Lesson 09 · The track reader - Build a MIDI Parser

The track reader

Parsing a track means reading fields one after another while remembering where you are. Today you build a small stateful reader - a byte slice plus a position - that hands out bytes, 16-bit words, and VLQs and advances itself.

The goal

Build a reader over a byte slice that reads a byte and a VLQ, advancing its position.

Start here - the target

TO DO

Scenario: The reader advances its position as it reads

Givena reader over the bytes 0x81 0x00 0x3C with position 0

Whena VLQ is read, then a single byte

Thenthe VLQ is 128 and the position is now 2

Andthe next byte read is 0x3C and the position is now 3

Background

Every event in a track is a delta-time followed by some bytes, then another event, with no length prefix on the individual events - so you read the body left to right, and each read has to leave the cursor exactly where the next one starts. A tiny reader carrying the buffer and a pos index captures that: NextByte returns one byte and steps forward by one; ReadVLQ decodes a variable-length quantity and steps forward by however many bytes it spanned. (Call the byte method NextByte, not ReadByte - a ReadByte() byte signature clashes with Go’s io.ByteReader convention and trips the linter.)

This wraps the pure helpers you already wrote into something stateful and convenient - the two facets to confirm are that a multi-byte VLQ advances the position by the right amount (two, for 0x81 0x00) and that the following NextByte picks up exactly where it left off. From here on, parsing an event is just a sequence of reader calls, and the reader keeps the bookkeeping honest.

Make it work

type Reader struct {
  buf []byte
  pos int
}
// name it NextByte, not ReadByte: a ReadByte() byte signature trips go vet
func (r *Reader) NextByte() byte { b := r.buf[r.pos]; r.pos++; return b }
func (r *Reader) ReadVLQ() uint32 {
  v, n := decodeVLQ(r.buf[r.pos:]) // reuse lesson 7
  r.pos += n
  return v
}

CheckpointDONE

You have a reader that advances as it reads bytes and VLQs. Commit and stop here.