Lesson 03 · Multi-byte varints - Build a Protobuf Decoder

Multi-byte varints

Values above 127 spill across several bytes, each carrying seven more bits in little-endian group order. Today you extend the reader to keep consuming bytes while the continuation flag is set, and decode the canonical example 150.

The goal

Decode a varint that spans multiple bytes by combining 7-bit groups little-endian.

Start here - the target

TO DO

Scenario: A larger integer spans several varint bytes

Givena reader over the bytes 0x96, 0x01

WhenReadVarint is called

Thenit returns 150 and the position is now 2

Andthe bytes 0xAC, 0x02 decode to 300, and 0xFF, 0x01 decode to 255

Background

When a value needs more than 7 bits, protobuf keeps emitting bytes, each with the continuation flag 0x80 set, until a final byte with the flag clear. The groups arrive least-significant first (little-endian by group): the first byte holds bits 0 to 6, the next holds bits 7 to 13, and so on. To rebuild the number, mask each byte to its low 7 bits, shift it left by a growing multiple of 7, and OR it in.

Walk 0x96, 0x01: the first byte is 1001_0110, continuation set, payload 001_0110 which is 22; the second is 0000_0001, continuation clear, payload 1 shifted left by 7 which is 128. Add them: 128 + 22 is 150. That single example - 150 encodes to 0x96 0x01 - is the one every protobuf tutorial opens with, and now your reader produces it. The loop naturally handles the one-byte case from the last lesson too, so this replaces the earlier body.

Make it work

func (r *Reader) ReadVarint() uint64 {
  var result uint64
  var shift uint
  for {
    b := r.ReadByte()
    result |= uint64(b&0x7F) << shift // lowest group comes first
    if b&0x80 == 0 { break }          // continuation bit clear: done
    shift += 7
  }
  return result
}

CheckpointDONE

You can decode varints of any length. Commit and stop here.