# Finds the diff position in the new diff that corresponds to the same location
# specified by the provided position in the old diff.
module Gitlab
  module Diff
    class PositionTracer
      attr_accessor :project
      attr_accessor :old_diff_refs
      attr_accessor :new_diff_refs
      attr_accessor :paths

      def initialize(project:, old_diff_refs:, new_diff_refs:, paths: nil)
        @project = project
        @old_diff_refs = old_diff_refs
        @new_diff_refs = new_diff_refs
        @paths = paths
      end

      def trace(ab_position)
        return unless old_diff_refs&.complete? && new_diff_refs&.complete?
        return unless ab_position.diff_refs == old_diff_refs

        # Suppose we have an MR with source branch `feature` and target branch `master`.
        # When the MR was created, the head of `master` was commit A, and the
        # head of `feature` was commit B, resulting in the original diff A->B.
        # Since creation, `master` was updated to C.
        # Now `feature` is being updated to D, and the newly generated MR diff is C->D.
        # It is possible that C and D are direct decendants of A and B respectively,
        # but this isn't necessarily the case as rebases and merges come into play.
        #
        # Suppose we have a diff note on the original diff A->B. Now that the MR
        # is updated, we need to find out what line in C->D corresponds to the
        # line the note was originally created on, so that we can update the diff note's
        # records and continue to display it in the right place in the diffs.
        # If we cannot find this line in the new diff, this means the diff note is now
        # outdated, and we will display that fact to the user.
        #
        # In the new diff, the file the diff note was originally created on may
        # have been renamed, deleted or even created, if the file existed in A and B,
        # but was removed in C, and restored in D.
        #
        # Every diff note stores a Position object that defines a specific location,
        # identified by paths and line numbers, within a specific diff, identified
        # by start, head and base commit ids.
        #
        # For diff notes for diff A->B, the position looks like this:
        # Position
        #   start_sha - ID of commit A
        #   head_sha - ID of commit B
        #   base_sha - ID of base commit of A and B
        #   old_path - path as of A (nil if file was newly created)
        #   new_path - path as of B (nil if file was deleted)
        #   old_line - line number as of A (nil if file was newly created)
        #   new_line - line number as of B (nil if file was deleted)
        #
        # We can easily update `start_sha` and `head_sha` to hold the IDs of
        # commits C and D, and can trivially determine `base_sha` based on those,
        # but need to find the paths and line numbers as of C and D.
        #
        # If the file was unchanged or newly created in A->B, the path as of D can be found
        # by generating diff B->D ("head to head"), finding the diff file with
        # `diff_file.old_path == position.new_path`, and taking `diff_file.new_path`.
        # The path as of C can be found by taking diff C->D, finding the diff file
        # with that same `new_path` and taking `diff_file.old_path`.
        # The line number as of D can be found by using the LineMapper on diff B->D
        # and providing the line number as of B.
        # The line number as of C can be found by using the LineMapper on diff C->D
        # and providing the line number as of D.
        #
        # If the file was deleted in A->B, the path as of C can be found
        # by generating diff A->C ("base to base"), finding the diff file with
        # `diff_file.old_path == position.old_path`, and taking `diff_file.new_path`.
        # The path as of D can be found by taking diff C->D, finding the diff file
        # with `old_path` set to that `diff_file.new_path` and taking `diff_file.new_path`.
        # The line number as of C can be found by using the LineMapper on diff A->C
        # and providing the line number as of A.
        # The line number as of D can be found by using the LineMapper on diff C->D
        # and providing the line number as of C.

        if ab_position.added?
          trace_added_line(ab_position)
        elsif ab_position.removed?
          trace_removed_line(ab_position)
        else # unchanged
          trace_unchanged_line(ab_position)
        end
      end

      private

      def trace_added_line(ab_position)
        b_path = ab_position.new_path
        b_line = ab_position.new_line

        bd_diff = bd_diffs.diff_file_with_old_path(b_path)

        d_path = bd_diff&.new_path || b_path
        d_line = LineMapper.new(bd_diff).old_to_new(b_line)

        if d_line
          cd_diff = cd_diffs.diff_file_with_new_path(d_path)

          c_path = cd_diff&.old_path || d_path
          c_line = LineMapper.new(cd_diff).new_to_old(d_line)

          if c_line
            # If the line is still in D but also in C, it has turned from an
            # added line into an unchanged one.
            new_position = position(cd_diff, c_line, d_line)
            if valid_position?(new_position)
              # If the line is still in the MR, we don't treat this as outdated.
              { position: new_position, outdated: false }
            else
              # If the line is no longer in the MR, we unfortunately cannot show
              # the current state on the CD diff, so we treat it as outdated.
              ac_diff = ac_diffs.diff_file_with_new_path(c_path)

