(function( factory ) {
	if (typeof define !== 'undefined' && define.amd) {
		define([], factory);
	} else if (typeof module !== 'undefined' && module.exports) {
		module.exports = factory();
	} else {
		window.scrollMonitor = factory();
	}
})(function() {

	var scrollTop = function() {
		return window.pageYOffset ||
			(document.documentElement && document.documentElement.scrollTop) ||
			document.body.scrollTop;
	};

	var exports = {};

	var watchers = [];

	var VISIBILITYCHANGE = 'visibilityChange';
	var ENTERVIEWPORT = 'enterViewport';
	var FULLYENTERVIEWPORT = 'fullyEnterViewport';
	var EXITVIEWPORT = 'exitViewport';
	var PARTIALLYEXITVIEWPORT = 'partiallyExitViewport';
	var LOCATIONCHANGE = 'locationChange';
	var STATECHANGE = 'stateChange';

	var eventTypes = [
		VISIBILITYCHANGE,
		ENTERVIEWPORT,
		FULLYENTERVIEWPORT,
		EXITVIEWPORT,
		PARTIALLYEXITVIEWPORT,
		LOCATIONCHANGE,
		STATECHANGE
	];

	var defaultOffsets = {top: 0, bottom: 0};

	var getViewportHeight = function() {
		return window.innerHeight || document.documentElement.clientHeight;
	};

	var getDocumentHeight = function() {
		// jQuery approach
		// whichever is greatest
		return Math.max(
			document.body.scrollHeight, document.documentElement.scrollHeight,
			document.body.offsetHeight, document.documentElement.offsetHeight,
			document.documentElement.clientHeight
		);
	};

	exports.viewportTop = null;
	exports.viewportBottom = null;
	exports.documentHeight = null;
	exports.viewportHeight = getViewportHeight();

	var previousDocumentHeight;
	var latestEvent;

	var calculateViewportI;
	function calculateViewport() {
		exports.viewportTop = scrollTop();
		exports.viewportBottom = exports.viewportTop + exports.viewportHeight;
		exports.documentHeight = getDocumentHeight();
		if (exports.documentHeight !== previousDocumentHeight) {
			calculateViewportI = watchers.length;
			while( calculateViewportI-- ) {
				watchers[calculateViewportI].recalculateLocation();
			}
			previousDocumentHeight = exports.documentHeight;
		}
	}

	function recalculateWatchLocationsAndTrigger() {
		exports.viewportHeight = getViewportHeight();
		calculateViewport();
		updateAndTriggerWatchers();
	}

	var recalculateAndTriggerTimer;
	function debouncedRecalcuateAndTrigger() {
		clearTimeout(recalculateAndTriggerTimer);
		recalculateAndTriggerTimer = setTimeout( recalculateWatchLocationsAndTrigger, 100 );
	}

	var updateAndTriggerWatchersI;
	function updateAndTriggerWatchers() {
		// update all watchers then trigger the events so one can rely on another being up to date.
		updateAndTriggerWatchersI = watchers.length;
		var i = 0;
		while( i < updateAndTriggerWatchersI ) {
			watchers[i].update();
			i++;
		}

		updateAndTriggerWatchersI = watchers.length;
		var j = 0,
			newLength;
		while( j < updateAndTriggerWatchersI ) {
			watchers[j].triggerCallbacks();

			newLength = watchers.length;

			if (newLength < updateAndTriggerWatchersI) {
				j -= updateAndTriggerWatchersI - newLength;
			}

			updateAndTriggerWatchersI = newLength;
			j++;
		}

	}

	function ElementWatcher( watchItem, offsets ) {
		var self = this;

		this.watchItem = watchItem;

		this.uid = Math.random().toString(36).substring(8);

		if (!offsets) {
			this.offsets = defaultOffsets;
		} else if (offsets === +offsets) {
			this.offsets = {top: offsets, bottom: offsets};
		} else {
			this.offsets = {
				top: offsets.top || defaultOffsets.top,
				bottom: offsets.bottom || defaultOffsets.bottom
			};
		}

		this.callbacks = {}; // {callback: function, isOne: true }

		for (var i = 0, j = eventTypes.length; i < j; i++) {
			self.callbacks[eventTypes[i]] = [];
		}

