ruby_recommender.rb

class User
  def similarity_with(user)
    # Array#& is the set intersection operator.
    agreements = (self.likes & user.likes).size
    agreements += (self.dislikes & user.dislikes).size

    disagreements = (self.likes & user.dislikes).size
    disagreements += (self.dislikes & user.likes).size

    # Array#| is the set union operator
    total = (self.likes + self.dislikes) | (user.likes + user.dislikes)

    return (agreements - disagreements) / total.size.to_f
  end

  def prediction_for(item)
    hive_mind_sum = 0.0
    rated_by = item.liked_by.size + item.disliked_by.size

    item.liked_by.each { |u| hive_mind_sum += self.similarity_with(u) }
    item.disliked_by.each { |u| hive_mind_sum -= self.similarity_with(u) }

    return hive_mind_sum / rated_by.to_f
  end
end