number with an odd number of spaces
fn test_valid_number_with_an_odd_number_of_spaces() {
process_valid_case("234 567 891 234", true);
}``````
``````fn valid_format(code: &str) -> bool {
code.chars()
.any(|char| !(char.is_ascii_whitespace() || char.is_ascii_digit()))
|| code.trim() == "0"
}

/// Check a Luhn checksum.
pub fn is_valid(code: &str) -> bool {
if valid_format(code) {
return false;
}

code.chars()
.rev()
.filter(|char| char.is_ascii_digit())
.enumerate()
.map(|(index, char)| {
let digit = char.to_digit(10).unwrap();
let mut total = digit;

if index % 2 == 1 {
total *= 2;

if total > 9 {
total -= 9;
}
}

total
})
.sum::<u32>()
% 10
== 0
}``````