code: def _DoubleDecoder():
    
  
    local_unpack = struct.unpack
  
    def InnerDecode(buffer, pos):
      # We expect a 64-bit value in little-endian byte order.  Bit 1 is the sign
      # bit, bits 2-12 represent the exponent, and bits 13-64 are the significand.
      new_pos = pos + 8
      double_bytes = buffer[pos:new_pos]
  
      # If this value has all its exponent bits set and at least one significand
      # bit set, it's not a number.  In Python 2.4, struct.unpack will treat it
      # as inf or -inf.  To avoid that, we treat it specially.
      if ((double_bytes[7:8] in b'\x7F\xFF')
          and (double_bytes[6:7] >= b'\xF0')
          and (double_bytes[0:7] != b'\x00\x00\x00\x00\x00\x00\xF0')):
        return (_NAN, new_pos)
  
      # Note that we expect someone up-stack to catch struct.error and convert
      # it to _DecodeError -- this way we don't have to set up exception-
      # handling blocks every time we parse one value.
      result = local_unpack('<d', double_bytes)[0]
      return (result, new_pos)
    return _SimpleDecoder(wire_format.WIRETYPE_FIXED64, InnerDecode)
docs: Returns a decoder for a double field.
  
    This code works around a bug in struct.unpack for not-a-number.