exercise: add luhn (#33)

exercism/rust/luhn/.exercism/config.json

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+{
+  "authors": [
+    "IanWhitney"
+  ],
+  "contributors": [
+    "AvasDream",
+    "bitfield",
+    "coriolinus",
+    "cwhakes",
+    "efx",
+    "ErikSchierboom",
+    "gibfahn",
+    "idealhack",
+    "lutostag",
+    "mkantor",
+    "navossoc",
+    "nfiles",
+    "petertseng",
+    "rofrol",
+    "stkent",
+    "stringparser",
+    "workingjubilee",
+    "xakon",
+    "ZapAnton"
+  ],
+  "files": {
+    "solution": [
+      "src/lib.rs",
+      "Cargo.toml"
+    ],
+    "test": [
+      "tests/luhn.rs"
+    ],
+    "example": [
+      ".meta/example.rs"
+    ]
+  },
+  "blurb": "Given a number determine whether or not it is valid per the Luhn formula.",
+  "source": "The Luhn Algorithm on Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Luhn_algorithm"
+}

exercism/rust/luhn/.exercism/metadata.json

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+{"track":"rust","exercise":"luhn","id":"6f6daeed40434ca1b9e1def3b6dbed69","url":"https://exercism.org/tracks/rust/exercises/luhn","handle":"ruancomelli","is_requester":true,"auto_approve":false}

exercism/rust/luhn/.gitignore

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+# Generated by Cargo
+# will have compiled files and executables
+/target/
+**/*.rs.bk
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here http://doc.crates.io/guide.html#cargotoml-vs-cargolock
+Cargo.lock

exercism/rust/luhn/Cargo.toml

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+[package]
+edition = "2021"
+name = "luhn"
+version = "1.6.1"

exercism/rust/luhn/HELP.md

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+# Help
+
+## Running the tests
+
+Execute the tests with:
+
+```bash
+$ 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 _only ignored_ tests without editing the tests source file, use:
+
+```bash
+$ cargo test -- --ignored
+```
+
+If you are using Rust 1.51 or later, you can run _all_ tests with
+
+```bash
+$ cargo test -- --include-ignored
+```
+
+To run a specific test, for example `some_test`, you can use:
+
+```bash
+$ cargo test some_test
+```
+
+If the specific test is ignored, use:
+
+```bash
+$ cargo test some_test -- --ignored
+```
+
+To learn more about Rust tests refer to the online [test documentation][rust-tests].
+
+[rust-tests]: https://doc.rust-lang.org/book/ch11-02-running-tests.html
+
+## Submitting your solution
+
+You can submit your solution using the `exercism submit src/lib.rs Cargo.toml` command.
+This command will upload your solution to the Exercism website and print the solution page's URL.
+
+It's possible to submit an incomplete solution which allows you to:
+
+- See how others have completed the exercise
+- Request help from a mentor
+
+## Need to get help?
+
+If you'd like help solving the exercise, check the following pages:
+
+- The [Rust track's documentation](https://exercism.org/docs/tracks/rust)
+- The [Rust track's programming category on the forum](https://forum.exercism.org/c/programming/rust)
+- [Exercism's programming category on the forum](https://forum.exercism.org/c/programming/5)
+- The [Frequently Asked Questions](https://exercism.org/docs/using/faqs)
+
+Should those resources not suffice, you could submit your (incomplete) solution to request mentoring.
+
+## Rust Installation
+
+Refer to the [exercism help page][help-page] for Rust installation and learning
+resources.
+
+## 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 GitHub [track repository][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].
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.
+
+[help-page]: https://exercism.org/tracks/rust/learning
+[github]: https://github.com/exercism/rust
+[contribution guide]: https://exercism.org/docs/community/contributors

exercism/rust/luhn/README.md

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+# Luhn
+
+Welcome to Luhn on Exercism's Rust Track.
+If you need help running the tests or submitting your code, check out `HELP.md`.
+
+## Instructions
+
+Given a number determine whether or not it is valid per the Luhn formula.
+
+The [Luhn algorithm][luhn] 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
+
+```text
+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
+
+```text
+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:
+
+```text
+8569 6195 0383 3437
+```
+
+Then sum all of the digits:
+
+```text
+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
+
+```text
+8273 1232 7352 0569
+```
+
+Double the second digits, starting from the right
+
+```text
+7253 2262 5312 0539
+```
+
+Sum the digits
+
+```text
+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.
+
+[luhn]: https://en.wikipedia.org/wiki/Luhn_algorithm
+
+## Source
+
+### Created by
+
+- @IanWhitney
+
+### Contributed to by
+
+- @AvasDream
+- @bitfield
+- @coriolinus
+- @cwhakes
+- @efx
+- @ErikSchierboom
+- @gibfahn
+- @idealhack
+- @lutostag
+- @mkantor
+- @navossoc
+- @nfiles
+- @petertseng
+- @rofrol
+- @stkent
+- @stringparser
+- @workingjubilee
+- @xakon
+- @ZapAnton
+
+### Based on
+
+The Luhn Algorithm on Wikipedia - https://en.wikipedia.org/wiki/Luhn_algorithm

