Reddit ranking algorithms in Racket

reddit-ranking.rkt

#lang racket
(require "connive.rkt") 
; https://github.com/twfarland/connive

; Reddit ranking algorithms

; Story ranking (hot ranking)
; http://amix.dk/blog/post/19588
; Submission time greatly affects score
(:= epoch (current-seconds))

(:= (hot upvotes downvotes date-seconds)
  
  (:= time  (- date-seconds epoch))
  (:= score (- upvotes downvotes))
  (:= sign  (?? (> score 0)  1
                (> score 0)  -1
                             0))
  (:= order (?? (< score 1)  1
                             score))
  
  (+ (log order) (/ (* sign time) 45000)))


; Comment ranking (confidence sort using Wilson score)
; http://www.evanmiller.org/how-not-to-sort-by-average-rating.html
; Submission time doesn't affect score
(:= (confidence upvotes downvotes)
  
  (:= n (+ upvotes downvotes))
  
  (?? (= n 0) 0
      
      (let* ((z 1.0) ; 1.0 = 85%, 1.6 = 95% (sureness that comment will get to rank)
             (z^2 (* z z))
             (p (/ upvotes n)))
        
        (real-part (/ (sqrt (- ; use minus for lower bound
                             (+ p (/ z^2 (* 2 n))) 
                             (* z (/ (+ (* p (- 1 p)) (/ z^2 (* 4 n)) n)))))
                      (+ 1 (/ z^2 n)))))))
         

; generating test data
(struct story   (id user time body up down comments) #:transparent)
(struct comment (id user body up down) #:transparent)        
      
(:= users (vector "tim" "kat" "dave" "sally" "mitch" "walter" "george" "ingrid"))

(:= (random-user) 
   (vector-ref users (random (vector-length users))))

(:= (random-time) 
   (- epoch (random 1000)))

(:= stories
    (for/list ((s 10))
      (story s (random-user) (random-time) "" (random 300) (random 100)
             (for/list ((c 10)) 
               (comment c (random-user) "" (random 20) (random 20))))))


; applying ranking
(:= ((comparing f) x y) 
  (> (f x) (f y)))

(:= (story-heat story)
  (hot (story-up story) (story-down story) (story-time story)))

(:= (comment-confidence comment)
  (confidence (comment-up comment) (comment-down comment)))

(:= (rank-stories stories) 
  (sort stories (comparing story-heat)))

(:= (rank-comments comments)
  (sort comments (comparing comment-confidence)))


; try it out
(rank-comments (story-comments (list-ref stories 0)))

(map (λ (s) (list (story-up s) (story-down s) (story-time s))) 
     (rank-stories stories))