Implement Luhn (#247)

* Implement Luhn

Closes #237

This follows the current test suite as of right now

https://github.com/exercism/x-common/blob/41b9fe8e9c2513f4443e1cad1f286f53b2720ce9/exercises/luhn/canonical-data.json

* Place Luhn before Largest Series Product

I don't think my solution is that far from what people will do. The
[existing crate](https://lunemec.github.io/rust-luhn/src/luhn2/src/lib.rs.html)
uses another crate to handle digits > 10 after doubling. If people go
down that path their solutions will vary a bit from mine, but not by a
bunch.

So the solution mostly requires a good handle on

- Converting a string to digits
- Working with iteration

The conversion of a string to digits is also part of Largest Series
Product, so we could put the exercise to right before LSP to reinforce
that concept.

Author: IanWhitney

config.json

@@ -122,6 +122,15 @@
         "match"
       ]
     },
+    {
+      "slug": "luhn",
+      "difficulty": 1,
+      "topics": [
+        "str to digits",
+        "iterators",
+        "higher-order functions"
+      ]
+    },
     {
       "slug": "largest-series-product",
       "difficulty": 1,

exercises/luhn/.gitignore

@@ -0,0 +1,7 @@
+# Generated by Cargo
+# will have compiled files and executables
+/target/
+
+# 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

exercises/luhn/Cargo.toml

@@ -0,0 +1,3 @@
+[package]
+name = "luhn"
+version = "0.0.0"

exercises/luhn/example.rs

@@ -0,0 +1,13 @@
+pub fn is_valid(candidate: &str) -> bool {
+    if candidate.chars().any(|c| c.is_alphabetic()) || candidate.chars().count() == 1 {
+        return false;
+    }
+
+    candidate.chars()
+        .filter_map(|c| c.to_digit(10))
+        .rev()
+        .enumerate()
+        .map(|(index, digit)| if index % 2 == 0 { digit } else { digit * 2 })
+        .map(|digit| if digit > 9 { digit - 9 } else { digit })
+        .sum::<u32>() % 10 == 0
+}

exercises/luhn/tests/luhn.rs

@@ -0,0 +1,38 @@
+extern crate luhn;
+
+use luhn::*;
+
+#[test]
+fn single_digit_string_is_invalid() {
+    assert!(!is_valid("1"));
+}
+
+#[test]
+#[ignore]
+fn single_zero_string_is_invalid() {
+    assert!(!is_valid("0"));
+}
+
+#[test]
+#[ignore]
+fn valid_canadian_sin_is_valid() {
+    assert!(is_valid("046 454 286"));
+}
+
+#[test]
+#[ignore]
+fn invalid_canadian_sin_is_invalid() {
+    assert!(!is_valid("046 454 287"));
+}
+
+#[test]
+#[ignore]
+fn invalid_credit_card_is_invalid() {
+    assert!(!is_valid("8273 1232 7352 0569"));
+}
+
+#[test]
+#[ignore]
+fn strings_that_contain_non_digits_are_invalid() {
+    assert!(!is_valid("046a 454 286"));
+}