reddit 'hot' algorithm, in ruby, with typo fixed

gistfile1.rb

require 'date'

# Actually doesn't matter WHAT you choose as the epoch, it
# won't change the algorithm. Just don't change it after you
# have cached computed scores. Choose something before your first
# post to avoid annoying negative numbers. Choose something close
# to your first post to keep the numbers smaller. This is, I think,
# reddit's own epoch. 
$our_epoch = Time.local(2005, 12, 8, 7, 46, 43).to_time


def epoch_seconds(t)
  (t.to_i - $our_epoch.to_i).to_f
end


# date is a ruby Time
def hot(ups, downs, date)
    s = ups - downs
    displacement = Math.log( [s.abs, 1].max,  10 )

    sign = if s > 0
      1
    elsif s < 0
      -1
    else
      0
    end

    return (displacement * sign.to_f) + ( epoch_seconds(date) / 45000 )
end 

See http://bibwild.wordpress.com/2012/05/08/reddit-story-ranking-algorithm/