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.

1 files changed, 49 insertions, 3 deletions

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