# File lib/aws/glacier/archive_collection.rb, line 124
      def compute_tree_hash hashes

        digest = OpenSSL::Digest.new('sha256')

        until hashes.count == 1
          hashes = hashes.each_slice(2).map do |h1,h2|
            digest.reset
            if h2
              digest.update(h1)
              digest.update(h2)
              digest.digest
            else
              h1
            end
          end
        end

        hashes.first.bytes.map{|x| x.to_i.to_s(16).rjust(2,'0')}.join('')
      end