Support selective highlighting of lines65152-selective-highlight

Instead of highlighting all lines when not all of them are
needed, only highlight specific lines.

The `BlobPresenter#highlight` method has been updated to
support `since` and `to` params. These params will be used to
limit the content to be highlighted.

Modify `Gitlab::Highlight` to support `since` param which will
then be used to determine the starting line number.

7 files changed, 93 insertions, 29 deletions

diff --git a/app/presenters/blob_presenter.rb b/app/presenters/blob_presenter.rb
index 2cf3278d240..f85c1a237a6 100644
--- a/app/presenters/blob_presenter.rb
+++ b/app/presenters/blob_presenter.rb
@@ -3,12 +3,13 @@
 class BlobPresenter < Gitlab::View::Presenter::Delegated
   presents :blob
 
-  def highlight(plain: nil)
+  def highlight(since: nil, to: nil, plain: nil)
     load_all_blob_data
 
     Gitlab::Highlight.highlight(
       blob.path,
-      blob.data,
+      limited_blob_data(since: since, to: to),
+      since: since,
       language: blob.language_from_gitattributes,
       plain: plain
     )
@@ -23,4 +24,18 @@ class BlobPresenter < Gitlab::View::Presenter::Delegated
   def load_all_blob_data
     blob.load_all_data! if blob.respond_to?(:load_all_data!)
   end
+
+  def limited_blob_data(since: nil, to: nil)
+    return blob.data if since.blank? || to.blank?
+
+    limited_blob_lines(since, to).join
+  end
+
+  def limited_blob_lines(since, to)
+    all_lines[since - 1..to - 1]
+  end
+
+  def all_lines
+    @all_lines ||= blob.data.lines
+  end
 end
diff --git a/app/presenters/blobs/unfold_presenter.rb b/app/presenters/blobs/unfold_presenter.rb
index 21a1e1309e0..f4672d22fc9 100644
--- a/app/presenters/blobs/unfold_presenter.rb
+++ b/app/presenters/blobs/unfold_presenter.rb
@@ -21,20 +21,19 @@ module Blobs
       load_all_blob_data
 
       @subject = blob
-      @all_lines = blob.data.lines
       super(params)
 
       if full?
-        self.attributes = { since: 1, to: @all_lines.size, bottom: false, unfold: false, offset: 0, indent: 0 }
+        self.attributes = { since: 1, to: all_lines.size, bottom: false, unfold: false, offset: 0, indent: 0 }
       end
     end
 
     # Returns an array of Gitlab::Diff::Line with match line added
     def diff_lines
-      diff_lines = lines.map.with_index do |line, index|
-        full_line = limited_blob_lines[index].delete("\n")
+      diff_lines = limited_blob_lines(since, to).map.with_index do |line, index|
+        full_line = line.delete("\n")
 
-        Gitlab::Diff::Line.new(full_line, nil, nil, nil, nil, rich_text: line)
+        Gitlab::Diff::Line.new(full_line, nil, nil, nil, nil, rich_text: lines[index])
       end
 
       add_match_line(diff_lines)
@@ -43,7 +42,7 @@ module Blobs
     end
 
     def lines
-      @lines ||= limit(highlight.lines).map(&:html_safe)
+      @lines ||= highlight(since: since, to: to).lines.map(&:html_safe)
     end
 
     def match_line_text
@@ -59,7 +58,7 @@ module Blobs
     def add_match_line(diff_lines)
       return unless unfold?
 
-      if bottom? && to < @all_lines.size
+      if bottom? && to < all_lines.size
         old_pos = to - offset
         new_pos = to
       elsif since != 1
@@ -73,15 +72,5 @@ module Blobs
 
       bottom? ? diff_lines.push(match_line) : diff_lines.unshift(match_line)
     end
-
-    def limited_blob_lines
-      @limited_blob_lines ||= limit(@all_lines)
-    end
-
-    def limit(lines)
-      return lines if full?
-
-      lines[since - 1..to - 1]
-    end
   end
 end
diff --git a/changelogs/unreleased/65152-selective-highlight.yml b/changelogs/unreleased/65152-selective-highlight.yml
new file mode 100644
index 00000000000..371dbbd5924
--- /dev/null
+++ b/changelogs/unreleased/65152-selective-highlight.yml
@@ -0,0 +1,5 @@
+---
+title: Support selective highlighting of lines
+merge_request: 31361
+author:
+type: performance
diff --git a/lib/gitlab/highlight.rb b/lib/gitlab/highlight.rb
index 381f1dd4e55..1f49a26f0a2 100644
--- a/lib/gitlab/highlight.rb
+++ b/lib/gitlab/highlight.rb
@@ -6,15 +6,16 @@ module Gitlab
     TIMEOUT_FOREGROUND = 3.seconds
     MAXIMUM_TEXT_HIGHLIGHT_SIZE = 1.megabyte
 
