package main

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

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

type (
	AudioVersionId int
	LayerIndex     int

	header []byte
)

const (
	AudioVersionMPEG25 AudioVersionId = iota
	AudioVersionReserved
	AudioVersionMPEG2
	AudioVersionMPEG1
)

const (
	LayerIndexReserved LayerIndex = iota
	LayerIndexIII
	LayerIndexII
	LayerIndexI
)

var (
	bitRates = [5][16]uint32{
		{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]uint32{
		{11025, 12000, 8000, 0},
		{0, 0, 0, 0},
		{22050, 24000, 16000, 0},
		{44100, 48000, 32000, 0},
	}

	samplesPerFrame = [4][4]uint32{
		{0, 384, 1152, 576},
		{0, 0, 0, 0},
		{384, 1152, 576},
		{384, 1152, 1152},
	}
)

// TODO: tests
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) LayerIndex() LayerIndex {
	return LayerIndex((h[1] >> 1) & 0x3)
}

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

func (h header) SampleRate() uint32 {
	i := h.AudioVersionId()
	j := (h[2] >> 2) & 0x3
	return sampleRates[i][j]
}

func (h header) IsValid() bool {
	if !h.HasSyncWord() {
		return false
	}
	if h.AudioVersionId() == AudioVersionReserved {
		return false
	}
	if h.LayerIndex() == LayerIndexReserved {
		return false
	}
	if h.BitRate() == 0 {
		return false
	}

	return true
}

func (h header) BitRate() uint32 {
	// TODO: improve this
	var j int
	switch h.AudioVersionId() {
	case AudioVersionMPEG1:
		switch h.LayerIndex() {
		case LayerIndexIII:
			j = 2
		case LayerIndexII:
			j = 1
		case LayerIndexI:
			j = 0
		default:
			return 0
		}
	case AudioVersionReserved:
		return 0
	case AudioVersionMPEG2, AudioVersionMPEG25:
		switch h.LayerIndex() {
		case LayerIndexIII, LayerIndexII:
			j = 4
		case LayerIndexI:
			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)
		}

		h := header(buf)
		if h.IsValid() {
			fmt.Printf("Got header: %08b, audio version: %d, layer index: %d, bitrate: %d\n", h, h.AudioVersionId(), h.LayerIndex(), h.BitRate())
		} else {
			bufreader.Discard(3)
		}
		i++
	}
}