1 files changed, 47 insertions, 0 deletions
diff --git a/lib/gitlab/diff/position_tracer/file_strategy.rb b/lib/gitlab/diff/position_tracer/file_strategy.rb
new file mode 100644
index 00000000000..171d78bf46f
--- /dev/null
+++ b/lib/gitlab/diff/position_tracer/file_strategy.rb
@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Diff
+    class PositionTracer
+      class FileStrategy < BaseStrategy
+        def trace(position)
+          a_path = position.old_path
+          b_path = position.new_path
+
+          # If file exists in B->D (e.g. updated, renamed, removed), let the
+          # note become outdated.
+          bd_diff = bd_diffs.diff_file_with_old_path(b_path)
+
+          return { position: new_position(position, bd_diff), outdated: true } if bd_diff
+
+          # If file still exists in the new diff, update the position.
+          cd_diff = cd_diffs.diff_file_with_new_path(b_path)
+
+          return { position: new_position(position, cd_diff), outdated: false } if cd_diff
+
+          # If file exists in A->C (e.g. rebased and same changes were present
+          # in target branch), let the note become outdated.
+          ac_diff = ac_diffs.diff_file_with_old_path(a_path)
+
+          return { position: new_position(position, ac_diff), outdated: true } if ac_diff
+
+          # If ever there's a case that the file no longer exists in any diff,
+          # don't set a change position and let the note become outdated.
+          #
+          # This should never happen given the file should exist in one of the
+          # diffs above.
+          { outdated: true }
+        end
+
+        private
+
+        def new_position(position, diff_file)
+          Position.new(
+            diff_file: diff_file,
+            position_type: position.position_type
+          )
+        end
+      end
+    end
+  end
+end