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Avatar of bkhl

bkhl's solution

to Luhn in the Rust Track

Published at Jan 25 2020 · 3 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 1488 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_ 1_8_ 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 2478 0383 3437

Then sum all of the digits:

8+5+6+9+2+4+7+8+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]
fn test_a_valid_canadian_sin() {
    process_valid_case("055 444 285", true);
}

#[test]
#[ignore]
fn test_invalid_canadian_sin() {
    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("055£ 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
fn test_valid_strings_with_a_nondigit_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);
}
pub fn is_valid(input: &str) -> bool {
    let digits = match input
        .chars()
        .filter(|c| !c.is_whitespace())
        .map(|c| c.to_digit(10))
        .rev()
        .collect::<Option<Vec<u32>>>()
    {
        Some(digits) => digits,
        _ => return false,
    };

    if digits.len() < 2 {
        return false;
    }

    let sum = digits
        .into_iter()
        .zip([false, true].iter().cycle())
        .fold(0, |acc, (digit, even)| {
            if *even {
                if digit < 5 {
                    acc + digit * 2
                } else {
                    acc + digit * 2 - 9
                }
            } else {
                acc + digit
            }
        });

    sum % 10 == 0
}

Community comments

Find this solution interesting? Ask the author a question to learn more.
Avatar of bkhl

@barskern trait version, as promised. :-)

Avatar of barskern

@bkhl commented:

@barskern trait version, as promised. :-)

This is looking good! :)

Hmm I thought a bit about the warning and you handle that in the str impl and you cannot really represent 0000 in a u64 anyways, so I think it's okay ?

Avatar of singpolyma-shopify

Why not? Any self == 0 will have had all-zero digits, so it's valid unless it was too short. I would say such should be valid in this case

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