/**
 * Used to run asynchronous tasks sequentially (ajax mainly). An asynchronous
 * task is composed of different callback types: onRun, onSuccess, onError
 */
define([ "core", "underscore" ], function(core) {

	var asyncRunner = {};

	var taskQueue = [];
	var currentTask = undefined;
	var currentTaskRunning = false;
	var currentTaskStartTime = 0;

	asyncRunner.createTask = function() {
		var task = {};
		task.finished = false;
		task.timeout = ASYNC_TASK_DEFAULT_TIMEOUT;
		task.retryCounter = 0;
		/**
		 * onRun callbacks are called by chain(). These callbacks have to call
		 * chain() themselves to chain with next onRun callback or error() to
		 * throw an exception or retry() to restart the task.
		 */
		// Run callbacks
		task.runCallbacks = [];
		task.onRun = function(callback) {
			task.runCallbacks.push(callback);
		};
		/**
		 * onSuccess callbacks are called when every onRun callbacks have
		 * succeed.
		 */
		task.successCallbacks = [];
		task.onSuccess = function(callback) {
			task.successCallbacks.push(callback);
		};
		/**
		 * onError callbacks are called when error() is called in a onRun
		 * callback.
		 */
		task.errorCallbacks = [];
		task.onError = function(callback) {
			task.errorCallbacks.push(callback);
		};
		/**
		 * chain() calls the next onRun callback or the onSuccess callbacks when
		 * finished. The optional callback parameter can be used to add a onRun
		 * callback during execution.
		 */
		task.chain = function(callback) {
			if (task.finished === true) {
				return;
			}
			// If first execution
			if (task.queue === undefined) {
				// Create a copy of the onRun callbacks
				task.queue = task.runCallbacks.slice();
			}
			// If a callback is passed as a parameter
			if(callback !== undefined) {
				callback();
				return;
			}
			// If all callbacks have been run
			if (task.queue.length === 0) {
				// Run the onSuccess callbacks
				runSafe(task, task.successCallbacks);
				return;
			}
			// Run the next callback
			var runCallback = task.queue.shift();
			runCallback();
		};
		/**
		 * error() calls the onError callbacks with the error parameter and ends
		 * the task by throwing an exception.
		 */
		task.error = function(error) {
			if (task.finished === true) {
				return;
			}
			error = error || "Unknown error";
			runSafe(task, task.errorCallbacks, error);
			// Exit the current call stack
			throw error;
		};
		/**
		 * retry() can be called in an onRun callback to restart the task
		 */
		task.retry = function() {
			if (task.finished === true) {
				return;
			}
			task.queue = undefined;
			if (task.retryCounter === 5) {
				task.error(new Error("Maximum retry number reached"));
				return;
			}
			// Implement an exponential backoff
			var delay = Math.pow(2, task.retryCounter++) * 1000;
			currentTaskStartTime = core.currentTime + delay;
			currentTaskRunning = false;
			asyncRunner.runTask();
		};
		return task;
	};

	// Run the next task in the queue if any and no other running
	function runTask() {

		// If there is a task currently running
		if (currentTaskRunning === true) {
			// If the current task takes too long
			if (currentTaskStartTime + currentTask.timeout < core.currentTime) {
				var errorMsg = "A timeout occurred.";
				core.showError(errorMsg);
				currentTask.error(new Error(errorMsg));
			}
			return;
		}

		if (currentTask === undefined) {
			// If no task in the queue
			if (taskQueue.length === 0) {
				return;
			}

			// Dequeue an enqueued task
			currentTask = taskQueue.shift();
			currentTaskStartTime = core.currentTime;
			core.showWorkingIndicator(true);
		}

		// Run the task
		if (currentTaskStartTime <= core.currentTime) {
			currentTaskRunning = true;
			currentTask.chain();
		}
	}

	asyncRunner.runTask = function() {
		// Use defer to avoid stack overflow
		_.defer(runTask);
	};
	// Run runTask function periodically
	core.addPeriodicCallback(asyncRunner.runTask);

	function runSafe(task, callbacks, param) {
		try {
			_.each(callbacks, function(callback) {
				callback(param);
			});
		} finally {
			task.finished = true;
			if (currentTask === task) {
				currentTask = undefined;
				currentTaskRunning = false;
			}
			if (taskQueue.length === 0) {
				core.showWorkingIndicator(false);
			} else {
				asyncRunner.runTask();
			}
		}
	}

	// Add a task to the queue
	asyncRunner.addTask = function(task) {
		taskQueue.push(task);
		asyncRunner.runTask();
	};

	// Change current task timeout
	asyncRunner.setCurrentTaskTimeout = function(timeout) {
		if (currentTask !== undefined) {
			currentTask.timeout = timeout;
		}
	};

	return asyncRunner;
});