FEATURE: experiment with hot sort order (#25274)

This introduces a new experimental hot sort ordering. 

It attempts to float top conversations by first prioritizing a  topics with lots of recent activity (likes and users responding) 

The schedule that updates hot topics is disabled unless the hidden site setting: `experimental_hot_topics` is enabled. 

You can control "decay" with `hot_topic_gravity` and `recency` with `hot_topics_recent_days` 

Data is stored in the new `topic_hot_scores` table and you can check it out on the `/hot` route once 
enabled. 
---------

Co-authored-by: Penar Musaraj <pmusaraj@gmail.com>

app/models/topic_hot_score.rb

@@ -0,0 +1,140 @@
+# frozen_string_literal: true
+
+class TopicHotScore < ActiveRecord::Base
+  belongs_to :topic
+
+  DEFAULT_BATCH_SIZE = 1000
+
+  def self.update_scores(max = DEFAULT_BATCH_SIZE)
+    # score is
+    # (total likes - 1) / (age in hours + 2) ^ gravity
+
+    # 1. insert a new record if one does not exist (up to batch size)
+    # 2. update recently created (up to batch size)
+    # 3. update all top scoring topics (up to batch size)
+
+    now = Time.zone.now
+
+    args = {
+      now: now,
+      gravity: SiteSetting.hot_topics_gravity,
+      max: max,
+      private_message: Archetype.private_message,
+      recent_cutoff: now - SiteSetting.hot_topics_recent_days.days,
+    }
+
+    # insert up to BATCH_SIZE records that are missing from table
+    DB.exec(<<~SQL, args)
+      INSERT INTO topic_hot_scores (
+        topic_id,
+        score,
+        recent_likes,
+        recent_posters,
+        created_at,
+        updated_at
+      )
+      SELECT
+        topics.id,
+        0.0,
+        0,
+        0,
+        :now,
+        :now
+
+      FROM topics
+      LEFT OUTER JOIN topic_hot_scores ON topic_hot_scores.topic_id = topics.id
+      WHERE topic_hot_scores.topic_id IS NULL
+        AND topics.deleted_at IS NULL
+        AND topics.archetype <> :private_message
+        AND topics.created_at <= :now
+      ORDER BY topics.bumped_at desc
+      LIMIT :max
+    SQL
+
+    # update recent counts for batch
+    DB.exec(<<~SQL, args)
+      UPDATE topic_hot_scores thsOrig
+      SET
+          recent_likes = COALESCE(new_values.likes_count, 0),
+          recent_posters = COALESCE(new_values.unique_participants, 0),
+          recent_first_bumped_at = COALESCE(new_values.first_bumped_at, ths.recent_first_bumped_at)
+      FROM
+        topic_hot_scores ths
+        LEFT OUTER JOIN
+        (
+          SELECT
+              t.id AS topic_id,
+              COUNT(DISTINCT p.user_id) AS unique_participants,
+              (
+                SELECT COUNT(*)
+                FROM post_actions pa
+                JOIN posts p2 ON p2.id = pa.post_id
+                WHERE p2.topic_id = t.id
+                  AND pa.post_action_type_id = 2 -- action_type for 'like'
+                  AND pa.created_at >= :recent_cutoff
+                  AND pa.deleted_at IS NULL
+              ) AS likes_count,
+              MIN(p.created_at) AS first_bumped_at
+          FROM
+              topics t
+          JOIN
+              posts p ON t.id = p.topic_id
+          WHERE
+              p.created_at >= :recent_cutoff
+              AND t.archetype <> 'private_message'
+              AND t.deleted_at IS NULL
+              AND p.deleted_at IS NULL
+              AND t.created_at <= :now
+              AND t.bumped_at >= :recent_cutoff
+              AND p.created_at < :now
+              AND p.created_at >= :recent_cutoff
+          GROUP BY
+              t.id
+        ) AS new_values
+      ON ths.topic_id = new_values.topic_id
+
+      WHERE thsOrig.topic_id = ths.topic_id
+    SQL
+
+    # update up to BATCH_SIZE records that are out of date based on age
+    # we need an extra index for this
+    DB.exec(<<~SQL, args)
+      UPDATE topic_hot_scores ths
+      SET score = topics.like_count /
+        (EXTRACT(EPOCH FROM (:now - topics.created_at)) / 3600 + 2) ^ :gravity
+ +
+        CASE WHEN ths.recent_first_bumped_at IS NULL THEN 0 ELSE
+          (ths.recent_likes + ths.recent_posters) /
+          (EXTRACT(EPOCH FROM (:now - recent_first_bumped_at)) / 3600 + 2) ^ :gravity
+        END
+        ,
+        updated_at = :now
+
+      FROM topics
+      WHERE topics.id IN (
+        SELECT topic_id FROM topic_hot_scores
+        ORDER BY score DESC, recent_first_bumped_at DESC NULLS LAST
+        LIMIT :max
+      ) AND ths.topic_id = topics.id
+    SQL
+  end
+end
+
+# == Schema Information
+#
+# Table name: topic_hot_scores
+#
+#  id                     :bigint           not null, primary key
+#  topic_id               :integer          not null
+#  score                  :float            default(0.0), not null
+#  recent_likes           :integer          default(0), not null
+#  recent_posters         :integer          default(0), not null
+#  recent_first_bumped_at :datetime
+#  created_at             :datetime         not null
+#  updated_at             :datetime         not null
+#
+# Indexes
+#
+#  index_topic_hot_scores_on_score_and_topic_id  (score,topic_id) UNIQUE
+#  index_topic_hot_scores_on_topic_id            (topic_id) UNIQUE
+#