1 files changed, 65 insertions, 0 deletions

diff --git a/app/services/discussions/capture_diff_note_position_service.rb b/app/services/discussions/capture_diff_note_position_service.rb
new file mode 100644
index 00000000000..273a60f7e55
--- /dev/null
+++ b/app/services/discussions/capture_diff_note_position_service.rb
@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+module Discussions
+  class CaptureDiffNotePositionService
+    def initialize(merge_request, paths)
+      @project = merge_request.project
+      @tracer = build_tracer(merge_request, paths)
+    end
+
+    def execute(discussion)
+      # The service has been implemented for text only
+      # The impact of image notes on this service is being investigated in
+      # https://gitlab.com/gitlab-org/gitlab/-/issues/213989
+      return unless discussion.on_text?
+
+      result = tracer&.trace(discussion.position)
+      return unless result
+
+      position = result[:position]
+
+      # Currently position data is copied across all notes of a discussion
+      # It makes sense to store a position only for the first note instead
+      # Within the newly introduced table we can start doing just that
+      DiffNotePosition.create_or_update_for(discussion.notes.first,
+        diff_type: :head,
+        position: position,
+        line_code: position.line_code(project.repository))
+    end
+
+    private
+
+    attr_reader :tracer, :project
+
+    def build_tracer(merge_request, paths)
+      return if paths.blank?
+
+      old_diff_refs, new_diff_refs = build_diff_refs(merge_request)
+
+      return unless old_diff_refs && new_diff_refs
+
+      Gitlab::Diff::PositionTracer.new(
+        project: project,
+        old_diff_refs: old_diff_refs,
+        new_diff_refs: new_diff_refs,
+        paths: paths.uniq)
+    end
+
+    def build_diff_refs(merge_request)
+      merge_ref_head = merge_request.merge_ref_head
+      return unless merge_ref_head
+
+      start_sha, base_sha = merge_ref_head.parent_ids
+      new_diff_refs = Gitlab::Diff::DiffRefs.new(
+        base_sha: base_sha,
+        start_sha: start_sha,
+        head_sha: merge_ref_head.id)
+
+      old_diff_refs = merge_request.diff_refs
+
+      return if new_diff_refs == old_diff_refs
+
+      [old_diff_refs, new_diff_refs]
+    end
+  end
+end