132 lines
2.5 KiB
Go
132 lines
2.5 KiB
Go
package main
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import (
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"bufio"
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"errors"
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"fmt"
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"io"
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"log"
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"os"
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)
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const fname = "/home/rob/home-office/example.mp3"
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type (
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AudioVersionId int
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LayerIndex int
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header uint32
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)
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const (
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AudioVersionMPEG25 AudioVersionId = iota
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AudioVersionReserved
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AudioVersionMPEG2
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AudioVersionMPEG1
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)
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const (
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LayerIndexReserved LayerIndex = iota
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LayerIndexIII
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LayerIndexII
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LayerIndexI
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)
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var sampleRates = [5][16]uint32{
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{0, 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448, 0},
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{0, 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384, 0},
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{0, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 0},
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{0, 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256, 0},
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{0, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, 0},
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}
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func (h header) AudioVersionId() AudioVersionId {
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return AudioVersionId(((h >> 8) & 0x18) >> 3)
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}
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func (h header) LayerIndex() LayerIndex {
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return LayerIndex(((h >> 8) & 0x6) >> 1)
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}
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func (h header) IsPrivate() bool {
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return (h>>8)&1 == 1
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}
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func (h header) SampleRate() uint32 {
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var j int
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switch h.AudioVersionId() {
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case AudioVersionMPEG1:
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switch h.LayerIndex() {
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case LayerIndexIII:
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j = 2
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case LayerIndexII:
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j = 1
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case LayerIndexI:
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j = 0
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default:
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return 0
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}
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case AudioVersionReserved:
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return 0
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case AudioVersionMPEG2, AudioVersionMPEG25:
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switch h.LayerIndex() {
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case LayerIndexIII, LayerIndexII:
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j = 4
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case LayerIndexI:
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j = 3
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default:
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return 0
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}
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}
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k := ((h >> 20) & 0xf)
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return sampleRates[j][k]
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}
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func main() {
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f, err := os.Open(fname)
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if err != nil {
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log.Fatal(err)
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}
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bufreader := bufio.NewReader(f)
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buf := make([]byte, 4)
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var i int
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for {
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_, err := io.ReadFull(bufreader, buf)
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if err != nil {
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if err == io.EOF {
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break
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}
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log.Fatal(err)
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}
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header, err := newHeaderFromBytes(buf)
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if err == nil {
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fmt.Printf("Got header: %08X, audio version: %d, layer index: %d\n", header, header.AudioVersionId(), header.LayerIndex())
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}
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i++
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bufreader.Discard(3)
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}
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}
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func newHeaderFromBytes(buf []byte) (header, error) {
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if len(buf) < 4 {
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return 0, errors.New("insufficient bytes")
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}
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if buf[0] != 0xFF || buf[1]&0xE0 != 0xE0 {
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return 0, errors.New("no header")
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}
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// always little-endian for now
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n := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
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return newHeader(n)
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}
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// TODO use unsafe here? https://codereview.stackexchange.com/a/60161
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func newHeader(n uint32) (header, error) {
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return header(n), nil
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}
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