jquery 上下滚动广告

(function($){
$.fn.extend({
rollList:function(option){
option=$.extend({
direction:"up",
step:1,
time:23
},option);
var step_coe,scroll_coe,score_coe;
if(option.direction=="up")
{
step_coe=1;
scroll_coe=1;
score_coe=1;
}else
{
step_coe=-1;
scroll_coe=-1;
score_coe=0;
}
return this.each(function(){
var $this=$(this);
var _this=this;
var itemHeight;
var temp=$("<DIV> </DIV>");
$this.css("overflow","hidden").children()
.appendTo(temp);
$this.append(temp.clone(true)).append(temp);
itemHeight=$this.children();
itemHeight=itemHeight.eq(1).offset().top-itemHeight.eq(0).offset().top;
while($this.children(":last").offset().top-$this.offset().top<=$this.height())
$this.append(temp.clone(true));
var roll;
this.scrollTop=itemHeight*(1-score_coe);
roll=function (){
temp=setInterval(function(){
if(_this.scrollTop*scroll_coe>=itemHeight*score_coe)
{
_this.scrollTop=(_this.scrollTop-itemHeight)*scroll_coe;
}
_this.scrollTop+=option.step*step_coe;

},option.time);
}
$this.hover(function(){
clearInterval(temp);
},function(){
roll();
});
roll();
});
}
})
}(jQuery));

$(elem).rollList();