mp4 parsing

_mp4_parser_go.md

要約

• pinf, sinf, apID, purd にitunes の購入情報が入っている
• https://hydrogenaud.io/index.php/topic,99548.25.html
• https://metacpan.org/source/Audio::M4P::QuickTime#L558

+ pinf
|
+--+ schi
   |
   +-- user
   +-- cert
   +-- righ
   +-- name
   +-- chtb

調べる方法

• mp4dumpを使う

mp4dump [MP4FILE]

[ftyp] size=8+24
  major_brand = M4A 
  minor_version = 0
  compatible_brand = M4A 
  compatible_brand = mp42
  compatible_brand = isom
  compatible_brand = 
[moov] size=8+75216
  [mvhd] size=12+96
 ...

手動でバイナリを読むのに参考になったコード

https://github.com/abema/go-mp4

special padding とかまじかよ・・・。。このソースコード見てなかったら後数時間なやんでただろう https://github.com/SteveMarshall/mp4-quicktime/blob/master/atom.py#L33

main.go

package main

import (
	"log"
	"os"
	"encoding/binary"
	"encoding/hex"
	"io"
	"fmt"
)

// atomName => padding size
var boxAtomPaddings = map[string]int64{
	"moov": 0,
	"trak": 0,
	"mdia": 0,
	"minf": 0,
	"stsd": 8,
	"stbl": 0,
	"mp4a": 28,
}

func main() {
	f, err := os.Open("a.m4a")
	if err != nil {
		log.Fatalln(err)
	}
	dumper := hex.Dumper(os.Stdout)
	defer dumper.Close()

	if _, err := readAllAtoms(f); err != nil {
		log.Fatalln(err)
	}
}

type Atom struct {
	Name string
	DataStartPos int64
	DataLen int64
	Children map[string]*Atom
}

func readAllAtoms(r io.ReadSeeker) (map[string]*Atom, error) {
	atoms := make(map[string]*Atom)
	for {
		atom, err := readAtom(r)
		if err != nil {
			if err != io.EOF {
				return nil, err
			}
			break
		}
		atoms[atom.Name] = atom
	}
	return atoms, nil
}

func readAtom(r io.ReadSeeker) (*Atom, error) {
	var length uint32
	if err := binary.Read(r, binary.BigEndian, &length); err != nil {
		return nil, err
	}

	nameb := make([]byte, 4)
	if err := binary.Read(r, binary.BigEndian, &nameb); err != nil {
		return nil, err
	}
	name := string(nameb)

	startPos, err := r.Seek(0, io.SeekCurrent)
	if err !=  nil {
		return nil, err
	}
	dataLen := int64(length - 8)

	catoms := make(map[string]*Atom)
	pad, isBox := boxAtomPaddings[name]
	if isBox {
		r.Seek(pad, io.SeekCurrent)
		for {
			catom, err := readAtom(r)
			if err != nil {
				return nil, err
			}
			catoms[catom.Name] = catom

			cur, err := r.Seek(0, io.SeekCurrent)
			if err != nil {
				return nil, err
			}
			if cur >= startPos + dataLen {
				break
			}
		}
	} else {
		if _, err := r.Seek(dataLen, io.SeekCurrent); err != nil {
			return nil, err
		}
	}
	fmt.Printf("%s (start: %d, len: %d)\n", name, startPos, dataLen)

	return &Atom {
		Name: name,
		DataStartPos: startPos,
		DataLen: dataLen,
		Children: catoms,
	}, nil
}