# File lib/ttfunk/table/hmtx.rb, line 10
      def self.encode(hmtx, mapping)
        metrics = mapping.keys.sort.map do |new_id|
          metric = hmtx.for(mapping[new_id])
          [metric.advance_width, metric.left_side_bearing]
        end

        { :number_of_metrics => metrics.length,
          :table => metrics.flatten.pack("n*") }
      end