// scroller(el, duration)
//
// el : 目標錨點 ID
// duration : 持續時間，以毫秒為單位，越小越快

function intval(v){
  v=parseInt(v);
  return isNaN(v)?0:v;
}

function getPos(e){
  var l=0;
  var t=0;
  var w=intval(e.style.width);
  var h=intval(e.style.height);
  var wb=e.offsetWidth;
  var hb=e.offsetHeight;
  while(e.offsetParent){
	l+=e.offsetLeft+(e.currentStyle?intval(e.currentStyle.borderLeftWidth):0);
	t+=e.offsetTop+(e.currentStyle?intval(e.currentStyle.borderTopWidth):0);
	e=e.offsetParent;
  }
  l+=e.offsetLeft+(e.currentStyle?intval(e.currentStyle.borderLeftWidth):0);
  t+=e.offsetTop+(e.currentStyle?intval(e.currentStyle.borderTopWidth):0);
  return{x:l,y:t,w:w,h:h,wb:wb,hb:hb};
}

function getScroll(){
  var t,l,w,h;
  if(document.documentElement&&document.documentElement.scrollTop){
	t=document.documentElement.scrollTop;
	l=document.documentElement.scrollLeft;
	w=document.documentElement.scrollWidth;
	h=document.documentElement.scrollHeight;
  } else if(document.body){
	t=document.body.scrollTop;
	l=document.body.scrollLeft;
	w=document.body.scrollWidth;
	h=document.body.scrollHeight;
  }
  return{t:t,l:l,w:w,h:h};
}

function scroller(el,duration){
  if(typeof el!='object'){
	  el=document.getElementById(el);
  }
  
  if(!el)return;
  var z=this;
  z.el=el;
  z.p=getPos(el);
  z.s=getScroll();
  z.clear=function(){
	window.clearInterval(z.timer);
	z.timer=null
  };
  z.t=(new Date).getTime();
  z.step=function(){
	var t=(new Date).getTime();
	var p=(t-z.t)/duration;
	if(t>=duration+z.t){
		z.clear();
		window.setTimeout(function(){z.scroll(z.p.y,z.p.x)},13);
	} else {
		st=((-Math.cos(p*Math.PI)/2)+0.5)*(z.p.y-z.s.t)+z.s.t;
		sl=((-Math.cos(p*Math.PI)/2)+0.5)*(z.p.x-z.s.l)+z.s.l;
		z.scroll(st,sl);
	}
  };
  
z.scroll=function(t,l){
  window.scrollTo(l,t)};
  z.timer=window.setInterval(function(){z.step();},13);
}