		this.locked = false;

		var wasInViewport;
		var wasFullyInViewport;
		var wasAboveViewport;
		var wasBelowViewport;

		var listenerToTriggerListI;
		var listener;
		function triggerCallbackArray( listeners ) {
			if (listeners.length === 0) {
				return;
			}
			listenerToTriggerListI = listeners.length;
			while( listenerToTriggerListI-- ) {
				listener = listeners[listenerToTriggerListI];
				listener.callback.call( self, latestEvent );
				if (listener.isOne) {
					listeners.splice(listenerToTriggerListI, 1);
				}
			}
		}
		this.triggerCallbacks = function triggerCallbacks() {

			if (this.isInViewport && !wasInViewport) {
				triggerCallbackArray( this.callbacks[ENTERVIEWPORT] );
			}
			if (this.isFullyInViewport && !wasFullyInViewport) {
				triggerCallbackArray( this.callbacks[FULLYENTERVIEWPORT] );
			}


			if (this.isAboveViewport !== wasAboveViewport &&
				this.isBelowViewport !== wasBelowViewport) {

				triggerCallbackArray( this.callbacks[VISIBILITYCHANGE] );

				// if you skip completely past this element
				if (!wasFullyInViewport && !this.isFullyInViewport) {
					triggerCallbackArray( this.callbacks[FULLYENTERVIEWPORT] );
					triggerCallbackArray( this.callbacks[PARTIALLYEXITVIEWPORT] );
				}
				if (!wasInViewport && !this.isInViewport) {
					triggerCallbackArray( this.callbacks[ENTERVIEWPORT] );
					triggerCallbackArray( this.callbacks[EXITVIEWPORT] );
				}
			}

			if (!this.isFullyInViewport && wasFullyInViewport) {
				triggerCallbackArray( this.callbacks[PARTIALLYEXITVIEWPORT] );
			}
			if (!this.isInViewport && wasInViewport) {
				triggerCallbackArray( this.callbacks[EXITVIEWPORT] );
			}
			if (this.isInViewport !== wasInViewport) {
				triggerCallbackArray( this.callbacks[VISIBILITYCHANGE] );
			}
			switch( true ) {
				case wasInViewport !== this.isInViewport:
				case wasFullyInViewport !== this.isFullyInViewport:
				case wasAboveViewport !== this.isAboveViewport:
				case wasBelowViewport !== this.isBelowViewport:
					triggerCallbackArray( this.callbacks[STATECHANGE] );
			}

			wasInViewport = this.isInViewport;
			wasFullyInViewport = this.isFullyInViewport;
			wasAboveViewport = this.isAboveViewport;
			wasBelowViewport = this.isBelowViewport;

		};

		this.recalculateLocation = function() {
			if (this.locked) {
				return;
			}
			var previousTop = this.top;
			var previousBottom = this.bottom;
			if (this.watchItem.nodeName) { // a dom element
				var cachedDisplay = this.watchItem.style.display;
				if (cachedDisplay === 'none') {
					this.watchItem.style.display = '';
				}

				var boundingRect = this.watchItem.getBoundingClientRect();
				this.top = boundingRect.top + exports.viewportTop;
				this.bottom = boundingRect.bottom + exports.viewportTop;

				if (cachedDisplay === 'none') {
					this.watchItem.style.display = cachedDisplay;
				}

			} else if (this.watchItem === +this.watchItem) { // number
				if (this.watchItem > 0) {
					this.top = this.bottom = this.watchItem;
				} else {
					this.top = this.bottom = exports.documentHeight - this.watchItem;
				}

			} else { // an object with a top and bottom property
				this.top = this.watchItem.top;
				this.bottom = this.watchItem.bottom;
			}

			this.top -= this.offsets.top;
			this.bottom += this.offsets.bottom;
			this.height = this.bottom - this.top;

			if ( (previousTop !== undefined || previousBottom !== undefined) && (this.top !== previousTop || this.bottom !== previousBottom) ) {
				triggerCallbackArray( this.callbacks[LOCATIONCHANGE] );
			}
		};

		this.recalculateLocation();
		this.update();

		wasInViewport = this.isInViewport;
		wasFullyInViewport = this.isFullyInViewport;
		wasAboveViewport = this.isAboveViewport;
		wasBelowViewport = this.isBelowViewport;
	}

