This isn't reddit. But both Reddit and HN are open-source, so you should be able to discover the algorithm. HN source is here at http://ycombinator.com/arc/arc2.tar and Reddit's can be found at http://reddit.com/code/ .
Didn't mean to give the impression I don't know where I am. I realize I am not on Reddit currently - I don't hear the sound of wild dogs grinding their teeth.