# File lib/uuidtools.rb, line 193
    def self.timestamp_create(timestamp=nil)
      # We need a lock here to prevent two threads from ever
      # getting the same timestamp.
      @@mutex.synchronize do
        # Always use GMT to generate UUIDs.
        if timestamp.nil?
          gmt_timestamp = Time.now.gmtime
        else
          gmt_timestamp = timestamp.gmtime
        end
        # Convert to 100 nanosecond blocks
        gmt_timestamp_100_nanoseconds = (gmt_timestamp.tv_sec * 10000000) +
          (gmt_timestamp.tv_usec * 10) + 0x01B21DD213814000
        mac_address = self.mac_address
        node_id = 0
        if mac_address != nil
          nodes = mac_address.split(":").collect do |octet|
            octet.to_i(16)
          end
        else
          nodes = SecureRandom.random_bytes(6).unpack("C*")
          nodes[0] |= 0b00000001
        end
        for i in 0..5
          node_id += (nodes[i] << (40 - (i * 8)))
        end
        clock_sequence = @@last_clock_sequence
        if clock_sequence.nil?
          clock_sequence = self.convert_byte_string_to_int(
            SecureRandom.random_bytes(16)
          )
        end
        if @@last_node_id != nil && @@last_node_id != node_id
          # The node id has changed.  Change the clock id.
          clock_sequence = self.convert_byte_string_to_int(
            SecureRandom.random_bytes(16)
          )
        elsif @@last_timestamp != nil &&
            gmt_timestamp_100_nanoseconds <= @@last_timestamp
          clock_sequence = clock_sequence + 1
        end
        @@last_timestamp = gmt_timestamp_100_nanoseconds
        @@last_node_id = node_id
        @@last_clock_sequence = clock_sequence

        time_low = gmt_timestamp_100_nanoseconds & 0xFFFFFFFF
        time_mid = ((gmt_timestamp_100_nanoseconds >> 32) & 0xFFFF)
        time_hi_and_version = ((gmt_timestamp_100_nanoseconds >> 48) & 0x0FFF)
        time_hi_and_version |= (1 << 12)
        clock_seq_low = clock_sequence & 0xFF;
        clock_seq_hi_and_reserved = (clock_sequence & 0x3F00) >> 8
        clock_seq_hi_and_reserved |= 0x80

        return self.new(time_low, time_mid, time_hi_and_version,
          clock_seq_hi_and_reserved, clock_seq_low, nodes)
      end
    end