He glossed over it, but this code does include URL-free posts dropping off faster. See the stuff dealing with "nourl-factor*". I don't know arc at all, but it appears that having no URL multiplies your final score by a factor of .4, meaning that it's ranked almost 3x lower than it otherwise would be. That surprises me; I've noticed that Ask HN get rated lower, but it doesn't seem that extreme.

So is Hacker News is a fork of news.arc, rather than straight news.arc? I figured it was, but never heard that officially (since people refer to news.arc as the HN "source code").

Edit: Also, the "lightweight" thing is interesting. There's something in place that sees if the post is a "rallying cry" or is mostly made of images. Additionally, if you link directly to an image file, or to some list of domains that have been deemed lightweight, that'll get marked as lightweight as well. Lightweight posts have a .3 factor, meaning that they're even more deflated than URL-free posts.

Ah yes, you're right, the 'nourl' is there, it's in the arc bit, but I couldn't find that in the graphs or in the python code.

The current HN algorithm probably follows the same basic idea, but the implementation is more complex because of spammers.

I did a fast test of how placements would look like with the vanilla HN algorithm using the current frontpage: http://paste.plurk.com/show/316811/

The code to generate the new sorting: http://paste.plurk.com/show/316812/

As you can see the rankings are not the same, but very similar...

Flags are the main factor that you've missed I'm not quite sure how to measure them reliably but from what I've seen in terms of 'jumps' downward on a heavily flagged post it looked like 6 flags will take you off the homepage on to page two when you have about 20 upvotes. Maybe that will allow you to model it.

I have two will take you off when you are under 9 upvotes.

How did you know how many flags it had?

Counting the jumps down the home page. Would be nice if PG could confirm this either way (right or wrong), but I'm fairly sure that's what it is. It wouldn't make sense any other way (it was this post: http://news.ycombinator.com/item?id=1759548 ) it got to the first spot on the homepage very quickly and then dropped in six steps off the homepage.

19 points, six steps, that's averaging 3 points per step, so it's like a single flag counts for 3 'downvotes' on an article or something close to that.

That's all guesswork of course.