The finale decodes one real protobuf message that exercises every piece at once - a string, an int, a nested message, a packed repeated list, and an unknown field - into exact structured values, then encodes it back to equivalent bytes. Every layer proves itself together.
Decode a full message to exact values, preserve its unknown field, and round-trip it back to bytes.
This is the promise the whole project was built to keep: a real Protocol Buffers
codec. The capstone message packs one of everything - a string name, an
int32 age, a nested home message with its own zip string, a packed repeated
scores list, and an unknown field 9 the descriptor never mentions. Decoding it
walks every path you built: varints for the tags and the age, a length-delimited
string for the name, recursion into the nested message, unpacking the packed
[7, 8, 9], proto3 defaults for anything absent, and stashing field 9’s raw bytes.
Then the round-trip closes the loop. Encoding the decoded message emits the known fields in ascending number order and appends the preserved unknown bytes, and the result is byte-for-byte identical to the 23 bytes you started with. That exact match is only reachable if varints, tags, wire types, scalars, nesting, packing, defaults, and unknown-field preservation all agree. From a cursor that read one byte, you have built the honest core of the format that gRPC and countless systems move data with - a decoder and encoder that are entirely yours, and that speak the real protobuf wire format.
fields, _ := ParseFields(NewReader(msg))m, _ := Decode(personDesc, fields) // name, age, home{zip}, scores, +unknownsout := Encode(personDesc, m) // fields in number order, then unknown bytes// out is byte-for-byte equal to msg