Slowly Scroll To Top ?

I've started seeing this on a few pages and I'd like to add it to mine. I'd like to add a back to top button where it doesn't just jump right to the top, but acts like the user were actually using the scroll bar.

Here are some well done examples: [url] and [url]

I like the first one better (where it shows an up arrow if you've scrolled far enough), and I found what I think is the javascript responsible (though I'm not sure if it's exact right to take it.) let alone, i'm not sure how i would even implement it yet.

[url]

I've got a basic example working on my own site, I just don't know how to stop it, (right now it just keeps going and going even if i try to scroll back down.) i'd like to be able to tell if the user reached the top, then stop it, but from what I've looked, i can't find a way to check where exactly they are in my page.

I figure, if i can get mine to stop, I can fancify it later with the image that shows if you've moved down and what not, right now I'd like to just stop it when it reaches the top.