Add twelve days of christmas example

twelve_days_of_christmas/Cargo.toml

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+[package]
+name = "twelve_days_of_christmas"
+version = "0.1.0"
+authors = ["Timothy Warren <twarren@nabancard.com>"]
+edition = "2018"
+
+[dependencies]

twelve_days_of_christmas/src/main.rs

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+const ORDINALS: [&str; 12] = [
+    "first",
+    "second",
+    "third",
+    "fourth",
+    "fifth",
+    "sixth",
+    "seventh",
+    "eighth",
+    "ninth",
+    "tenth",
+    "eleventh",
+    "twelfth",
+];
+
+const GIFTS: [&str; 12] = [
+    "Twelve drummers drumming",
+    "Eleven pipers piping",
+    "Ten lords a-leaping",
+    "Nine ladies dancing",
+    "Eight maids a-milking",
+    "Seven swans a-swimming",
+    "Six geese a-laying",
+    "Five golden rings",
+    "Four calling birds",
+    "Three french hens",
+    "Two turtle doves",
+    "and a partridge in a pear tree.",
+];
+
+fn format_verse(day: &str, gifts: &str) -> String {
+    format!("On the {} day of Christmas, my true love gave to me: \n{}\n", day, gifts)
+}
+
+fn main() {
+    for n in 0..12 {
+        let verse: String = match n {
+            0 => format_verse("first", "A partridge in a pear tree."),
+            _ => {
+                // The highest index is 11, and we want
+                // the highest index to start, so we can
+                // easily have the cumulative list each verse.
+                let gift_index = 11 - n;
+
+                // Get the slice of the array of gifts, from
+                // the index to the end of the array
+                let gifts = &GIFTS[gift_index..]
+                    .to_vec() // So we have the join method that returns a string
+                    .join(",\n");
+
+                format_verse(ORDINALS[n], gifts)
+            }
+        };
+
+        // For some reason, the println! macro only takes string literals
+        println!("{}", verse);
+    }
+}