Reddit ranking algorithm in Ruby

gistfile1.rb

#!/Users/dustyeike/.rvm/rubies/ruby-1.9.2-p290/bin/ruby
require "rubygems"
require "test/unit"
require "date"

EPOCH = Date.new(1970, 1, 1)

#
# Returns the number of seconds from the epoch to date
def epoch_seconds date
  td = date - EPOCH
  Time.at(td).day * 86400 + Time.at(td).sec + (Time.at(td).usec).to_f / 1000000
end

def score ups, down
  ups - down
end

#
# The hot formula. Should match the equivalent function in postgres.
def hot ups, down, date
  s = score ups, down
  order = Math.log([s.abs, 1].max, 10)
  
  if s > 0
    sign = 1
  elsif s < 0
    sign = -1
  else
    sign = 0
  end

  seconds = epoch_seconds(date) - 1134028003
  (order + sign * seconds / 45000).round
end

class TestRedditAlgoritym < Test::Unit::TestCase
  def setup
    today = DateTime.now
    @now = Date.new(today.year, today.month, today.day)
  end
  
  def test_epoch_seconds
    assert_equal(epoch_seconds(@now), 2678403.0)
  end
  
  def test_score
    assert_equal(score(10, 5), 5)
  end
  
  def test_hot
    assert_equal(hot(10,5,@now), -25140)
  end
end

I'm trying to understand the reddit ranking algorithm, and came accross this gist.

How can hot(10,5,@now) equal -25140 for any value of @now? And yet, indeed it seems to, I just tried it, and with my different now calculated, well, now, it's still -25140. Seems to indicate something's not right?

Also I think you have a sign reversed somehow, shouldn't more upvotes than downvotes be positive rather than negative?

But even when done right, the ranking algorithm (which I got from the same blog posts you prob did and have been trying to play with too) doesn't seem to actually match what reddit does, I think those blog posts must have left something out; it seems to put way too much emphasis on votes and not enough on time of submission, while actual reddit does the reverse, doesn't it?

Have you figured out anything illuminating?

As this is the 2nd result in Google and the gist has a typo and won't work properly, I should post the corrected gist in ruby(not made by me):

https://gist.github.com/jrochkind/2636355