Lesson 06 · Decoding a tag - Build a Protobuf Decoder

Decoding a tag

Every field in a message begins with a tag, itself a varint, that packs together which field this is and how its value is encoded. Today you split a tag into its field number and wire type, the routing decision the whole decoder pivots on.

The goal

Decode a tag varint into a field number and a wire type.

Start here - the target

TO DO

Scenario: A tag splits into field number and wire type

Giventhe tag byte 0x08 (which is the varint 8)

WhenDecodeTag reads it

Thenthe field number is 1 and the wire type is 0 (Varint)

Andtag 0x0A gives field 1 wire type 2 (Len), tag 0x10 gives field 2 wire type 0, and tag 0x1D gives field 3 wire type 5 (I32)

Background

A message is a flat list of fields, and each field starts with a tag. The tag is encoded as a varint whose bits pack two things: the low 3 bits are the wire type, and everything above them is the field number. So field_number << 3 | wire_type builds a tag, and tag >> 3 with tag & 7 takes it apart. The wire type tells the decoder how to read the value that follows without knowing anything about the schema.

There are four wire types in current protobuf: 0 Varint (integers, bools, enums), 1 I64 (a fixed 8 bytes), 2 Len (a length-delimited chunk: strings, bytes, embedded messages, packed fields), and 5 I32 (a fixed 4 bytes). Types 3 and 4 were the deprecated group markers and we skip them. Walk 0x08: as a varint it is 8, 8 >> 3 is 1 and 8 & 7 is 0, so field 1, wire type Varint. And 0x0A is 10: field 1, wire type Len - the tag you will see in front of almost every string.

Make it work

const (
  WireVarint = 0
  WireI64    = 1
  WireLen    = 2
  WireI32    = 5
)
// low 3 bits are the wire type, the rest is the field number
func (r *Reader) DecodeTag() (field int, wire int, err error) {
  tag, err := r.ReadVarint()
  // field = tag >> 3 ; wire = tag & 7
  return
}

CheckpointDONE

You can decode a tag into a field number and wire type. Commit and stop here.