Returns a decoder for a group field.

def GroupDecoder(field_number, is_repeated, is_packed, key, new_default):
  """Returns a decoder for a group field."""

  end_tag_bytes = encoder.TagBytes(field_number,
                                   wire_format.WIRETYPE_END_GROUP)
  end_tag_len = len(end_tag_bytes)

  assert not is_packed
  if is_repeated:
    tag_bytes = encoder.TagBytes(field_number,
                                 wire_format.WIRETYPE_START_GROUP)
    tag_len = len(tag_bytes)
    def DecodeRepeatedField(buffer, pos, end, message, field_dict):
      value = field_dict.get(key)
      if value is None:
        value = field_dict.setdefault(key, new_default(message))
      while 1:
        value = field_dict.get(key)
        if value is None:
          value = field_dict.setdefault(key, new_default(message))
        # Read sub-message.
        pos = value.add()._InternalParse(buffer, pos, end)
        # Read end tag.
        new_pos = pos+end_tag_len
        if buffer[pos:new_pos] != end_tag_bytes or new_pos > end:
          raise _DecodeError('Missing group end tag.')
        # Predict that the next tag is another copy of the same repeated field.
        pos = new_pos + tag_len
        if buffer[new_pos:pos] != tag_bytes or new_pos == end:
          # Prediction failed.  Return.
          return new_pos
    return DecodeRepeatedField
  else:
    def DecodeField(buffer, pos, end, message, field_dict):
      value = field_dict.get(key)
      if value is None:
        value = field_dict.setdefault(key, new_default(message))
      # Read sub-message.
      pos = value._InternalParse(buffer, pos, end)
      # Read end tag.
      new_pos = pos+end_tag_len
      if buffer[pos:new_pos] != end_tag_bytes or new_pos > end:
        raise _DecodeError('Missing group end tag.')
      return new_pos
    return DecodeField