compute_asset_host

# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 276
      def compute_asset_host(source = "", options = {})
        request = self.request if respond_to?(:request)
        host = options[:host]
        host ||= config.asset_host if defined? config.asset_host

        if host
          if host.respond_to?(:call)
            arity = host.respond_to?(:arity) ? host.arity : host.method(:call).arity
            args = [source]
            args << request if request && (arity > 1 || arity < 0)
            host = host.call(*args)
          elsif host.include?("%d")
            host = host % (Zlib.crc32(source) % 4)
          end
        end

        host ||= request.base_url if request && options[:protocol] == :request
        return unless host

        if URI_REGEXP.match?(host)
          host
        else
          protocol = options[:protocol] || config.default_asset_host_protocol || (request ? :request : :relative)
          case protocol
          when :relative
            "//#{host}"
          when :request
            "#{request.protocol}#{host}"
          else
            "#{protocol}://#{host}"
          end
        end
      end