def _ModifiedEncoder(wire_type, encode_value, compute_value_size, modify_value):
  """Like SimpleEncoder but additionally invokes modify_value on every value
  before passing it to encode_value.  Usually modify_value is ZigZagEncode."""

  def SpecificEncoder(field_number, is_repeated, is_packed):
    if is_packed:
      tag_bytes = TagBytes(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
      local_EncodeVarint = _EncodeVarint
      def EncodePackedField(write, value, deterministic):
        write(tag_bytes)
        size = 0
        for element in value:
          size += compute_value_size(modify_value(element))
        local_EncodeVarint(write, size, deterministic)
        for element in value:
          encode_value(write, modify_value(element), deterministic)
      return EncodePackedField
    elif is_repeated:
      tag_bytes = TagBytes(field_number, wire_type)
      def EncodeRepeatedField(write, value, deterministic):
        for element in value:
          write(tag_bytes)
          encode_value(write, modify_value(element), deterministic)
      return EncodeRepeatedField
    else:
      tag_bytes = TagBytes(field_number, wire_type)
      def EncodeField(write, value, deterministic):
        write(tag_bytes)
        return encode_value(write, modify_value(value), deterministic)
      return EncodeField

  return SpecificEncoder
Like SimpleEncoder but additionally invokes modify_value on every value
  before passing it to encode_value.  Usually modify_value is ZigZagEncode.
deps/protobuf/python/google/protobuf/internal/encoder.py