avframes/main.go

package main

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"log"
	"os"
)

const fname = "/home/rob/share/home-office/example.mp3"

type (
	AudioVersionId int
	LayerIndex     int

	header uint32
)

const (
	AudioVersionMPEG25 AudioVersionId = iota
	AudioVersionReserved
	AudioVersionMPEG2
	AudioVersionMPEG1
)

const (
	LayerIndexReserved LayerIndex = iota
	LayerIndexIII
	LayerIndexII
	LayerIndexI
)

func (h header) AudioVersionId() AudioVersionId {
	b := h >> 8
	if b&0x18 == 0x18 {
		return AudioVersionMPEG1
	}
	if b&0x10 == 0x10 {
		return AudioVersionMPEG2
	}
	if b&0x8 == 0x8 {
		return AudioVersionReserved
	}
	return AudioVersionMPEG25
}

func (h header) LayerIndex() LayerIndex {
	b := h >> 8
	if b&0x6 == 0x6 {
		return LayerIndexI
	}
	if b&0x4 == 0x4 {
		return LayerIndexII
	}
	if b&0x2 == 0x2 {
		return LayerIndexIII
	}
	return LayerIndexReserved
}

func main() {
	f, err := os.Open(fname)
	if err != nil {
		log.Fatal(err)
	}

	bufreader := bufio.NewReader(f)
	buf := make([]byte, 4)

	var i int
	for {
		_, err := io.ReadFull(bufreader, buf)
		if err != nil {
			if err == io.EOF {
				break
			}
			log.Fatal(err)
		}

		header, err := newHeaderFromBytes(buf)

		if err == nil {
			fmt.Printf("Got header: %08X, audio version: %d\n", header, header.AudioVersionId())
		}
		i++

		bufreader.Discard(3)
	}
}

func newHeaderFromBytes(buf []byte) (header, error) {
	if len(buf) < 4 {
		return 0, errors.New("insufficient bytes")
	}
	if buf[0] != 0xFF || buf[1]&0xE0 != 0xE0 {
		return 0, errors.New("no header")
	}

	// always little-endian for now
	n := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
	return newHeader(n)
}

// TODO use unsafe here? https://codereview.stackexchange.com/a/60161
func newHeader(n uint32) (header, error) {
	return header(n), nil
}