support MPEG-2 audio

pkg/codecs/mpeg1audio/frame_header.go

@@ -5,45 +5,115 @@ import (
 )
 
 // http://www.mp3-tech.org/programmer/frame_header.html
-var bitrates = map[uint8]map[uint8]int{
-	2: {
-		0b1:    32000,
-		0b10:   48000,
-		0b11:   56000,
-		0b100:  64000,
-		0b101:  80000,
-		0b110:  96000,
-		0b111:  112000,
-		0b1000: 128000,
-		0b1001: 160000,
-		0b1010: 192000,
-		0b1011: 224000,
-		0b1100: 256000,
-		0b1101: 320000,
-		0b1110: 384000,
+var bitrates = [][][]int{
+	// MPEG-1
+	{
+		// layer 1
+		{},
+		// layer 2
+		{
+			32000,
+			48000,
+			56000,
+			64000,
+			80000,
+			96000,
+			112000,
+			128000,
+			160000,
+			192000,
+			224000,
+			256000,
+			320000,
+			384000,
+		},
+		// layer 3
+		{
+			32000,
+			40000,
+			48000,
+			56000,
+			64000,
+			80000,
+			96000,
+			112000,
+			128000,
+			160000,
+			192000,
+			224000,
+			256000,
+			320000,
+		},
 	},
-	3: {
-		0b1:    32000,
-		0b10:   40000,
-		0b11:   48000,
-		0b100:  56000,
-		0b101:  64000,
-		0b110:  80000,
-		0b111:  96000,
-		0b1000: 112000,
-		0b1001: 128000,
-		0b1010: 160000,
-		0b1011: 192000,
-		0b1100: 224000,
-		0b1101: 256000,
-		0b1110: 320000,
+	// MPEG-2
+	{
+		// layer 1
+		{},
+		// layer 2
+		{
+			8000,
+			16000,
+			24000,
+			32000,
+			40000,
+			48000,
+			56000,
+			64000,
+			80000,
+			96000,
+			112000,
+			128000,
+			144000,
+			160000,
+		},
+		// layer 3
+		{
+			8000,
+			16000,
+			24000,
+			32000,
+			40000,
+			48000,
+			56000,
+			64000,
+			80000,
+			96000,
+			112000,
+			128000,
+			144000,
+			160000,
+		},
 	},
 }
 
-var sampleRates = map[uint8]int{
-	0:    44100,
-	0b1:  48000,
-	0b10: 32000,
+var sampleRates = [][]int{
+	// MPEG-1
+	{
+		44100,
+		48000,
+		32000,
+	},
+	// MPEG-2
+	{
+		22050,
+		24000,
+		16000,
+	},
+}
+
+var samplesPerFrame = [][]int{
+	// MPEG-1
+	{
+		384,
+		1152,
+		1152,
+	},
+	// MPEG-2
+	{
+		384,
+		1152,
+		576,
+	},
 }
 
 // ChannelMode is a channel mode of a MPEG-1/2 audio frame.
@@ -80,35 +150,38 @@ func (h *FrameHeader) Unmarshal(buf []byte) error {
 	}
 
 	h.MPEG2 = ((buf[1] >> 3) & 0x01) == 0
-	h.Layer = 4 - ((buf[1] >> 1) & 0b11)
 
-	switch {
-	case !h.MPEG2 && h.Layer == 2:
-	case !h.MPEG2 && h.Layer == 3:
-	default:
-		return fmt.Errorf("unsupported MPEG version or layer: %v %v", h.MPEG2, h.Layer)
+	var mpegIndex int
+	if h.MPEG2 {
+		mpegIndex = 1
+	} else {
+		mpegIndex = 0
+	}
+
+	h.Layer = 4 - ((buf[1] >> 1) & 0b11)
+	if h.Layer <= 1 || h.Layer >= 4 {
+		return fmt.Errorf("unsupported MPEG layer: %v", h.Layer)
 	}
 
-	bitrateIndex := buf[2] >> 4
-	var ok bool
-	h.Bitrate, ok = bitrates[h.Layer][bitrateIndex]
-	if !ok {
+	bitrateIndex := (buf[2] >> 4)
+	if bitrateIndex == 0 || bitrateIndex >= 15 {
 		return fmt.Errorf("invalid bitrate")
 	}
+	h.Bitrate = bitrates[mpegIndex][h.Layer-1][bitrateIndex-1]
 
 	sampleRateIndex := (buf[2] >> 2) & 0b11
-	h.SampleRate, ok = sampleRates[sampleRateIndex]
-	if !ok {
+	if sampleRateIndex >= 3 {
 		return fmt.Errorf("invalid sample rate")
 	}
+	h.SampleRate = sampleRates[mpegIndex][sampleRateIndex]
 
 	h.Padding = ((buf[2] >> 1) & 0b1) != 0
 	h.ChannelMode = ChannelMode(buf[3] >> 6)
 
 	return nil
 }
 
-// FrameLen returns the length of the frame associated to the header.
+// FrameLen returns the length of the frame associated with the header.
 func (h FrameHeader) FrameLen() int {
 	if h.Padding {
 		return (144 * h.Bitrate / h.SampleRate) + 1
@@ -118,16 +191,12 @@ func (h FrameHeader) FrameLen() int {
 
 // SampleCount returns the number of samples contained into the frame.
 func (h FrameHeader) SampleCount() int {
-	/*
-		MPEG-1:
-		* layer 1: 384
-		* layer 2: 1152
-		* layer 3: 1152
-
-		MPEG-2:
-		* layer 1: 384
-		* layer 2: 1152
-		* layer 3: 576
-	*/
-	return 1152
+	var mpegIndex int
+	if h.MPEG2 {
+		mpegIndex = 1
+	} else {
+		mpegIndex = 0
+	}
+
+	return samplesPerFrame[mpegIndex][h.Layer-1]
 }