Lesson 04 · Encoding a varint - Build a Protobuf Decoder

Encoding a varint

Decoding is only half a codec; to write protobuf you need to turn an integer back into varint bytes. Today you build the encoder and confirm it is the exact inverse of the reader on the canonical values.

The goal

Encode an unsigned integer into varint bytes, the inverse of the reader.

Start here - the target

TO DO

Scenario: An integer encodes to the same bytes it decoded from

Giventhe value 150

WhenAppendVarint encodes it

Thenit produces the bytes 0x96, 0x01

And300 encodes to 0xAC, 0x02; the value 0 encodes to a single 0x00; and 127 encodes to a single 0x7F

Background

Encoding reverses the read: peel off the low 7 bits, and if anything remains, set the continuation flag 0x80 and shift the value right by 7 to move to the next group. When the remaining value is small enough to fit in 7 bits, write it with the flag clear and stop. Note that 0 is a special little case worth pinning: the loop never runs, so a single 0x00 byte is emitted, and a varint is never zero bytes long.

Because the encoder walks the same 7-bit groups the reader rebuilt, it is the exact inverse: encode 150 and you get 0x96 0x01; feed those bytes to last lesson’s reader and you get 150 back. That round-trip property is the backbone of the whole project, and appending onto a caller-supplied buffer is the pattern every later encoder follows, so you can build a message up field by field.

Make it work

// emit 7 bits at a time, low group first, setting 0x80 while more remain
func AppendVarint(buf []byte, v uint64) []byte {
  for v >= 0x80 {
    buf = append(buf, byte(v)|0x80) // low 7 bits + continuation
    v >>= 7
  }
  return append(buf, byte(v))       // final group, continuation clear
}

CheckpointDONE

You can encode any unsigned integer to varint bytes. Commit and stop here.