# frozen_string_literal: true

module MergeRequests
  # OldestPerCommitFinder is used to retrieve the oldest merge requests for
  # every given commit, grouped per commit SHA.
  #
  # This finder is useful when you need to efficiently retrieve the first/oldest
  # merge requests for multiple commits, and you want to do so in batches;
  # instead of running a query for every commit.
  class OldestPerCommitFinder
    def initialize(project)
      @project = project
    end

    # Returns a Hash that maps a commit ID to the oldest merge request that
    # introduced that commit.
    def execute(commits)
      mapping = {}
      shas = commits.map(&:id)

      # To include merge requests by the commit SHA, we don't need to go through
      # any diff rows.
      #
      # We can't squeeze all this into a single query, as the diff based data
      # relies on a GROUP BY. On the other hand, retrieving MRs by their merge
      # SHAs separately is much easier, and plenty fast.
      @project
        .merge_requests
        .preload_target_project
        .by_merge_commit_sha(shas)
        .each do |mr|
          # Merge SHAs can't be in the merge request itself. It _is_ possible a
          # newer merge request includes the merge commit, but in that case we
          # still want the oldest merge request.
          mapping[mr.merge_commit_sha] = mr
        end

      remaining = shas - mapping.keys

      return mapping if remaining.empty?

      id_rows = MergeRequestDiffCommit
        .oldest_merge_request_id_per_commit(@project.id, remaining)

      mrs = MergeRequest
        .preload_target_project
        .id_in(id_rows.map { |r| r[:merge_request_id] })
        .index_by(&:id)

      id_rows.each do |row|
        if (mr = mrs[row[:merge_request_id]])
          mapping[row[:sha]] = mr
        end
      end

      mapping
    end
  end
end