DEV: adjustments to hot algorithm (#25517)

1. Serial likers will just like a bunch of posts on the same topic, this will
heavily inflate hot score. To avoid artificial "heat" generated by one user only count
the first like on the topic within the recent_cutoff range per topic

2. When looking at recent topics prefer "unique likers", defer to total likes on
older topics cause we do not have an easy count for unique likers

3. Stop taking 1 off like_count, it is not needed - platforms like reddit
allow you to like own post so they need to remove it.

app/models/topic_hot_score.rb

@@ -66,7 +66,7 @@ class TopicHotScore < ActiveRecord::Base
               t.id AS topic_id,
               COUNT(DISTINCT p.user_id) AS unique_participants,
               (
-                SELECT COUNT(*)
+                SELECT COUNT(distinct pa.user_id)
                 FROM post_actions pa
                 JOIN posts p2 ON p2.id = pa.post_id
                 WHERE p2.topic_id = t.id
@@ -100,7 +100,10 @@ class TopicHotScore < ActiveRecord::Base
     # we need an extra index for this
     DB.exec(<<~SQL, args)
       UPDATE topic_hot_scores ths
-      SET score = (topics.like_count - 1) /
+      SET score = (
+        CASE WHEN topics.created_at > :recent_cutoff
+          THEN ths.recent_likes ELSE topics.like_count END
+        ) /
         (EXTRACT(EPOCH FROM (:now - topics.created_at)) / 3600 + 2) ^ :gravity
  +
         CASE WHEN ths.recent_first_bumped_at IS NULL THEN 0 ELSE

spec/models/topic_hot_scores_spec.rb

@@ -4,6 +4,7 @@ RSpec.describe TopicHotScore do
   describe ".update_scores" do
     fab!(:user)
     fab!(:user2) { Fabricate(:user) }
+    fab!(:user3) { Fabricate(:user) }
 
     it "can correctly update like counts and post counts and account for activity" do
       freeze_time
@@ -24,35 +25,57 @@ RSpec.describe TopicHotScore do
       PostActionCreator.like(user2, new_reply)
       PostActionCreator.like(user, newer_reply)
 
+      # user 3 likes two posts, but we should only count 1
+      # this avoids a single user from trivially inflating hot scores
+      PostActionCreator.like(user3, new_reply)
+      PostActionCreator.like(user3, newer_reply)
+
       TopicHotScore.update_scores
 
       hot_scoring = TopicHotScore.find_by(topic_id: topic.id)
 
-      expect(hot_scoring.recent_likes).to eq(2)
+      expect(hot_scoring.recent_likes).to eq(3)
       expect(hot_scoring.recent_posters).to eq(2)
       expect(hot_scoring.recent_first_bumped_at).to eq_time(new_reply.created_at)
-      expect(hot_scoring.score).to be_within(0.001).of(1.219)
+      expect(hot_scoring.score).to be_within(0.001).of(1.771)
 
       expect(TopicHotScore.find_by(topic_id: -1).recent_likes).to eq(0)
     end
 
+    it "prefers recent_likes to topic like count for recent topics" do
+      freeze_time
+
+      topic = Fabricate(:topic, created_at: 1.hour.ago)
+      post = Fabricate(:post, topic: topic, created_at: 1.minute.ago)
+      PostActionCreator.like(user, post)
+
+      TopicHotScore.update_scores
+      score = TopicHotScore.find_by(topic_id: topic.id).score
+
+      topic.update!(like_count: 100)
+
+      TopicHotScore.update_scores
+
+      expect(TopicHotScore.find_by(topic_id: topic.id).score).to be_within(0.001).of(score)
+    end
+
     it "can correctly set scores for topics" do
       freeze_time
 
-      topic1 = Fabricate(:topic, like_count: 3, created_at: 1.hour.ago)
+      topic1 = Fabricate(:topic, like_count: 3, created_at: 2.weeks.ago)
-      topic2 = Fabricate(:topic, like_count: 10, created_at: 3.hour.ago)
+      topic2 = Fabricate(:topic, like_count: 10, created_at: 2.weeks.ago)
 
       TopicHotScore.update_scores
 
-      expect(TopicHotScore.find_by(topic_id: topic1.id).score).to be_within(0.001).of(0.535)
+      expect(TopicHotScore.find_by(topic_id: topic1.id).score).to be_within(0.001).of(0.002)
-      expect(TopicHotScore.find_by(topic_id: topic2.id).score).to be_within(0.001).of(1.304)
+      expect(TopicHotScore.find_by(topic_id: topic2.id).score).to be_within(0.001).of(0.009)
 
-      freeze_time(2.hours.from_now)
+      freeze_time(6.weeks.from_now)
 
       TopicHotScore.update_scores
 
-      expect(TopicHotScore.find_by(topic_id: topic1.id).score).to be_within(0.001).of(0.289)
+      expect(TopicHotScore.find_by(topic_id: topic1.id).score).to be_within(0.0001).of(0.0005)
-      expect(TopicHotScore.find_by(topic_id: topic2.id).score).to be_within(0.001).of(0.871)
+      expect(TopicHotScore.find_by(topic_id: topic2.id).score).to be_within(0.001).of(0.001)
     end
   end
 end