class SassC::Engine

def source_map
  raise NotRenderedError unless @source_map
  url = URL.parse(source_map_file_url || file_url)
  data = JSON.parse(@source_map)
  data['file'] = URL.file_urls_to_relative_url(output_url, url) if output_url
  data['sources'].map! do |source|
    if source.start_with?(Protocol::FILE)
      URL.file_urls_to_relative_url(source, url)
    else
      source
    end
  end
  JSON.generate(data)
end