// bouncydiv.js - v 1.0 
// 
// by Dejapong L. Suwaratana
// www.dejapong.com
// released 3/21/2009
//
// This script makes a div scroll down with the screen. Its most useful for 
// advertising blocks that you want to always be in front of the user. To use 
// this script, set the variable 'scrollerId' to the id of the div you want to 
// be bouncy. Remember to also set the div's style position to 
// "position:relative;" as this will allow the div to move. Then simply add the 
// javascript file to your webpage. 
//
// Feel free to use this as you see fit. Credit would be appreciated or at least 
// just a link to dejapong.com if you can find it in your heart to do so. 
// 
// Have fun. 


    var bouncyTime = 2                  //seconds - depends on computer some slower computer will take longer
    var stickyness = 4;                 //if we are using the easy method, this restricts bouncyness
    var useRealDHO = false              //use realistic dynamic harmonic oscillations. This can be slower.
        //these variables are used with realistic DHO. this is overkill for most webapps. 
        var mass = 1;                   //mass of object affects momentum
        var dampingConst =3;            //think of this as friction
        var frequency = 3;              //cycles per second

    var scrollerId = 'advertblock'      //id of the div to scroll
    var offset = null;                  //keep at least this far from the top
    var waitTime = '600'               //Time to wait before starting animation (milliseconds)

    var startPoint;                 
    var endPoint; 
    var intval 
    var timeout    
    var t = 0;

    

    //helper function
    function byId(id){
        return document.getElementById(id);
    }

    //trim 'px' helper function
	function trimPx(strg){
        strip=/[px,pt,em]/gi;
        strg?null:strg="0px";
        strg.replace(strip,'');
        return parseInt(strg);
    }

    //kick off our animation
    function init(){
        //if we have an animation already in progress, cancel it 
        (intval?window.clearInterval(intval):null)
        //reset our time to zero
        t = 0;
        //get the starting point by finding the top of our div
        startPoint = trimPx(byId(scrollerId).style.top);
        endPoint = f_scrollTop()+(offset/2);
        
        intval = window.setInterval("oscillate()",10);
    }

    function oscillate(){ 
        //if we are at the end of our time //then stop the animation//else, increment the time for our equation
        (t == 1000?window.clearInterval(intval):t += 10/bouncyTime)
        //scale it for our equation
        ts = t/100;
        //if we are using real Damped harmonic oscillation
        if (useRealDHO){
            od = dampingConst/(2*mass);
            oprime = Math.sqrt(frequency*frequency - od*od);
            moveto = (startPoint-endPoint) * Math.pow(Math.E,-ts*od) * Math.cos(2*Math.PI+(ts*oprime)) + endPoint ;   
        }else{
            //else, do the quick method
            moveto =  (startPoint-endPoint) * Math.pow(Math.E,-ts) * Math.cos(2*Math.PI*(ts/stickyness)) + endPoint;
        }   
        //keep it from going up too far
        moveto < 0 ? moveto = 0 : null; 
        byId(scrollerId).style.top = Math.round(moveto) + "px";
    }

    //return the location of the scroll in any browser
    function f_scrollTop() {
    	return f_filterResults (
    		window.pageYOffset ? window.pageYOffset : 0,
    		document.documentElement ? document.documentElement.scrollTop : 0,
    		document.body ? document.body.scrollTop : 0
    	);
    }

    //helper function to return scroll location
    function f_filterResults(n_win, n_docel, n_body) {
    	var n_result = n_win ? n_win : 0;
    	if (n_docel && (!n_result || (n_result > n_docel)))
    		n_result = n_docel;
    	return n_body && (!n_result || (n_result > n_body)) ? n_body : n_result;
    }

    //whenever the scrolling happens, wait half a second and kick start our animation    
    window.onscroll = function(evt){
        //if the offset is not set, then set it 
        offset ? null : offset = document.getElementById(scrollerId).offsetTop * -1 + 30;  
        (timeout?window.clearTimeout(timeout):null);
        timeout = window.setTimeout("init()",waitTime);
    };
    
  //  window.onload += function(evt){
  function startBounce(){
        //if the offset is not set, then set it 
        offset ? null : offset = document.getElementById(scrollerId).offsetTop * -1 +30;  
        (timeout?window.clearTimeout(timeout):null);
        timeout = window.setTimeout("init()",waitTime);
    };