Adds initial hot score implementation for debates

app/models/debate.rb

@@ -21,6 +21,8 @@ class Debate < ActiveRecord::Base
   before_validation :sanitize_description
   before_validation :sanitize_tag_list
 
+  before_save :calculate_hot_score
+
   scope :sorted_for_moderation, -> { order(flags_count: :desc, updated_at: :desc) }
   scope :pending_flag_review, -> { where(ignored_flag_at: nil, hidden_at: nil) }
   scope :with_ignored_flag, -> { where("ignored_flag_at IS NOT NULL AND hidden_at IS NULL") }
@@ -102,6 +104,24 @@ class Debate < ActiveRecord::Base
     update(ignored_flag_at: Time.now)
   end
 
+  def calculate_hot_score
+    z          = 1.96 # Normal distribution with a confidence of 0.95
+    time_unit  = 12.hours
+    start      = Time.new(2015, 6, 15)
+
+    n   = cached_votes_total + comments_count / 3
+    pos = cached_votes_up + comments_count / 3
+
+    phat = 1.0 * pos / n
+
+    weigted_score = n == 0 ? 0 :
+      (phat + z*z/(2*n) - z * Math.sqrt((phat*(1-phat)+z*z/(4*n))/n))/(1+z*z/n)
+
+    age_in_units = ((created_at || Time.now) - start) / time_unit
+
+    self.hot_score = (age_in_units**3 + weigted_score * 1000).round
+  end
+
   protected
 
   def sanitize_description
@@ -111,5 +131,4 @@ class Debate < ActiveRecord::Base
   def sanitize_tag_list
     self.tag_list = TagSanitizer.new.sanitize_tag_list(self.tag_list)
   end
-
 end

spec/factories.rb

@@ -71,6 +71,10 @@ FactoryGirl.define do
         Flag.flag(FactoryGirl.create(:user), debate)
       end
     end
+
+    trait :with_hot_score do
+      before(:save) { |d| d.calculate_hot_score }
+    end
   end
 
   factory :vote do

spec/models/debate_spec.rb

@@ -181,4 +181,38 @@ describe Debate do
     end
   end
 
+  describe '#hot_score' do
+    let(:now) { Time.now }
+
+    it "gets bigger for newer debates" do
+      old = create(:debate, :with_hot_score, created_at: now - 1.day)
+      new = create(:debate, :with_hot_score, created_at: now)
+      expect(new.hot_score).to be > old.hot_score
+    end
+
+    it "gets bigger for debates with more comments" do
+      more_comments = create(:debate, :with_hot_score, created_at: now, comments_count: 10)
+      less_comments = create(:debate, :with_hot_score, created_at: now, comments_count: 1)
+      expect(more_comments.hot_score).to be > less_comments.hot_score
+    end
+
+    it "gets bigger for debates with more positive votes" do
+      more_likes = create(:debate, :with_hot_score, created_at: now, cached_votes_total: 10, cached_votes_up: 5)
+      less_likes = create(:debate, :with_hot_score, created_at: now, cached_votes_total: 10, cached_votes_up: 1)
+      expect(more_likes.hot_score).to be > less_likes.hot_score
+    end
+
+    it "gets bigger for debates with more confidence" do
+      more_confidence = create(:debate, :with_hot_score, created_at: now, cached_votes_total: 1000, cached_votes_up: 700)
+      less_confidence = create(:debate, :with_hot_score, created_at: now, cached_votes_total: 10, cached_votes_up: 9)
+      expect(more_confidence.hot_score).to be > less_confidence.hot_score
+    end
+
+    it "decays older debates, even if they have more votes" do
+      older_more_voted = create(:debate, :with_hot_score, created_at: now - 2.days, cached_votes_total: 1000, cached_votes_up: 900)
+      new_less_voted   = create(:debate, :with_hot_score, created_at: now, cached_votes_total: 10, cached_votes_up: 9)
+      expect(new_less_voted.hot_score).to be > older_more_voted.hot_score
+    end
+  end
+
 end