discourse/app/services/topic_summarization.rb

# frozen_string_literal: true

class TopicSummarization
  def initialize(strategy)
    @strategy = strategy
  end

  def summarize(topic, user, opts = {}, &on_partial_blk)
    existing_summary = SummarySection.find_by(target: topic, meta_section_id: nil)

    # Existing summary shouldn't be nil in this scenario because the controller checks its existence.
    return if !user && !existing_summary

    targets_data = summary_targets(topic).pluck(:post_number, :raw, :username)

    current_topic_sha = build_sha(targets_data.map(&:first))
    can_summarize = Summarization::Base.can_request_summary_for?(user)

    if use_cached?(existing_summary, can_summarize, current_topic_sha, !!opts[:skip_age_check])
      # It's important that we signal a cached summary is outdated
      if can_summarize && new_targets?(existing_summary, current_topic_sha)
        existing_summary.mark_as_outdated
      end

      return existing_summary
    end

    delete_cached_summaries_of(topic) if existing_summary

    content = {
      resource_path: "#{Discourse.base_path}/t/-/#{topic.id}",
      content_title: topic.title,
      contents: [],
    }

    targets_data.map do |(pn, raw, username)|
      content[:contents] << { poster: username, id: pn, text: raw }
    end

    summarization_result = strategy.summarize(content, &on_partial_blk)

    cache_summary(summarization_result, targets_data.map(&:first), topic)
  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 delete_cached_summaries_of(topic)
    SummarySection.where(target: topic).destroy_all
  end

  # For users without permissions to generate a summary or fresh summaries, we return what we have cached.
  def use_cached?(existing_summary, can_summarize, current_sha, skip_age_check)
    existing_summary &&
      !(
        can_summarize && new_targets?(existing_summary, current_sha) &&
          (skip_age_check || existing_summary.created_at < 1.hour.ago)
      )
  end

  def new_targets?(summary, current_sha)
    summary.original_content_sha != current_sha
  end

  def cache_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: build_sha(post_numbers),
      )

    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: build_sha(chunk[:ids]),
        meta_section_id: main_summary.id,
      )
    end

    # Calling reload here ensures Postgres' discrete range type is applied.
    # an inclusive [1,2] ranges is stored as [1,3).
    # Read more about this here: https://www.postgresql.org/docs/current/rangetypes.html#RANGETYPES-DISCRETE
    main_summary.reload
  end

  def build_sha(ids)
    Digest::SHA256.hexdigest(ids.join)
  end
end