DEV: Cache summarization strategy results. (#22230)

Updates the interface for implementing summarization strategies and adds a cache layer to summarize topics once.

The cache stores the final summary and each chunk used to build it, which will be useful when we have to extend or rebuild it.

app/services/topic_summarization.rb

@@ -0,0 +1,90 @@
+# frozen_string_literal: true
+
+class TopicSummarization
+  def initialize(strategy)
+    @strategy = strategy
+  end
+
+  def summarize(topic)
+    DistributedMutex.synchronize("toppic_summarization_#{topic.id}") do
+      existing_summary = SummarySection.find_by(target: topic, meta_section_id: nil)
+      return existing_summary.summarized_text if existing_summary
+
+      content = {
+        resource_path: "#{Discourse.base_path}/t/-/#{topic.id}",
+        content_title: topic.title,
+        contents: [],
+      }
+
+      targets_data = summary_targets(topic).pluck(:post_number, :raw, :username)
+
+      targets_data.map do |(pn, raw, username)|
+        content[:contents] << { poster: username, id: pn, text: raw }
+      end
+
+      summarization_result = strategy.summarize(content)
+
+      cached_summary(summarization_result, targets_data.map(&:first), topic)
+
+      summarization_result[:summary]
+    end
+  end
+
+  def summary_targets(topic)
+    topic.has_summary? ? best_replies(topic) : pick_selection(topic)
+  end
+
+  private
+
+  attr_reader :strategy
+
+  def best_replies(topic)
+    Post
+      .summary(topic.id)
+      .where("post_type = ?", Post.types[:regular])
+      .where("NOT hidden")
+      .joins(:user)
+      .order(:post_number)
+  end
+
+  def pick_selection(topic)
+    posts =
+      Post
+        .where(topic_id: topic.id)
+        .where("post_type = ?", Post.types[:regular])
+        .where("NOT hidden")
+        .order(:post_number)
+
+    post_numbers = posts.limit(5).pluck(:post_number)
+    post_numbers += posts.reorder("posts.score desc").limit(50).pluck(:post_number)
+    post_numbers += posts.reorder("post_number desc").limit(5).pluck(:post_number)
+
+    Post
+      .where(topic_id: topic.id)
+      .joins(:user)
+      .where("post_number in (?)", post_numbers)
+      .order(:post_number)
+  end
+
+  def cached_summary(result, post_numbers, topic)
+    main_summary =
+      SummarySection.create!(
+        target: topic,
+        algorithm: strategy.model,
+        content_range: (post_numbers.first..post_numbers.last),
+        summarized_text: result[:summary],
+        original_content_sha: Digest::SHA256.hexdigest(post_numbers.join),
+      )
+
+    result[:chunks].each do |chunk|
+      SummarySection.create!(
+        target: topic,
+        algorithm: strategy.model,
+        content_range: chunk[:ids].min..chunk[:ids].max,
+        summarized_text: chunk[:summary],
+        original_content_sha: Digest::SHA256.hexdigest(chunk[:ids].join),
+        meta_section_id: main_summary.id,
+      )
+    end
+  end
+end