Lesson 02 · A chunk type - Build a MIDI Parser

A chunk type

Every chunk in the file starts with a 4-byte type tag - four ASCII letters like MThd or MTrk. Today you read those four bytes as a short string, which is how you recognise each kind of chunk from here on.

The goal

Read 4 bytes as a four-character ASCII chunk type.

Start here - the target

TO DO

Scenario: Four ASCII bytes decode to a chunk type string

Giventhe four bytes 0x4D 0x54 0x68 0x64

Whenthey are read as a four-character type

Thenthe type is the string "MThd"

Andthe bytes 0x4D 0x54 0x72 0x6B read the same way give "MTrk"

Background

A Standard MIDI File is built entirely from chunks, and each chunk begins with a 4-byte type tag: four printable ASCII bytes read straight through as a string. 0x4D 0x54 0x68 0x64 is M T h d, the header chunk that opens every file. 0x4D 0x54 0x72 0x6B is M T r k, a track chunk. Those are the only two types the standard defines, but a well-behaved reader must tolerate others.

There is no endianness question here - just four bytes copied into a string in order. Pairing this with yesterday’s length reader gives you both halves of a chunk header, which you assemble tomorrow.

Make it work

// a chunk type is just 4 ASCII bytes turned into a string
func readType(b []byte) string {
  return string(b[0:4])
}

CheckpointDONE

You can read a four-character chunk type. Commit and stop here.