-    def self.highlight(blob_name, blob_content, language: nil, plain: false)
-      new(blob_name, blob_content, language: language)
+    def self.highlight(blob_name, blob_content, since: nil, language: nil, plain: false)
+      new(blob_name, blob_content, since: since, language: language)
         .highlight(blob_content, continue: false, plain: plain)
     end
 
     attr_reader :blob_name
 
-    def initialize(blob_name, blob_content, language: nil)
+    def initialize(blob_name, blob_content, since: nil, language: nil)
       @formatter = Rouge::Formatters::HTMLGitlab
+      @since = since
       @language = language
       @blob_name = blob_name
       @blob_content = blob_content
@@ -53,13 +54,13 @@ module Gitlab
     end
 
     def highlight_plain(text)
-      @formatter.format(Rouge::Lexers::PlainText.lex(text)).html_safe
+      @formatter.format(Rouge::Lexers::PlainText.lex(text), since: @since).html_safe
     end
 
     def highlight_rich(text, continue: true)
       tag = lexer.tag
       tokens = lexer.lex(text, continue: continue)
-      Timeout.timeout(timeout_time) { @formatter.format(tokens, tag: tag).html_safe }
+      Timeout.timeout(timeout_time) { @formatter.format(tokens, tag: tag, since: @since).html_safe }
     rescue Timeout::Error => e
       Gitlab::Sentry.track_exception(e)
       highlight_plain(text)
diff --git a/lib/rouge/formatters/html_gitlab.rb b/lib/rouge/formatters/html_gitlab.rb
index e2a7d3ef5ba..0d4ac504428 100644
--- a/lib/rouge/formatters/html_gitlab.rb
+++ b/lib/rouge/formatters/html_gitlab.rb
@@ -8,8 +8,8 @@ module Rouge
       # Creates a new <tt>Rouge::Formatter::HTMLGitlab</tt> instance.
       #
       # [+tag+]     The tag (language) of the lexer used to generate the formatted tokens
-      def initialize(tag: nil)
-        @line_number = 1
+      def initialize(tag: nil, since: nil)
+        @line_number = since || 1
         @tag = tag
       end
 
diff --git a/spec/lib/gitlab/highlight_spec.rb b/spec/lib/gitlab/highlight_spec.rb
index 4676db6b8d8..a410e4eab45 100644
--- a/spec/lib/gitlab/highlight_spec.rb
+++ b/spec/lib/gitlab/highlight_spec.rb
@@ -62,6 +62,14 @@ describe Gitlab::Highlight do
       expect(lines[2].text).to eq('        """')
     end
 
+    context 'since param is present' do
+      it 'highlights with the LC starting from "since" param' do
+        lines = described_class.highlight(file_name, content, since: 2).lines
+
+        expect(lines[0]).to include('LC2')
+      end
+    end
+
     context 'diff highlighting' do
       let(:file_name) { 'test.diff' }
       let(:content) { "+aaa\n+bbb\n- ccc\n ddd\n"}
diff --git a/spec/presenters/blob_presenter_spec.rb b/spec/presenters/blob_presenter_spec.rb
index eacf383be7d..95db2ba6a0d 100644
--- a/spec/presenters/blob_presenter_spec.rb
+++ b/spec/presenters/blob_presenter_spec.rb
@@ -28,24 +28,70 @@ describe BlobPresenter, :seed_helper do
     subject { described_class.new(blob) }
 
     it 'returns highlighted content' do
-      expect(Gitlab::Highlight).to receive(:highlight).with('files/ruby/regex.rb', git_blob.data, plain: nil, language: nil)
+      expect(Gitlab::Highlight)
+        .to receive(:highlight)
+        .with(
+          'files/ruby/regex.rb',
+          git_blob.data,
+          since: nil,
+          plain: nil,
+          language: nil
+        )
 
       subject.highlight
     end
 
     it 'returns plain content when :plain is true' do
-      expect(Gitlab::Highlight).to receive(:highlight).with('files/ruby/regex.rb', git_blob.data, plain: true, language: nil)
+      expect(Gitlab::Highlight)
+        .to receive(:highlight)
+        .with(
+          'files/ruby/regex.rb',
+          git_blob.data,
+          since: nil,
+          plain: true,
+          language: nil
+        )
 
       subject.highlight(plain: true)
     end
 
+    context '"since" and "to" are present' do
+      before do
+        allow(git_blob)
+          .to receive(:data)
+          .and_return("line one\nline two\nline 3\nline 4")
+      end
+
+      it 'returns limited highlighted content' do
+        expect(Gitlab::Highlight)
+          .to receive(:highlight)
+          .with(
+            'files/ruby/regex.rb',
+            "line two\nline 3\n",
+            since: 2,
+            language: nil,
+            plain: nil
+          )
+
+        subject.highlight(since: 2, to: 3)
+      end
+    end
+
     context 'gitlab-language contains a match' do
       before do
         allow(blob).to receive(:language_from_gitattributes).and_return('ruby')
       end
 
       it 'passes language to inner call' do
-        expect(Gitlab::Highlight).to receive(:highlight).with('files/ruby/regex.rb', git_blob.data, plain: nil, language: 'ruby')
+        expect(Gitlab::Highlight)
+          .to receive(:highlight)
+          .with(
+            'files/ruby/regex.rb',
+            git_blob.data,
+            since: nil,
+            plain: nil,
+            language: 'ruby'
+          )
 
         subject.highlight
       end