Add a basic Queue data structure

Implemented as a linked list.

Terrain2/Algorithms.swift

@@ -222,11 +222,11 @@ public class DiamondSquareGenerator: TerrainGenerator {
         }
 
         func breadthFirstSearch(visit: (Box) -> (Void)) {
-            var queue = [self]
-            while queue.count > 0 {
-                let box = queue.removeFirst()
+            var queue = Queue<Box>()
+            queue.enqueue(item: self)
+            while let box = queue.dequeue() {
                 visit(box)
-                queue.append(contentsOf: box.subdivisions)
+                queue.enqueue(items: box.subdivisions)
             }
         }
     }

Terrain2/Queue.swift

@@ -0,0 +1,54 @@
+//
+//  Queue.swift
+//  Terrain2
+//
+//  Created by Eryn Wells on 11/10/18.
+//  Copyright © 2018 Eryn Wells. All rights reserved.
+//
+
+import Foundation
+
+struct Queue<Element> {
+    class Node<Element> {
+        var item: Element
+        var next: Node?
+
+        init(item i: Element) {
+            item = i
+        }
+    }
+
+    var count: Int = 0
+    var head: Node<Element>?
+    var tail: Node<Element>?
+
+    mutating func enqueue(item: Element) {
+        let newNode = Node(item: item)
+        if head == nil {
+            head = newNode
+            tail = newNode
+        } else {
+            tail!.next = newNode
+            tail = newNode
+        }
+        count += 1
+    }
+
+    mutating func enqueue<S>(items: S) where Element == S.Element, S : Sequence {
+        for i in items {
+            enqueue(item: i)
+        }
+    }
+
+    mutating func dequeue() -> Element? {
+        guard let oldHead = head else {
+            return nil
+        }
+        head = oldHead.next
+        if head == nil {
+            tail = nil
+        }
+        count -= 1
+        return oldHead.item
+    }
+}