154 lines
2.7 KiB
Go
154 lines
2.7 KiB
Go
package mpeg
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import "fmt"
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type (
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AudioVersionId int
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Layer int
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)
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type Decoder struct{}
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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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LayerReserved Layer = iota
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LayerIII
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LayerII
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LayerI
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)
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var (
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bitRates = [5][16]int{
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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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sampleRates = [4][4]int{
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{11025, 12000, 8000, 0},
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{0, 0, 0, 0},
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{22050, 24000, 16000, 0},
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{44100, 48000, 32000, 0},
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}
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samplesPerFrame = [4][4]int{
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{0, 576, 1152, 384},
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{0, 0, 0, 0},
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{0, 576, 1152, 384},
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{0, 1152, 1152, 384},
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}
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)
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type Header []byte
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func (h Header) HasSyncWord() bool {
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return h[0] == 0xff && h[1]&0xe0 == 0xe0
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}
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func (h Header) AudioVersionId() AudioVersionId {
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return AudioVersionId((h[1] >> 3) & 0x3)
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}
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func (h Header) Layer() Layer {
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return Layer((h[1] >> 1) & 0x3)
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}
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func (h Header) IsProtected() bool {
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return h[1]&1 == 1
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}
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func (h Header) String() string {
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return fmt.Sprintf("%08b", h)
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}
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func (h Header) SamplesPerFrame() int {
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return samplesPerFrame[h.AudioVersionId()][h.Layer()]
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}
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func (h Header) SampleRate() int {
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i := h.AudioVersionId()
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j := (h[2] >> 2) & 0x3
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return sampleRates[i][j]
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}
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func (h Header) PaddingBytes() int {
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if isPadded := (h[2] >> 1) & 1; isPadded == 0 {
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return 0
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}
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switch h.Layer() {
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case LayerI:
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return 4
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case LayerII, LayerIII:
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return 1
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default:
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return 0
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}
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}
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func (h Header) IsValid() bool {
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if len(h) < 4 {
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return false
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}
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if !h.HasSyncWord() {
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return false
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}
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if h.AudioVersionId() == AudioVersionReserved {
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return false
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}
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if h.Layer() == LayerReserved {
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return false
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}
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if h.BitRate() == 0 {
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return false
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}
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if h.SampleRate() == 0 {
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return false
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}
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return true
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}
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func (h Header) Len() int {
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// TODO premultiply bitrates?
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bps := float64(h.SamplesPerFrame()) / 8
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return int((bps*float64(h.BitRate()*1000))/float64(h.SampleRate()) + float64(h.PaddingBytes()))
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}
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func (h Header) BitRate() int {
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// TODO: improve this
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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.Layer() {
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case LayerIII:
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j = 2
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case LayerII:
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j = 1
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case LayerI:
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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.Layer() {
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case LayerIII, LayerII:
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j = 4
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case LayerI:
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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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return bitRates[j][h[2]>>4]
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}
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