Added helper JS queue.js

queue/queue.js

@@ -0,0 +1,70 @@
+/*
+
+Queue.js
+
+A function to represent a queue
+
+Created by Stephen Morley - http://code.stephenmorley.org/ - and released under
+the terms of the CC0 1.0 Universal legal code:
+
+http://creativecommons.org/publicdomain/zero/1.0/legalcode
+
+*/
+
+/* Creates a new queue. A queue is a first-in-first-out (FIFO) data structure -
+ * items are added to the end of the queue and removed from the front.
+ */
+function Queue(){
+
+  // initialise the queue and offset
+  var queue  = [];
+  var offset = 0;
+
+  // Returns the length of the queue.
+  this.getLength = function(){
+    return (queue.length - offset);
+  }
+
+  // Returns true if the queue is empty, and false otherwise.
+  this.isEmpty = function(){
+    return (queue.length == 0);
+  }
+
+  /* Enqueues the specified item. The parameter is:
+   *
+   * item - the item to enqueue
+   */
+  this.enqueue = function(item){
+    queue.push(item);
+  }
+
+  /* Dequeues an item and returns it. If the queue is empty, the value
+   * 'undefined' is returned.
+   */
+  this.dequeue = function(){
+
+    // if the queue is empty, return immediately
+    if (queue.length == 0) return undefined;
+
+    // store the item at the front of the queue
+    var item = queue[offset];
+
+    // increment the offset and remove the free space if necessary
+    if (++ offset * 2 >= queue.length){
+      queue  = queue.slice(offset);
+      offset = 0;
+    }
+
+    // return the dequeued item
+    return item;
+
+  }
+
+  /* Returns the item at the front of the queue (without dequeuing it). If the
+   * queue is empty then undefined is returned.
+   */
+  this.peek = function(){
+    return (queue.length > 0 ? queue[offset] : undefined);
+  }
+
+}