              { position: position(ac_diff, nil, c_line), outdated: true }
            end
          else
            # If the line is still in D and not in C, it is still added.
            { position: position(cd_diff, nil, d_line), outdated: false }
          end
        else
          # If the line is no longer in D, it has been removed from the MR.
          { position: position(bd_diff, b_line, nil), outdated: true }
        end
      end

      def trace_removed_line(ab_position)
        a_path = ab_position.old_path
        a_line = ab_position.old_line

        ac_diff = ac_diffs.diff_file_with_old_path(a_path)

        c_path = ac_diff&.new_path || a_path
        c_line = LineMapper.new(ac_diff).old_to_new(a_line)

        if c_line
          cd_diff = cd_diffs.diff_file_with_old_path(c_path)

          d_path = cd_diff&.new_path || c_path
          d_line = LineMapper.new(cd_diff).old_to_new(c_line)

          if d_line
            # If the line is still in C but also in D, it has turned from a
            # removed line into an unchanged one.
            bd_diff = bd_diffs.diff_file_with_new_path(d_path)

            { position: position(bd_diff, nil, d_line), outdated: true }
          else
            # If the line is still in C and not in D, it is still removed.
            { position: position(cd_diff, c_line, nil), outdated: false }
          end
        else
          # If the line is no longer in C, it has been removed outside of the MR.
          { position: position(ac_diff, a_line, nil), outdated: true }
        end
      end

      def trace_unchanged_line(ab_position)
        a_path = ab_position.old_path
        a_line = ab_position.old_line
        b_path = ab_position.new_path
        b_line = ab_position.new_line

        ac_diff = ac_diffs.diff_file_with_old_path(a_path)

        c_path = ac_diff&.new_path || a_path
        c_line = LineMapper.new(ac_diff).old_to_new(a_line)

        bd_diff = bd_diffs.diff_file_with_old_path(b_path)

        d_line = LineMapper.new(bd_diff).old_to_new(b_line)

        cd_diff = cd_diffs.diff_file_with_old_path(c_path)

        if c_line && d_line
          # If the line is still in C and D, it is still unchanged.
          new_position = position(cd_diff, c_line, d_line)
          if valid_position?(new_position)
            # If the line is still in the MR, we don't treat this as outdated.
            { position: new_position, outdated: false }
          else
            # If the line is no longer in the MR, we unfortunately cannot show
            # the current state on the CD diff or any change on the BD diff,
            # so we treat it as outdated.
            { position: nil, outdated: true }
          end
        elsif d_line # && !c_line
          # If the line is still in D but no longer in C, it has turned from
          # an unchanged line into an added one.
          # We don't treat this as outdated since the line is still in the MR.
          { position: position(cd_diff, nil, d_line), outdated: false }
        else # !d_line && (c_line || !c_line)
          # If the line is no longer in D, it has turned from an unchanged line
          # into a removed one.
          { position: position(bd_diff, b_line, nil), outdated: true }
        end
      end

      def ac_diffs
        @ac_diffs ||= compare(
          old_diff_refs.base_sha || old_diff_refs.start_sha,
          new_diff_refs.base_sha || new_diff_refs.start_sha,
          straight: true
        )
      end

      def bd_diffs
        @bd_diffs ||= compare(old_diff_refs.head_sha, new_diff_refs.head_sha, straight: true)
      end

      def cd_diffs
        @cd_diffs ||= compare(new_diff_refs.start_sha, new_diff_refs.head_sha)
      end

      def compare(start_sha, head_sha, straight: false)
        compare = CompareService.new(project, head_sha).execute(project, start_sha, straight: straight)
        compare.diffs(paths: paths, expanded: true)
      end

      def position(diff_file, old_line, new_line)
        Position.new(diff_file: diff_file, old_line: old_line, new_line: new_line)
      end

      def valid_position?(position)
        !!position.diff_line(project.repository)
      end
    end
  end
end
