Lesson 14 · Strings and bytes - Build a Protobuf Decoder

Strings and bytes

Strings and byte fields are just Len values: a length prefix and that many raw bytes, read as UTF-8 text or kept as-is. Today you interpret a Len payload as a string and encode a full string field, tag and all.

The goal

Decode a Len payload as a string, and encode a string field with its tag and length.

Start here - the target

TO DO

Scenario: A string field is a tag, a length, and the bytes

Giventhe bytes 0x0A, 0x05, 0x68, 0x65, 0x6C, 0x6C, 0x6F

Whenthe field is decoded

Thenit is field 1, wire type Len, with the string value "hello"

Andencoding field 1 with the string "hi" produces the bytes 0x0A, 0x02, 0x68, 0x69, and an empty string encodes to just 0x0A, 0x00

Background

A string and a bytes field share the Len wire type and are identical on the wire: a varint length followed by that many bytes. The only difference is interpretation. bytes is the payload verbatim; string is those same bytes read as UTF-8 text. Since ReadBytes already hands you the payload, decoding a string is just wrapping it as text, and an empty payload is a legal empty string.

Encoding a full field ties together everything from this chapter and the last: write the tag (field number plus the Len wire type), then the length as a varint, then the raw bytes. The classic worked example, field 1 with “hello”, serializes to 0x0A 0x05 followed by the five ASCII bytes of “hello”. Pin the empty-string case too - 0x0A 0x00 - because a zero length is a value, not an absence, a distinction the defaults lesson will lean on hard.

Make it work

// decode: the Len payload bytes are the string, no transformation
func asString(payload []byte) string { return string(payload) }
// encode a whole string field: tag, then length-prefixed bytes
func AppendStringField(buf []byte, field int, s string) []byte {
  buf = AppendTag(buf, field, WireLen)
  buf = AppendVarint(buf, uint64(len(s)))
  return append(buf, s...)
}

CheckpointDONE

You can decode and encode string and bytes fields. Commit and stop here.