/* Creates a clone of an element and lets it track the mouse as it moves
----------------------------------------------------------------------------------------------------------------------*/

function MouseFollower(sourceEl, options) {
	this.options = options = options || {};
	this.sourceEl = sourceEl;
	this.parentEl = options.parentEl ? $(options.parentEl) : sourceEl.parent(); // default to sourceEl's parent
}


MouseFollower.prototype = {

	options: null,

	sourceEl: null, // the element that will be cloned and made to look like it is dragging
	el: null, // the clone of `sourceEl` that will track the mouse
	parentEl: null, // the element that `el` (the clone) will be attached to

	// the initial position of el, relative to the offset parent. made to match the initial offset of sourceEl
	top0: null,
	left0: null,

	// the initial position of the mouse
	mouseY0: null,
	mouseX0: null,

	// the number of pixels the mouse has moved from its initial position
	topDelta: null,
	leftDelta: null,

	mousemoveProxy: null, // document mousemove handler, bound to the MouseFollower's `this`

	isFollowing: false,
	isHidden: false,
	isAnimating: false, // doing the revert animation?


	// Causes the element to start following the mouse
	start: function(ev) {
		if (!this.isFollowing) {
			this.isFollowing = true;

			this.mouseY0 = ev.pageY;
			this.mouseX0 = ev.pageX;
			this.topDelta = 0;
			this.leftDelta = 0;

			if (!this.isHidden) {
				this.updatePosition();
			}

			$(document).on('mousemove', this.mousemoveProxy = $.proxy(this, 'mousemove'));
		}
	},


	// Causes the element to stop following the mouse. If shouldRevert is true, will animate back to original position.
	// `callback` gets invoked when the animation is complete. If no animation, it is invoked immediately.
	stop: function(shouldRevert, callback) {
		var _this = this;
		var revertDuration = this.options.revertDuration;

		function complete() {
			this.isAnimating = false;
			_this.destroyEl();

			this.top0 = this.left0 = null; // reset state for future updatePosition calls

			if (callback) {
				callback();
			}
		}

		if (this.isFollowing && !this.isAnimating) { // disallow more than one stop animation at a time
			this.isFollowing = false;

			$(document).off('mousemove', this.mousemoveProxy);

			if (shouldRevert && revertDuration && !this.isHidden) { // do a revert animation?
				this.isAnimating = true;
				this.el.animate({
					top: this.top0,
					left: this.left0
				}, {
					duration: revertDuration,
					complete: complete
				});
			}
			else {
				complete();
			}
		}
	},


	// Gets the tracking element. Create it if necessary
	getEl: function() {
		var el = this.el;

		if (!el) {
			this.sourceEl.width(); // hack to force IE8 to compute correct bounding box
			el = this.el = this.sourceEl.clone()
				.css({
					position: 'absolute',
					visibility: '', // in case original element was hidden (commonly through hideEvents())
					display: this.isHidden ? 'none' : '', // for when initially hidden
					margin: 0,
					right: 'auto', // erase and set width instead
					bottom: 'auto', // erase and set height instead
					width: this.sourceEl.width(), // explicit height in case there was a 'right' value
					height: this.sourceEl.height(), // explicit width in case there was a 'bottom' value
					opacity: this.options.opacity || '',
					zIndex: this.options.zIndex
				})
				.appendTo(this.parentEl);
		}

		return el;
	},


	// Removes the tracking element if it has already been created
	destroyEl: function() {
		if (this.el) {
			this.el.remove();
			this.el = null;
		}
	},


	// Update the CSS position of the tracking element
	updatePosition: function() {
		var sourceOffset;
		var origin;

		this.getEl(); // ensure this.el

		// make sure origin info was computed
		if (this.top0 === null) {
			this.sourceEl.width(); // hack to force IE8 to compute correct bounding box
			sourceOffset = this.sourceEl.offset();
			origin = this.el.offsetParent().offset();
			this.top0 = sourceOffset.top - origin.top;
			this.left0 = sourceOffset.left - origin.left;
		}

		this.el.css({
			top: this.top0 + this.topDelta,
			left: this.left0 + this.leftDelta
		});
	},


	// Gets called when the user moves the mouse
	mousemove: function(ev) {
		this.topDelta = ev.pageY - this.mouseY0;
		this.leftDelta = ev.pageX - this.mouseX0;

		if (!this.isHidden) {
			this.updatePosition();
		}
	},


	// Temporarily makes the tracking element invisible. Can be called before following starts
	hide: function() {
		if (!this.isHidden) {
			this.isHidden = true;
			if (this.el) {
				this.el.hide();
			}
		}
	},


	// Show the tracking element after it has been temporarily hidden
	show: function() {
		if (this.isHidden) {
			this.isHidden = false;
			this.updatePosition();
			this.getEl().show();
		}
	}

};