	ElementWatcher.prototype = {
		on: function( event, callback, isOne ) {

			// trigger the event if it applies to the element right now.
			switch( true ) {
				case event === VISIBILITYCHANGE && !this.isInViewport && this.isAboveViewport:
				case event === ENTERVIEWPORT && this.isInViewport:
				case event === FULLYENTERVIEWPORT && this.isFullyInViewport:
				case event === EXITVIEWPORT && this.isAboveViewport && !this.isInViewport:
				case event === PARTIALLYEXITVIEWPORT && this.isAboveViewport:
					callback.call( this, latestEvent );
					if (isOne) {
						return;
					}
			}

			if (this.callbacks[event]) {
				this.callbacks[event].push({callback: callback, isOne: isOne||false});
			} else {
				throw new Error('Tried to add a scroll monitor listener of type '+event+'. Your options are: '+eventTypes.join(', '));
			}
		},
		off: function( event, callback ) {
			if (this.callbacks[event]) {
				for (var i = 0, item; item = this.callbacks[event][i]; i++) {
					if (item.callback === callback) {
						this.callbacks[event].splice(i, 1);
						break;
					}
				}
			} else {
				throw new Error('Tried to remove a scroll monitor listener of type '+event+'. Your options are: '+eventTypes.join(', '));
			}
		},
		one: function( event, callback ) {
			this.on( event, callback, true);
		},
		recalculateSize: function() {
			this.height = this.watchItem.offsetHeight + this.offsets.top + this.offsets.bottom;
			this.bottom = this.top + this.height;
		},
		update: function() {
			this.isAboveViewport = this.top < exports.viewportTop;
			this.isBelowViewport = this.bottom > exports.viewportBottom;

			this.isInViewport = (this.top <= exports.viewportBottom && this.bottom >= exports.viewportTop);
			this.isFullyInViewport = (this.top >= exports.viewportTop && this.bottom <= exports.viewportBottom) ||
								 (this.isAboveViewport && this.isBelowViewport);

		},
		destroy: function() {
			var index = -1,
				self  = this;

			for (var i = 0; i < watchers.length; i++) {
				if (this.uid == watchers[i].uid) {
					index = i;
					break;
				}
			}

			if (index == -1) {
				index = watchers.indexOf(this);
			}

			watchers.splice(index, 1);
			for (var i = 0, j = eventTypes.length; i < j; i++) {
				self.callbacks[eventTypes[i]].length = 0;
			}
		},
		// prevent recalculating the element location
		lock: function() {
			this.locked = true;
		},
		unlock: function() {
			this.locked = false;
		}
	};

	var eventHandlerFactory = function (type) {
		return function( callback, isOne ) {
			this.on.call(this, type, callback, isOne);
		};
	};

	for (var i = 0, j = eventTypes.length; i < j; i++) {
		var type =  eventTypes[i];
		ElementWatcher.prototype[type] = eventHandlerFactory(type);
	}

	try {
		calculateViewport();
	} catch (e) {
		try {
			window.$(calculateViewport);
		} catch (e) {
			throw new Error('If you must put scrollMonitor in the <head>, you must use jQuery.');
		}
	}

	function scrollMonitorListener(event) {
		latestEvent = event;
		calculateViewport();
		updateAndTriggerWatchers();
	}

	if (window.addEventListener) {
		window.addEventListener('scroll', scrollMonitorListener);
		window.addEventListener('resize', debouncedRecalcuateAndTrigger);
	} else {
		// Old IE support
		window.attachEvent('onscroll', scrollMonitorListener);
		window.attachEvent('onresize', debouncedRecalcuateAndTrigger);
	}

	exports.beget = exports.create = function( element, offsets ) {
		if (typeof element === 'string') {
			element = document.querySelector(element);
		} else if (element && element.length > 0) {
			element = element[0];
		}

		var watcher = new ElementWatcher( element, offsets );
		watchers.push(watcher);
		watcher.update();
		return watcher;
	};

	exports.update = function() {
		latestEvent = null;
		calculateViewport();
		updateAndTriggerWatchers();
	};
	exports.recalculateLocations = function() {
		exports.documentHeight = 0;
		exports.update();
	};

	return exports;
});