Duplicate the behaviour and refactor for use with parallel diff.

app/helpers/commits_helper.rb

@@ -23,6 +23,13 @@ module CommitsHelper
       end
   end
 
+  def side_diff_line(diff, index)
+    Gitlab::DiffParser.new(diff.diff.lines.to_a, diff.new_path)
+      .each_for_parallel do |full_line, type, line_code, line_new, line_old, next_line|
+        yield(full_line, type, line_code, line_new, line_old, next_line)
+      end
+  end
+
   def each_diff_line_near(diff, index, expected_line_code)
     max_number_of_lines = 16
 

app/views/projects/commits/_parallel_view.html.haml

 / Side-by-side diff view
 %div.text-file
   %table
-    - each_diff_line(diff, index) do |line, type, line_code, line_new, line_old, raw_line, e|
+    - side_diff_line(diff, index) do |line, type, line_code, line_new, line_old, next_line|
+      - next if type == 'new'
       %tr.line_holder.parallel{ id: line_code, class: "#{type}" }
         - if type != 'match'
           %td.old_line
-            = link_to raw(type == "new" ? " " : line_old), "##{line_code}", id: line_code
+            = link_to raw(line_old), "##{line_code}", id: line_code
             - if type == 'old'
-              %td.line_content{class: "parallel noteable_line old #{line_code}", "line_code" => line_code}= line
+              %td.line_content{class: "parallel noteable_line old #{line_code}", "line_code" => line_code}= raw line
             - else
               %td.line_content.parallel= line
           %td.new_line{data: {linenumber: line_new}}
-            = link_to raw(type == "old" ? " " : line_new) , "##{line_code}", id: line_code
-            - if type == 'new'
-              %td.line_content.parallel{class: "noteable_line #{type} #{line_code}", "line_code" => line_code}= line
+            = link_to raw(line_new) , "##{line_code}", id: line_code
+            - if type == 'old'
+              %td.line_content.parallel{class: "noteable_line new #{line_code}", "line_code" => line_code}= raw next_line
             - else
               %td.line_content.parallel= line

lib/gitlab/diff_parser.rb

@@ -50,6 +50,51 @@ module Gitlab
       end
     end
 
+    def each_for_parallel
+      line_old = 1
+      line_new = 1
+      type = nil
+
+      lines_arr = ::Gitlab::InlineDiff.processing lines
+
+      lines_arr.each_cons(2) do |line, next_line|
+        raw_line = line.dup
+
+        next if filename?(line)
+
+        full_line = html_escape(line.gsub(/\n/, ''))
+        full_line = ::Gitlab::InlineDiff.replace_markers full_line
+
+        next_line = html_escape(next_line.gsub(/\n/, ''))
+        next_line = ::Gitlab::InlineDiff.replace_markers next_line
+
+        if line.match(/^@@ -/)
+          type = "match"
+
+          line_old = line.match(/\-[0-9]*/)[0].to_i.abs rescue 0
+          line_new = line.match(/\+[0-9]*/)[0].to_i.abs rescue 0
+
+          next if line_old == 1 && line_new == 1 #top of file
+          yield(full_line, type, nil, line_new, line_old)
+          next
+        else
+          type = identification_type(line)
+          line_code = generate_line_code(new_path, line_new, line_old)
+          yield(full_line, type, line_code, line_new, line_old, next_line)
+        end
+
+
+        if line[0] == "+"
+          line_new += 1
+        elsif line[0] == "-"
+          line_old += 1
+        else
+          line_new += 1
+          line_old += 1
+        end
+      end
+    end
+
     def empty?
       @lines.empty?
     end