af_DragNDrop = function() {

	var that = this;
	
	// offset inside clicked element
	that.mouseOffset = {};
	// all drop targets
	that.dropTargets = {};
	
	// container which is actually being moved around
	that.dragContainer = null;
	// element that is currently being dragged
	that.currentlyDragging = null;
	// drag-Id
	that.currentId = '';
	// dragging already initialized?
	that.draggingInitialized = false;
	// position, where we started dragging
	that.startPosition = {};
	
	that.dragRegex = /\bdrag_(.*?)\b/;
	that.dropRegex = /\bdrop_(.*?)\b/;
// @TODO: maybe this can be replaced by manually setting the regex.global flag?
	that.dropRegexG = /\bdrop_(.*?)\b/g;
	
	that.load = function() {
		
		// create drag container
		that.dragContainer = document.createElement('div');
		that.dragContainer.className = 'dragcontainer';
		// better solution for this?
		document.getElementsByTagName('body')[0].appendChild(that.dragContainer);
		
		// find draggable elements
		var elements = af_CSS.getElementsByClass(that.dragRegex);
		for (var i = 0; i < elements.length; ++i) {
			af_Events.addEventListener(elements[i], 'mousedown', that.startDrag);
			af_Events.addEventListener(elements[i], 'click', that.interceptClick);
//			af_Events.addEventListener(elements[i], 'click', function(event) {alert('Clicked!'); });
		}
		
		// find drop targets
		var elements = af_CSS.getElementsByClass(that.dropRegex);
		var match;
		for (var i = 0; i < elements.length; ++i) {
			// an elment can serve as a drop target for differend IDs
			while (match = that.dropRegexG.exec(elements[i].className)) {
				var id = match[1];
				that.dropTargets[id] = that.dropTargets[id] || new Array();
				that.dropTargets[id].push(elements[i]);
			}
		}
	}
	
	that.startDrag = function(event) {
		event = event || window.event;
		
		var mpos = af_Events.getMousePosition(event);
		var epos = af_CSS.getPosition(this);
		
		// record mouse offset
		that.mouseOffset = {
			x: mpos.x - epos.x,
			y: mpos.y - epos.y
		}
		
		that.startPosition = mpos;
		that.currentlyDragging = this;
		that.draggingInitialized = false;
		
		af_Events.stopEvent(event);
		return false;
	}
	
	that.initDrag = function(event) {
		
		// find drag id
		var match = that.dragRegex.exec(that.currentlyDragging.className);
		if (!match || !match[1]) {
			return false;
		}
		that.currentId = match[1];
		
		// are there any drop targets?
		if (!that.dropTargets[that.currentId]) {
			return false;
		}
		
		// highlight drop targets
		for (var i = 0; i < that.dropTargets[that.currentId].length; ++i) {
			af_CSS.addClass(that.dropTargets[that.currentId][i], 'active_' + that.currentId);
		}
		
		var mpos = af_Events.getMousePosition(event);
		
		// prepare drag container
		that.dragContainer.innerHTML = '';
		that.dragContainer.style.top = mpos.y - that.mouseOffset.y + 'px';
		that.dragContainer.style.left = mpos.x - that.mouseOffset.x + 'px';
		// copy element into drag container
		var clone = that.currentlyDragging.cloneNode(true)
		var dims = af_CSS.getDimensions(that.currentlyDragging);
		clone.style.width = dims.width + 'px';
		clone.style.height = dims.height + 'px';
		that.dragContainer.appendChild(clone);
		that.dragContainer.style.display = 'block';
		// mark element as being dragged
		af_CSS.addClass(that.currentlyDragging, 'dragging');
		that.draggingInitialized = true;
		
		return true;
	}
	
	that.runDrag = function(event) {
		if (!that.currentlyDragging) {
			return;
		}
		event = event || window.event;
		
		if (!that.draggingInitialized) {
			// first mouse move, set up dragging
			if (!that.initDrag(event)) {
				af_Events.stopEvent(event);
				return false;
			}
		}
		
		var mpos = af_Events.getMousePosition(event);
		
		// move drag container		
		that.dragContainer.style.left = (mpos.x - that.mouseOffset.x) + 'px';
		that.dragContainer.style.top = (mpos.y - that.mouseOffset.y) + 'px';
		
		var dropTarget = null;
		for (var i = 0; i < that.dropTargets[that.currentId].length; ++i) {
			if (
				that.dropTargets[that.currentId][i] != that.currentlyDragging &&
				that.testBoundingBox(that.getBoundingBox(that.dropTargets[that.currentId][i]), mpos)
			) {
				af_CSS.addClass(that.dropTargets[that.currentId][i], 'hover_' + that.currentId);
			} else {
				af_CSS.removeClass(that.dropTargets[that.currentId][i], 'hover_' + that.currentId);
			}
		}
		
		// required for Internet Explorer:
		return false;
	}
	
	that.endDrag = function(event) {
		if (!that.draggingInitialized) {
			that.currentlyDragging = null;
		}
		if (!that.currentlyDragging) {
			return;
		}
		event = event || window.event;
		
		// find drop target
		if (!that.dropTargets[that.currentId]) {
			return;
		}
		var dropTarget = null;
		var mpos = af_Events.getMousePosition(event);
		for (var i = 0; i < that.dropTargets[that.currentId].length; ++i) {
			if (that.testBoundingBox(that.getBoundingBox(that.dropTargets[that.currentId][i]), mpos)) {
				dropTarget = that.dropTargets[that.currentId][i];
				break;
			}
		}
			
		// reset highlighted drop targets
		for (var i = 0; i < that.dropTargets[that.currentId].length; ++i) {
			af_CSS.removeClass(that.dropTargets[that.currentId][i], 'active_' + that.currentId);
			af_CSS.removeClass(that.dropTargets[that.currentId][i], 'hover_' + that.currentId);
		}
		
		// backup element
		var element = that.currentlyDragging;
		
		// stop dragging
		af_CSS.removeClass(that.currentlyDragging, 'dragging');
		that.currentlyDragging = null;		
		that.dragContainer.style.display = 'none';
		that.dragContainer.style.top = 0;
		that.dragContainer.style.left = 0;
		that.dragContainer.innerHTML = '';
		
		af_Events.stopEvent(event);
		
		if (dropTarget && that['drop' + that.currentId]) {
			return that['drop' + that.currentId](element, dropTarget);
		}
		
		return false;
	}
	
	// intercepts clicks if mouse has moved
	that.interceptClick = function(event) {
		event = event || window.event;
		var mpos = af_Events.getMousePosition(event);
		if (mpos.x  != that.startPosition.x || mpos.y != that.startPosition.y) {
			// mouse was moved => dragging took place => intercept click 
			af_Events.stopEvent(event);
			return false;
		}
		return true;
	}
	
	that.getBoundingBox = function(element) {
		var coords = af_CSS.getPosition(element);
		var dimensions = af_CSS.getDimensions(element);
		return {
			xMin: coords.x,
			yMin: coords.y,
			xMax: coords.x + dimensions.width,
			yMax: coords.y + dimensions.height
		}
	}
	
	that.testBoundingBox = function(bbox, mpos) {
		return (
			mpos.x > bbox.xMin &&
			mpos.x < bbox.xMax &&
			mpos.y > bbox.yMin &&
			mpos.y < bbox.yMax
		);
	}
	
	af_Events.addOnEvent(window, 'load', that.load);
	af_Events.addEventListener(document, 'mousemove', that.runDrag);
	af_Events.addEventListener(document, 'mouseup', that.endDrag);
}