Remove improved_merge_diff_highlighting feature flag

Introduced in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52499
FF issue: https://gitlab.com/gitlab-org/gitlab/-/issues/299884

* Remove deprecated diff detection logic

changelogs/unreleased/299884_remove_diff_highlighting_feature_flag.yml

+---
+title: Remove improved_merge_diff_highlighting feature flag
+merge_request: 55771
+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: true

lib/gitlab/diff/highlight.rb

@@ -3,13 +3,12 @@
 module Gitlab
   module Diff
     class Highlight
-      attr_reader :diff_file, :diff_lines, :raw_lines, :repository, :project
+      attr_reader :diff_file, :diff_lines, :raw_lines, :repository
 
       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
@@ -67,7 +66,7 @@ module Gitlab
       end
 
       def inline_diffs
-        @inline_diffs ||= InlineDiff.for_lines(@raw_lines, project: project)
+        @inline_diffs ||= InlineDiff.for_lines(@raw_lines)
       end
 
       def old_lines

lib/gitlab/diff/highlight_cache.rb

@@ -7,8 +7,7 @@ module Gitlab
       include Gitlab::Utils::StrongMemoize
 
       EXPIRATION = 1.week
-      VERSION = 1
-      NEXT_VERSION = 2
+      VERSION = 2
 
       delegate :diffable,     to: :@diff_collection
       delegate :diff_options, to: :@diff_collection
@@ -70,20 +69,12 @@ 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, default_enabled: :yaml)
-          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

@@ -11,19 +11,15 @@ module Gitlab
         @offset = offset
       end
 
-      def inline_diffs(project: nil)
+      def inline_diffs
         # Skip inline diff if empty line was replaced with content
         return if old_line == ""
 
-        if Feature.enabled?(:improved_merge_diff_highlighting, project, default_enabled: :yaml)
-          CharDiff.new(old_line, new_line).changed_ranges(offset: offset)
-        else
-          deprecated_diff
-        end
+        CharDiff.new(old_line, new_line).changed_ranges(offset: offset)
       end
 
       class << self
-        def for_lines(lines, project: nil)
+        def for_lines(lines)
           pair_selector = Gitlab::Diff::PairSelector.new(lines)
 
           inline_diffs = []
@@ -32,7 +28,7 @@ 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(project: project)
+            old_diffs, new_diffs = new(old_line, new_line, offset: 1).inline_diffs
 
             inline_diffs[old_index] = old_diffs
             inline_diffs[new_index] = new_diffs
@@ -41,46 +37,6 @@ module Gitlab
           inline_diffs
         end
       end
-
-      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
-
-        length = 0
-        (0..max_length - 1).each do |pos|
-          old_char = a[pos]
-          new_char = b[pos]
-
-          break if old_char != new_char
-
-          length += 1
-        end
-
-        length
-      end
-
-      def longest_common_suffix(a, b) # rubocop:disable Naming/UncommunicativeMethodParamName
-        longest_common_prefix(a.reverse, b.reverse)
-      end
     end
   end
 end

spec/lib/gitlab/diff/highlight_cache_spec.rb

@@ -237,18 +237,8 @@ RSpec.describe Gitlab::Diff::HighlightCache, :clean_gitlab_redis_cache do
   describe '#key' do
     subject { cache.key }
 
-    it 'returns the next version of the cache' do
+    it 'returns cache key' 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

@@ -52,17 +52,6 @@ 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