/** Adapted and extended from the work of Stephen Morley - http://code.stephenmorley.org/javascript/queues/ 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. * * @return {void} */ function Updraft_Queue() { // Initialise the queue and offset. var queue = []; var offset = 0; var locked = false; /** * Returns the length of the queue. * * @returns {number} - the length of the queue */ this.get_length = function () { return (queue.length - offset); } /** * Query whether the queue is empty or not * * @returns {boolean} - returns true if the queue is empty, and false otherwise. */ this.is_empty = function () { return (queue.length == 0); } /** * Enqueues the specified item. The parameter is: * * @param {*} item The item to enqueue * * @return {void} */ this.enqueue = function (item) { queue.push(item); } /** * Returns the queue lock status * * @returns {boolean} - whether the queue is locked or not */ this.is_locked = function () { return locked; } /** * Attempt to get the queue lock * * @returns {boolean} - whether the attempt succeeded or not */ this.get_lock = function () { if (locked) { return false; } this.lock(); return true; } /** * Dequeues an item and returns it. If the queue is empty, the value * 'undefined' is returned. * * @returns {*} - returns and removes the item at the front of the queue, or undefined if the queue is empty */ 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; } /** * Lock the queue * * @returns {void} */ this.lock = function () { locked = true; } /** * Unlock the queue * * @returns {void} */ this.unlock = function () { locked = false; } /** * Returns the item at the front of the queue (without dequeuing it). If the * queue is empty then undefined is returned. * * @returns {*} - returns the item at the front of the queue, or undefined if the queue is empty */ this.peek = function () { return (queue.length > 0 ? queue[offset] : undefined); } /** * Replaces the item at the front of the queue (if any) * * @param {*} item The item to put at the front of the queue. * * @return {boolean} Whether or not the item was successfully replaced. */ this.replace_front = function (item) { if (queue.length < 1) { return false; } queue[offset] = item; return true; } }