Add feature flag `improved_merge_diff_highlighting`

* Provide `project` instance to Feature.enabled?
* Use old or new highlighting implementations based on the FF
* Generate a versioned cache based on FF

changelogs/unreleased/16950_improve_highlighting_for_diffs.yml

+---
+title: Improve highlighting for merge diffs
+merge_request: 52499
+author:
+type: added

config/feature_flags/development/improved_merge_diff_highlighting.yml

+---
+name: improved_merge_diff_highlighting
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52499
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/299884
+milestone: '13.9'
+type: development
+group: group::source code
+default_enabled: false

lib/gitlab/diff/highlight.rb

@@ -3,12 +3,13 @@
 module Gitlab
   module Diff
     class Highlight
-      attr_reader :diff_file, :diff_lines, :raw_lines, :repository
+      attr_reader :diff_file, :diff_lines, :raw_lines, :repository, :project
 
       delegate :old_path, :new_path, :old_sha, :new_sha, to: :diff_file, prefix: :diff
 
       def initialize(diff_lines, repository: nil)
         @repository = repository
+        @project = repository&.project
 
         if diff_lines.is_a?(Gitlab::Diff::File)
           @diff_file = diff_lines
@@ -66,7 +67,7 @@ module Gitlab
       end
 
       def inline_diffs
-        @inline_diffs ||= InlineDiff.for_lines(@raw_lines)
+        @inline_diffs ||= InlineDiff.for_lines(@raw_lines, project: project)
       end
 
       def old_lines

lib/gitlab/diff/highlight_cache.rb

@@ -8,6 +8,7 @@ module Gitlab
 
       EXPIRATION = 1.week
       VERSION = 1
+      NEXT_VERSION = 2
 
       delegate :diffable,     to: :@diff_collection
       delegate :diff_options, to: :@diff_collection
@@ -69,12 +70,20 @@ module Gitlab
 
       def key
         strong_memoize(:redis_key) do
-          ['highlighted-diff-files', diffable.cache_key, VERSION, diff_options].join(":")
+          ['highlighted-diff-files', diffable.cache_key, version, diff_options].join(":")
         end
       end
 
       private
 
+      def version
+        if Feature.enabled?(:improved_merge_diff_highlighting, diffable.project)
+          NEXT_VERSION
+        else
+          VERSION
+        end
+      end
+
       def set_highlighted_diff_lines(diff_file, content)
         diff_file.highlighted_diff_lines = content.map do |line|
           Gitlab::Diff::Line.safe_init_from_hash(line)

lib/gitlab/diff/inline_diff.rb

@@ -27,15 +27,19 @@ module Gitlab
         @offset = offset
       end
 
-      def inline_diffs
+      def inline_diffs(project: nil)
         # Skip inline diff if empty line was replaced with content
         return if old_line == ""
 
-        CharDiff.new(old_line, new_line).changed_ranges(offset: offset)
+        if Feature.enabled?(:improved_merge_diff_highlighting, project)
+          CharDiff.new(old_line, new_line).changed_ranges(offset: offset)
+        else
+          deprecated_diff
+        end
       end
 
       class << self
-        def for_lines(lines)
+        def for_lines(lines, project: nil)
           changed_line_pairs = find_changed_line_pairs(lines)
 
           inline_diffs = []
@@ -44,7 +48,7 @@ module Gitlab
             old_line = lines[old_index]
             new_line = lines[new_index]
 
-            old_diffs, new_diffs = new(old_line, new_line, offset: 1).inline_diffs
+            old_diffs, new_diffs = new(old_line, new_line, offset: 1).inline_diffs(project: project)
 
             inline_diffs[old_index] = old_diffs
             inline_diffs[new_index] = new_diffs
@@ -84,6 +88,24 @@ module Gitlab
 
       private
 
+      # See: https://gitlab.com/gitlab-org/gitlab/-/issues/299884
+      def deprecated_diff
+        lcp = longest_common_prefix(old_line, new_line)
+        lcs = longest_common_suffix(old_line[lcp..-1], new_line[lcp..-1])
+
+        lcp += offset
+        old_length = old_line.length + offset
+        new_length = new_line.length + offset
+
+        old_diff_range = lcp..(old_length - lcs - 1)
+        new_diff_range = lcp..(new_length - lcs - 1)
+
+        old_diffs = [old_diff_range] if old_diff_range.begin <= old_diff_range.end
+        new_diffs = [new_diff_range] if new_diff_range.begin <= new_diff_range.end
+
+        [old_diffs, new_diffs]
+      end
+
       def longest_common_prefix(a, b) # rubocop:disable Naming/UncommunicativeMethodParamName
         max_length = [a.length, b.length].max
 

spec/lib/gitlab/diff/highlight_cache_spec.rb

@@ -233,4 +233,22 @@ RSpec.describe Gitlab::Diff::HighlightCache, :clean_gitlab_redis_cache do
       cache.write_if_empty
     end
   end
+
+  describe '#key' do
+    subject { cache.key }
+
+    it 'returns the next version of the cache' do
+      is_expected.to start_with("highlighted-diff-files:#{cache.diffable.cache_key}:2")
+    end
+
+    context 'when feature flag is disabled' do
+      before do
+        stub_feature_flags(improved_merge_diff_highlighting: false)
+      end
+
+      it 'returns the original version of the cache' do
+        is_expected.to start_with("highlighted-diff-files:#{cache.diffable.cache_key}:1")
+      end
+    end
+  end
 end

spec/lib/gitlab/diff/inline_diff_spec.rb

@@ -52,6 +52,17 @@ RSpec.describe Gitlab::Diff::InlineDiff do
         expect(subject[0]).to eq([3..6])
         expect(subject[1]).to eq([3..3, 17..22])
       end
+
+      context 'when feature flag is disabled' do
+        before do
+          stub_feature_flags(improved_merge_diff_highlighting: false)
+        end
+
+        it 'finds all inline diffs' do
+          expect(subject[0]).to eq([3..19])
+          expect(subject[1]).to eq([3..22])
+        end
+      end
     end
   end