exercism/rust/luhn/src/lib.rs

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+/// Check a Luhn checksum.
+pub fn is_valid(code: &str) -> bool {
+    let trimmed = code.trim();
+
+    if let 0 | 1 = trimmed.len() {
+        return false;
+    }
+
+    trimmed
+        .chars()
+        .rev()
+        .filter(|c| !c.is_whitespace())
+        .zip([false, true].iter().cycle())
+        .try_fold(0, |sum, (c, &is_even_position)| {
+            c.to_digit(10).map(|digit| {
+                sum + if is_even_position {
+                    let twice = digit * 2;
+                    if twice >= 10 {
+                        twice - 9
+                    } else {
+                        twice
+                    }
+                } else {
+                    digit
+                }
+            })
+        })
+        .map_or(false, |sum| sum % 10 == 0)
+}

exercism/rust/luhn/tests/luhn.rs

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+use luhn::*;
+
+fn process_valid_case(number: &str, is_luhn_expected: bool) {
+    assert_eq!(is_valid(number), is_luhn_expected);
+}
+
+#[test]
+fn single_digit_strings_can_not_be_valid() {
+    process_valid_case("1", false);
+}
+
+#[test]
+fn a_single_zero_is_invalid() {
+    process_valid_case("0", false);
+}
+
+#[test]
+fn a_simple_valid_sin_that_remains_valid_if_reversed() {
+    process_valid_case("059", true);
+}
+
+#[test]
+fn a_simple_valid_sin_that_becomes_invalid_if_reversed() {
+    process_valid_case("59", true);
+}
+
+#[test]
+fn a_valid_canadian_sin() {
+    process_valid_case("055 444 285", true);
+}
+
+#[test]
+fn invalid_canadian_sin() {
+    process_valid_case("055 444 286", false);
+}
+
+#[test]
+fn invalid_credit_card() {
+    process_valid_case("8273 1232 7352 0569", false);
+}
+
+#[test]
+fn valid_number_with_an_even_number_of_digits() {
+    process_valid_case("095 245 88", true);
+}
+
+#[test]
+fn strings_that_contain_non_digits_are_invalid() {
+    process_valid_case("055a 444 285", false);
+}
+
+#[test]
+fn valid_strings_with_punctuation_included_become_invalid() {
+    process_valid_case("055-444-285", false);
+}
+
+#[test]
+fn symbols_are_invalid() {
+    process_valid_case("055£ 444$ 285", false);
+}
+
+#[test]
+fn single_zero_with_space_is_invalid() {
+    process_valid_case(" 0", false);
+}
+
+#[test]
+fn more_than_a_single_zero_is_valid() {
+    process_valid_case("0000 0", true);
+}
+
+#[test]
+fn input_digit_9_is_correctly_converted_to_output_digit_9() {
+    process_valid_case("091", true);
+}
+
+#[test]
+/// 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 using_ascii_value_for_doubled_nondigit_isnt_allowed() {
+    process_valid_case(":9", false);
+}
+
+#[test]
+/// valid strings with a non-digit added at the end become invalid
+fn valid_strings_with_a_nondigit_added_at_the_end_become_invalid() {
+    process_valid_case("059a", false);
+}
+
+#[test]
+/// valid strings with symbols included become invalid
+fn valid_strings_with_symbols_included_become_invalid() {
+    process_valid_case("055# 444$ 285", false);
+}
+
+#[test]
+/// 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 using_ascii_value_for_nondoubled_nondigit_isnt_allowed() {
+    process_valid_case("055b 444 285", false);
+}
+
+#[test]
+/// valid number with an odd number of spaces
+fn valid_number_with_an_odd_number_of_spaces() {
+    process_valid_case("234 567 891 234", true);
+}
+
+#[test]
+/// non-numeric, non-space char in the middle with a sum that's divisible by 10 isn't allowed
+fn invalid_char_in_middle_with_sum_divisible_by_10_isnt_allowed() {
+    process_valid_case("59%59", false);
+}
+
+#[test]
+/// unicode numeric characters are not allowed in a otherwise valid number
+fn valid_strings_with_numeric_unicode_characters_become_invalid() {
+    process_valid_case("1249①", false);
+}