A codec must close the loop by writing structured fields back to bytes. Today you encode a message from its named values in field-number order, the inverse of the schema-aware decode.
Encode named field values into a message, emitting fields in ascending number order.
Encoding a whole message pulls together every writer you have built. Walk the
descriptor’s fields in ascending field-number order (the canonical order a
protobuf encoder uses), and for each one emit its tag and value in the type’s wire
form: a string via the length-delimited field writer, an int32 as a tag plus a
varint, a fixed32 as a tag plus four little-endian bytes, and so on. Encoding
“Alice” and 30 reproduces exactly the 0x0A 0x05 ... bytes you first decoded.
Two rules keep the round-trip honest. First, omit any scalar equal to its
proto3 default, matching how real encoders drop zero-valued fields - decode will
put the default back. Second, append any preserved unknown fields’ raw bytes so
data you did not understand survives the trip. The property to confirm is
decode(encode(m)) == m: a message, encoded and decoded again, is unchanged. That
is the definition of a working codec, and you now have one.
// walk descriptor fields in ascending number, emit tag + value per type:// string -> AppendStringField ; int32 -> AppendTag + AppendVarint ; ...// omit a scalar equal to its default; append preserved unknown bytes last