Add Gitlab::Diff::LineMapper

1 files changed, 64 insertions, 0 deletions

diff --git a/lib/gitlab/diff/line_mapper.rb b/lib/gitlab/diff/line_mapper.rb
new file mode 100644
index 00000000000..bde5b4eedaa
--- /dev/null
+++ b/lib/gitlab/diff/line_mapper.rb
@@ -0,0 +1,64 @@
+# When provided a diff for a specific file, maps old line numbers to new line
+# numbers and back, to find out where a specific line in a file was moved by the
+# changes.
+module Gitlab
+  module Diff
+    class LineMapper
+      attr_accessor :diff_file
+
+      def initialize(diff_file)
+        @diff_file = diff_file
+      end
+
+      # Find new line number for old line number.
+      def old_to_new(old_line)
+        map_line_number(old_line, from: :old_line, to: :new_line)
+      end
+
+      # Find old line number for new line number.
+      def new_to_old(new_line)
+        map_line_number(new_line, from: :new_line, to: :old_line)
+      end
+
+      private
+
+      def diff_lines
+        @diff_lines ||= @diff_file.diff_lines
+      end
+
+      # Find old line number based on new line number.
+      def map_line_number(from_line, from:, to:)
+        # If no diff file could be found, the file wasn't changed, and the
+        # mapped line number is the same as the specified line number.
+        return from_line unless diff_file
+
+        # To find the mapped line number for the specified line number,
+        # we need to find:
+        # - The diff line with that exact line number, if it is in the diff context
+        # - The first diff line with a higher line number, if it falls between diff contexts
+        # - The last known diff line, if it falls after the last diff context
+        diff_line = diff_lines.find do |diff_line|
+          diff_from_line = diff_line.send(from)
+          diff_from_line && diff_from_line >= from_line
+        end
+        diff_line ||= diff_lines.last
+
+        # If no diff line could be found, the file wasn't changed, and the
+        # mapped line number is the same as the specified line number.
+        return from_line unless diff_line
+
+        diff_from_line = diff_line.send(from)
+        diff_to_line = diff_line.send(to)
+
+        # If the line was removed, there is no mapped line number.
+        return unless diff_to_line
+
+        # Because we may not have the diff line with the exact line number
+        # we were looking for, we need to adjust the mapped line number.
+        distance = diff_from_line - from_line
+
+        diff_to_line - distance
+      end
+    end
+  end
+end