尝试创建自定义jQuery文本滚轮

我想做一个自己的自定义js-text-scroller(我希望文本在div内向左滚动，然后一次又一次地重复滚动)。

但是我有一些问题。我真的不能指出我哪里做错了，或者哪里需要新的功能。所有我能得到的帮助都是受欢迎的。

这里有一个简单的例子:http://jsfiddle.net/Na373/

<div class="wrap">
   <div class="scroll">
       <ul class="active">
           <li>Hello World</li>
           <li>Hello World</li>
           <li>Hello World</li>
           <li>Hello World</li>
           <li>Hello World</li>
           <li>Hello World</li>
           <li>Hello World</li>
           <li>Hello World</li>
           <li>Hello World</li>
           <li>Hello World</li>
       </ul>
   </div>

JS:

$ul = $('.scroll ul');
//copys the existing ul and appends it after the existing one
$('.scroll').append($ul.clone().removeClass().addClass('not_active'));
function scroll() {
    $active = $('.scroll ul.active');
    $active.each(function() {
        $not_active = $('.scroll ul.not_active');
        // foreach time the function runs the ul goes "left: -1px;"
        var current_position = parseInt($(this).css('left'));
        var new_position = current_position - 1;
        $(this).css('left', new_position+'px');
        var parent_width = $(this).parent().outerWidth(); // gets the width of ".scroll"
        var width = $(this).outerWidth(); // gets the width of the "ul"
        var shown_all = current_position + width;
        // if the right corner of the "ul" are att the right corner of ".scroll"
        // this if statement executes and adds new existing class on the "ul.not_active"
        // and adds class ".active" instead
        // this means that there are 2 ul with the class active 
        //running simultaneously
        if (shown_all == parent_width) {
            $not_active.removeClass().addClass('active');
        }
        // here it checks if the "ul.active" have past its own length to left
        // Here im trying to make it "not_active" and put it after the existing ".active"
        //
        // <ul class="active">....</ul> ----> <ul class="not_active">...</ul>
        //
        // then put it after the other "ul.active"
        //
        //            <ul class="active
        // ---------> <ul class="not_active"></ul>
        //
        // So it all beginns from the beginning
        if (current_position == '-'+width) {
            $(this).removeClass().addClass('not_active').css('left', '0');
            $(this).insertAfter($('.scroll ul').next());
        }
    });
};
// And here I run the function
setInterval(scroll, 10);

.wrap {  
    position: relative;
    padding: 10px 10px 10px;
    height: 18px;
    background: #000;
    color: #fff;
    overflow: hidden;
    width: 500px;
}
.scroll {
   width: 500px;
   white-space: nowrap;
}
.scroll ul {
    left: 0;
    position: relative;
    margin: 0;
    padding: 0;
    display: inline;
    list-style: none;
}
.scroll ul li {
    padding: 0;
    margin: 0;
    display: inline;
}