Lesson 15 · Parsing a message into fields

Parsing a message into fields

A message is just fields back to back, so parsing one is a loop: read a tag, read its value by wire type, repeat until the bytes run out. Today you build that loop and return the message as an ordered list of raw fields.

The goal

Parse a whole message into an ordered list of (field number, wire type, value) records.

Start here - the target

TO DO

Scenario: A message decodes into its sequence of fields

Giventhe bytes 0x08, 0x96, 0x01, 0x12, 0x03, 0x61, 0x62, 0x63

WhenParseFields reads to the end of the buffer

Thenit returns two fields: field 1 wire type Varint with value 150, then field 2 wire type Len with the bytes 0x61, 0x62, 0x63

Andthe fields come back in the order they appear, and an empty input returns an empty list

Background

With tags, wire types, and every value reader built, parsing a message is the loop that ties them together: while the reader is not at the end, decode a tag into a field number and wire type, read the value the wire type prescribes, and record a field. A Varint value stores its integer; an I32 or I64 stores its fixed value; a Len value stores its raw payload bytes. The result is the message as an ordered list of raw fields, faithful to the byte order and not yet interpreted by any schema.

This raw list is the pivot of the whole decoder. Everything downstream is a view over it: grouping repeated fields, recursing into an embedded message, or matching field numbers against a descriptor. Keep the fields in wire order - protobuf allows the same field number to appear more than once, and which occurrence came last will matter when we resolve duplicates. An empty message is a legal message: zero fields.

Make it work

type Field struct {
  Number int
  Wire   int
  Varint uint64 // for Varint, I32, I64
  Bytes  []byte // for Len
}
func ParseFields(r *Reader) ([]Field, error) {
  var out []Field
  for !r.AtEnd() {
    num, wire, err := r.DecodeTag()
    // read the value per wire type, append a Field, continue
  }
  return out, nil
}

CheckpointDONE

You can parse a message into its list of fields. Commit and stop here.