Implement Luhn Trait

This implements the Trait-based API that I proposed here

exercism#247 (comment)

As pointed out by Peter, using traits like this is probably an
anti-pattern. I'm somewhat on the fence about demonstrating it.

Author: whit0694

config.json

@@ -140,6 +140,16 @@
         "higher-order functions"
       ]
     },
+    {
+      "slug": "luhn-trait",
+      "difficulty": 1,
+      "topics": [
+        "traits",
+        "str to digits",
+        "iterators",
+        "higher-order functions"
+      ]
+    },
     {
       "slug": "largest-series-product",
       "difficulty": 4,

exercises/luhn-trait/.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-trait/Cargo.toml

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

exercises/luhn-trait/HINTS.md

@@ -0,0 +1,16 @@
+# Luhn Validation with a Trait
+
+Before doing this exercise you should probably do the original Luhn exercise and its successor, Luhn From Trait 
+
+- `exercism fetch rust luhn`
+- `exercism fetch rust luhn-from`
+
+In the original Luhn exercise you only validated strings, but the Luhn algorithm can be applied integers as well.
+
+In Luhn From you implemented a From trait, which also required you to create a Luhn struct. 
+
+Instead of creating a Struct just to perform the validation, what if you you validated the primitives (i.e, String, u8, etc.) themselves?
+
+In this exercise you'll create and implement a custom [trait](https://doc.rust-lang.org/book/traits.html) that performs the validation
+
+Note: It is [not idiomatic Rust to implement traits on on primitives](https://doc.rust-lang.org/book/traits.html#rules-for-implementing-traits). In this exercise we're showing something that you _can_ do, not something you _should_ do. If you find yourself implementing traits on primitives, perhaps you have a case of [Primitive Obsession](http://wiki.c2.com/?PrimitiveObsession).

exercises/luhn-trait/example.rs

@@ -0,0 +1,55 @@
+pub trait Luhn {
+    fn valid_luhn(&self) -> bool;
+}
+
+impl Luhn for String {
+    fn valid_luhn(&self) -> bool {
+        if self.chars().any(|c| c.is_alphabetic()) || self.chars().count() == 1 {
+            return false;
+        }
+
+        self.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
+    }
+}
+
+impl Luhn for &'static str {
+    fn valid_luhn(&self) -> bool {
+        String::from(*self).valid_luhn()
+    }
+}
+
+impl Luhn for u8 {
+    fn valid_luhn(&self) -> bool {
+        self.to_string().valid_luhn()
+    }
+}
+
+impl Luhn for u16 {
+    fn valid_luhn(&self) -> bool {
+        self.to_string().valid_luhn()
+    }
+}
+
+impl Luhn for u32 {
+    fn valid_luhn(&self) -> bool {
+        self.to_string().valid_luhn()
+    }
+}
+
+impl Luhn for u64 {
+    fn valid_luhn(&self) -> bool {
+        self.to_string().valid_luhn()
+    }
+}
+
+impl Luhn for usize {
+    fn valid_luhn(&self) -> bool {
+        self.to_string().valid_luhn()
+    }
+}

exercises/luhn-trait/tests/luhn-trait.rs

@@ -0,0 +1,53 @@
+extern crate luhn_trait;
+
+use luhn_trait::*;
+
+#[test]
+fn you_can_validate_from_a_str() {
+    assert!("046 454 286".valid_luhn());
+    assert!(!"046 454 287".valid_luhn());
+}
+
+#[test]
+fn you_can_validate_from_a_string() {
+    assert!(String::from("046 454 286").valid_luhn());
+    assert!(!String::from("046 454 287").valid_luhn());
+}
+
+#[test]
+fn you_can_validate_from_a_u8() {
+    assert!(240u8.valid_luhn());
+    assert!(!241u8.valid_luhn());
+}
+
+#[test]
+fn you_can_validate_from_a_u16() {
+    let valid = 64_436u16;
+    let invalid = 64_437u16;
+    assert!(valid.valid_luhn());
+    assert!(!invalid.valid_luhn());
+}
+
+#[test]
+fn you_can_validate_from_a_u32() {
+    let valid = 46_454_286u32;
+    let invalid = 46_454_287u32;
+    assert!(valid.valid_luhn());
+    assert!(!invalid.valid_luhn());
+}
+
+#[test]
+fn you_can_validate_from_a_u64() {
+    let valid = 8273_1232_7352_0562u64;
+    let invalid = 8273_1232_7352_0569u64;
+    assert!(valid.valid_luhn());
+    assert!(!invalid.valid_luhn());
+}
+
+#[test]
+fn you_can_validate_from_a_usize() {
+    let valid = 8273_1232_7352_0562usize;
+    let invalid = 8273_1232_7352_0569usize;
+    assert!(valid.valid_luhn());
+    assert!(!invalid.valid_luhn());
+}