1 files changed, 58 insertions, 0 deletions
diff --git a/lib/gitlab/diff/pair_selector.rb b/lib/gitlab/diff/pair_selector.rb
new file mode 100644
index 00000000000..2e5ee3a7363
--- /dev/null
+++ b/lib/gitlab/diff/pair_selector.rb
@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Diff
+    class PairSelector
+      include Enumerable
+
+      # Regex to find a run of deleted lines followed by the same number of added lines
+      # rubocop: disable Lint/MixedRegexpCaptureTypes
+      LINE_PAIRS_PATTERN = %r{
+        # Runs start at the beginning of the string (the first line) or after a space (for an unchanged line)
+        (?:\A|\s)
+
+        # This matches a number of `-`s followed by the same number of `+`s through recursion
+        (?<del_ins>
+          -
+          \g<del_ins>?
+          \+
+        )
+
+        # Runs end at the end of the string (the last line) or before a space (for an unchanged line)
+        (?=\s|\z)
+      }x.freeze
+      # rubocop: enable Lint/MixedRegexpCaptureTypes
+
+      def initialize(lines)
+        @lines = lines
+      end
+
+      # Finds pairs of old/new line pairs that represent the same line that changed
+      # rubocop: disable CodeReuse/ActiveRecord
+      def each
+        # Prefixes of all diff lines, indicating their types
+        # For example: `" - +  -+  ---+++ --+  -++"`
+        line_prefixes = lines.each_with_object(+"") { |line, s| s << (line[0] || ' ') }.gsub(/[^ +-]/, ' ')
+
+        line_prefixes.scan(LINE_PAIRS_PATTERN) do
+          # For `"---+++"`, `begin_index == 0`, `end_index == 6`
+          begin_index, end_index = Regexp.last_match.offset(:del_ins)
+
+          # For `"---+++"`, `changed_line_count == 3`
+          changed_line_count = (end_index - begin_index) / 2
+
+          halfway_index = begin_index + changed_line_count
+          (begin_index...halfway_index).each do |i|
+            # For `"---+++"`, index 1 maps to 1 + 3 = 4
+            yield [i, i + changed_line_count]
+          end
+        end
+      end
+      # rubocop: enable CodeReuse/ActiveRecord
+
+      private
+
+      attr_reader :lines
+    end
+  end
+end