8

This is the Hacker News ranking algorithm, which I think is a simple way of ranking things, espcially if users are voting on items, but I really dnt understand this, can this be converted to PHP, so I can understand it fully? 

; Votes divided by the age in hours to the gravityth power.
; Would be interesting to scale gravity in a slider.


(= gravity* 1.8 timebase* 120 front-threshold* 1
           nourl-factor* .4 lightweight-factor* .17 gag-factor* .1)

        (def frontpage-rank (s (o scorefn realscore) (o gravity gravity*))
          (* (/ (let base (- (scorefn s) 1)
                  (if (> base 0) (expt base .8) base))
                (expt (/ (+ (item-age s) timebase*) 60) gravity))
             (if (no (in s!type 'story 'poll))  .8
                 (blank s!url)                  nourl-factor*
                 (mem 'bury s!keys)             .001
                                                (* (contro-factor s)
                                                   (if (mem 'gag s!keys)
                                                        gag-factor*
                                                       (lightweight s)
                                                        lightweight-factor*
                                                       1)))))
Gilles 'SO- stop being evil'
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getaway
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2 Answers2

12

Directly ripped from http://amix.dk/blog/post/19574 and translated to PHP from the Python:

function calculate_score($votes, $item_hour_age, $gravity=1.8){
    return ($votes - 1) / pow(($item_hour_age+2), $gravity);
}
Mark Elliot
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  • thank you for this great answer, can you just explain the $item_hour_age for me please, thanks , btw +1 upvote form me :)) – getaway Oct 20 '10 at 19:24
  • @getaway - `$item_hour_age` is age (amount of time since it was created) of the thing you're ranking counted in hours (starting at 0) – Mark Elliot Oct 20 '10 at 19:26
  • @MarkElliot, follow up question - how to order rows in a mysql select query based on this calculated algorithm outside of the mysql statement? – Andrew Samuelsen Dec 15 '11 at 04:59
  • @Andypandy: I don't think I understand your question; you want to order MySQL rows using this algorithm but dont want to let MySQL run the algorithm? – Mark Elliot Dec 15 '11 at 14:35
  • Can anyone explain, what is the gravity. In what cases does it increase and why is it used at all. I also have problems constructing my mysql query, I dont think pulling all the submissions from the db and then ranking them would be a good idea. Does HN limit it in anyway? Sorry for this pack of unconnected questions! – Nazar Mar 20 '12 at 18:04
6

There are write-ups about how this algorithm works. A quick search discovered: How Hacker News ranking algorithm works.

Lisp can make things seem more complicated than they really are.

tplaner
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