Support selective highlighting of lines65152-unfolded-lines-perf-improvement

Instead of highlighting all lines when not all of them
are needed, only highlight from the beginning up to
the specified limit.

The `BlobPresenter#highlight` method has been updated
to support `to` param. This param will be used to limit
the content to be highlighted.

2 files changed, 19 insertions, 7 deletions

diff --git a/app/presenters/blob_presenter.rb b/app/presenters/blob_presenter.rb
index 2cf3278d240..3a71d2b87f3 100644
--- a/app/presenters/blob_presenter.rb
+++ b/app/presenters/blob_presenter.rb
@@ -3,12 +3,12 @@
 class BlobPresenter < Gitlab::View::Presenter::Delegated
   presents :blob
 
-  def highlight(plain: nil)
+  def highlight(to: nil, plain: nil)
     load_all_blob_data
 
     Gitlab::Highlight.highlight(
       blob.path,
-      blob.data,
+      limited_blob_data(to: to),
       language: blob.language_from_gitattributes,
       plain: plain
     )
@@ -23,4 +23,18 @@ 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(to: nil)
+    return blob.data if to.blank?
+
+    # Even though we don't need all the lines at the start of the file (e.g
+    # viewing the middle part of a file), they still need to be highlighted
+    # to ensure that the succeeding lines can be formatted correctly (e.g.
+    # multi-line comments).
+    all_lines[0..to - 1].join
+  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 a256dd05a4d..487c6fe0757 100644
--- a/app/presenters/blobs/unfold_presenter.rb
+++ b/app/presenters/blobs/unfold_presenter.rb
@@ -26,8 +26,6 @@ module Blobs
       # so we can accurately show the rest of the diff when unfolding.
       load_all_blob_data
 
-      @all_lines = blob.data.lines
-
       handle_full_or_end!
     end
 
@@ -46,7 +44,7 @@ module Blobs
 
     def lines
       strong_memoize(:lines) do
-        limit(highlight.lines).map(&:html_safe)
+        limit(highlight(to: to).lines).map(&:html_safe)
       end
     end
 
@@ -76,7 +74,7 @@ module Blobs
 
     def all_lines_size
       strong_memoize(:all_lines_size) do
-        @all_lines.size
+        all_lines.size
       end
     end
 
@@ -101,7 +99,7 @@ module Blobs
 
     def limited_blob_lines
       strong_memoize(:limited_blob_lines) do
-        limit(@all_lines)
+        limit(all_lines)
       end
     end