Return a constructor for an encoder for a fixed-width field.

  Args:
      wire_type:  The field's wire type, for encoding tags.
      format:  The format string to pass to struct.pack().

def _StructPackEncoder(wire_type, format):
  """Return a constructor for an encoder for a fixed-width field.

  Args:
      wire_type:  The field's wire type, for encoding tags.
      format:  The format string to pass to struct.pack().
  """

  value_size = struct.calcsize(format)

  def SpecificEncoder(field_number, is_repeated, is_packed):
    local_struct_pack = struct.pack
    if is_packed:
      tag_bytes = TagBytes(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
      local_EncodeVarint = _EncodeVarint
      def EncodePackedField(write, value):
        write(tag_bytes)
        local_EncodeVarint(write, len(value) * value_size)
        for element in value:
          write(local_struct_pack(format, element))
      return EncodePackedField
    elif is_repeated:
      tag_bytes = TagBytes(field_number, wire_type)
      def EncodeRepeatedField(write, value):
        for element in value:
          write(tag_bytes)
          write(local_struct_pack(format, element))
      return EncodeRepeatedField
    else:
      tag_bytes = TagBytes(field_number, wire_type)
      def EncodeField(write, value):
        write(tag_bytes)
        return write(local_struct_pack(format, value))
      return EncodeField

  return SpecificEncoder