avframes/main.go

package main

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

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

type (
	AudioVersionId int
	Layer          int
)

type Decoder struct{}

const (
	AudioVersionMPEG25 AudioVersionId = iota
	AudioVersionReserved
	AudioVersionMPEG2
	AudioVersionMPEG1
)

const (
	LayerReserved Layer = iota
	LayerIII
	LayerII
	LayerI
)

var (
	bitRates = [5][16]int{
		{0, 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448, 0},
		{0, 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384, 0},
		{0, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 0},
		{0, 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256, 0},
		{0, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, 0},
	}

	sampleRates = [4][4]int{
		{11025, 12000, 8000, 0},
		{0, 0, 0, 0},
		{22050, 24000, 16000, 0},
		{44100, 48000, 32000, 0},
	}

	samplesPerFrame = [4][4]int{
		{0, 576, 1152, 384},
		{0, 0, 0, 0},
		{0, 576, 1152, 384},
		{0, 1152, 1152, 384},
	}
)

type header []byte

func (h header) HasSyncWord() bool {
	return h[0] == 0xff && h[1]&0xe0 == 0xe0
}

func (h header) AudioVersionId() AudioVersionId {
	return AudioVersionId((h[1] >> 3) & 0x3)
}

func (h header) Layer() Layer {
	return Layer((h[1] >> 1) & 0x3)
}

func (h header) IsProtected() bool {
	return h[1]&1 == 1
}

func (h header) String() string {
	return fmt.Sprintf("%08b", h)
}

func (h header) SamplesPerFrame() int {
	return samplesPerFrame[h.AudioVersionId()][h.Layer()]
}

func (h header) SampleRate() int {
	i := h.AudioVersionId()
	j := (h[2] >> 2) & 0x3
	return sampleRates[i][j]
}

func (h header) PaddingBytes() int {
	if isPadded := (h[2] >> 1) & 1; isPadded == 0 {
		return 0
	}
	switch h.Layer() {
	case LayerI:
		return 4
	case LayerII, LayerIII:
		return 1
	default:
		return 0
	}
}

func (h header) IsValid() bool {
	if len(h) < 4 {
		return false
	}
	if !h.HasSyncWord() {
		return false
	}
	if h.AudioVersionId() == AudioVersionReserved {
		return false
	}
	if h.Layer() == LayerReserved {
		return false
	}
	if h.BitRate() == 0 {
		return false
	}
	if h.SampleRate() == 0 {
		return false
	}

	return true
}

func (h header) Len() int {
	// TODO premultiply bitrates?
	bps := float64(h.SamplesPerFrame()) / 8
	return int((bps*float64(h.BitRate()*1000))/float64(h.SampleRate()) + float64(h.PaddingBytes()))
}

func (h header) BitRate() int {
	// TODO: improve this
	var j int
	switch h.AudioVersionId() {
	case AudioVersionMPEG1:
		switch h.Layer() {
		case LayerIII:
			j = 2
		case LayerII:
			j = 1
		case LayerI:
			j = 0
		default:
			return 0
		}
	case AudioVersionReserved:
		return 0
	case AudioVersionMPEG2, AudioVersionMPEG25:
		switch h.Layer() {
		case LayerIII, LayerII:
			j = 4
		case LayerI:
			j = 3
		default:
			return 0
		}
	}
	return bitRates[j][h[2]>>4]
}

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)
		}

		if h := header(buf); h.IsValid() {
			fmt.Printf("Got header: %v, audio version: %d, layer index: %d, bitrate: %d, samplerate: %d, samplesPerFrame: %d, padding: %d, size: %d\n", h, h.AudioVersionId(), h.Layer(), h.BitRate(), h.SampleRate(), h.SamplesPerFrame(), h.PaddingBytes(), h.Len())
			bufreader.Discard(h.Len() - 4)
		} else {
			fmt.Println("Discarding a byte")
			bufreader.Discard(3)
		}
		i++
	}
}