function sortable(elem) {
    switch(elem.tagName) {
    case "LI":
    case "DT":
    case "DD":
	return elem.className!="sep" && 
	       elem.parentNode.className=="sortable" ? elem : null;
    case "IMG":
	return sortable(elem.parentNode);
    }
    return null;
}

function move_element(elem,x,y) {
    elem.style.left=x+"px";
    elem.style.top=y+"px";
    elem.delta={x:x,y:y};
}

function adjust_refs(elem,dy) {
    //jsdebug("dy="+dy);
    move_element(elem,elem.delta.x,elem.delta.y-dy);
    elem.downAt.y+=dy;
    elem.range.lo-=dy;
    elem.range.hi-=dy;
}

function move_up(elem) {
    var prev=elem.previousSibling;
    if(prev) {
	var top=elem.offsetTop;
	var mid=prev.offsetTop+prev.offsetHeight/2;
	if(top<mid) {
	    elem.parentNode.insertBefore(elem,prev);
	    adjust_refs(elem,elem.offsetTop-top)
	}
	//else jsdebug("not yet, top="+top+", mid="+mid);
    }
    //else jsdebug("at top");
}

function move_down(elem) {
    var next=elem.nextSibling;
    if(next) {
	var top=elem.offsetTop;
	var bot=top+elem.offsetHeight;
	var mid=next.offsetTop+next.offsetHeight/2;
	if(bot>mid) {
	    next.parentNode.insertBefore(next,elem);
	    adjust_refs(elem,elem.offsetTop-top)
	}
	//else jsdebug("not yet, top="+top+", bot="+bot+", mid="+mid);
    }
    //else jsdebug("at bottom");
}

function swap(elem,dy) {
    if(dy>0) move_down(elem);
    else if(dy<0) move_up(elem);
}

function restrictTo(range,y) {
    return Math.min(range.hi,Math.max(range.lo,y));
}

function startDrag(event,elem) {
    //jsdebug("Start dragging");
    elem.style.position="relative";
    elem.delta || (elem.delta={x:0,y:0});
    elem.downAt={x:event.screenX-elem.delta.x,y:event.screenY-elem.delta.y};
    var list=elem.parentNode;
    var top=list.offsetTop-elem.offsetTop+elem.delta.y;
    elem.range={lo:top,hi:top+list.offsetHeight-elem.offsetHeight};
    elem.style.zIndex=1;
    document.onmousemove=function(event) {
	var dx=0/*event.screenX-elem.downAt.x*/;
	var dy=restrictTo(elem.range,event.screenY-elem.downAt.y);
	//jsdebug("dragging to "+dx+" "+dy+" "+show_props(elem.range,"range"));
	move_element(elem,dx,dy);
	//jsdebug("dragging to "+elem.offsetLeft+" "+elem.offsetTop);
	swap(elem,dy)
	return false;
    }
    document.onmouseup=function() {
	//jsdebug("dropped");
	elem.style.zIndex=0;
	move_element(elem,0,0);
	document.onmousemove=null;
	document.onmouseup=null;
	return false;
    }
    return false;
}

function mousedown(event) {
    var elem=sortable(event.target);
    if(elem) return startDrag(event,elem);
    //else jsdebug("Clicked"+taglist(event.target)/*+show_props(event,"event")*/);
}

//jsdebug=function() { };

function init() {
    document.onmousedown=mousedown;
    //var d=element("javascriptdebug");
    //if(d) jsdebug=function(msg) { d.innerHTML=msg; }
}

onload=init;

//Inspired by http://tool-man.org/examples/sorting.html