fix: improve ALAC M4A quality parsing

go_backend/metadata.go

@@ -1423,16 +1423,82 @@ func GetM4AQuality(filePath string) (AudioQuality, error) {
 	//   [28:32] samplerate (16.16 fixed-point)
 	sampleRate := int(buf[28])<<8 | int(buf[29])
 	bitDepth := int(buf[22])<<8 | int(buf[23])
-	if bitDepth <= 0 {
-		bitDepth = 16
-		if atomType == "alac" {
-			bitDepth = 24
+
+	if atomType == "alac" {
+		if alacBitDepth, alacSampleRate, ok := readALACSpecificConfig(f, sampleOffset, fileSize); ok {
+			if alacBitDepth > 0 {
+				bitDepth = alacBitDepth
+			}
+			if alacSampleRate > 0 {
+				sampleRate = alacSampleRate
+			}
 		}
 	}
 
+	if bitDepth <= 0 {
+		bitDepth = 16
+	}
+
 	return AudioQuality{BitDepth: bitDepth, SampleRate: sampleRate}, nil
 }
 
+func readALACSpecificConfig(f *os.File, sampleOffset, fileSize int64) (int, int, bool) {
+	if sampleOffset < 4 {
+		return 0, 0, false
+	}
+
+	sampleEntryHeader, err := readAtomHeaderAt(f, sampleOffset-4, fileSize)
+	if err != nil {
+		return 0, 0, false
+	}
+
+	childStart := sampleOffset + 32
+	childEnd := sampleEntryHeader.offset + sampleEntryHeader.size
+	if childStart >= childEnd {
+		return 0, 0, false
+	}
+
+	configHeader, found, err := findAtomInRange(f, childStart, childEnd-childStart, "alac", fileSize)
+	if err != nil || !found {
+		return 0, 0, false
+	}
+
+	payloadSize := configHeader.size - configHeader.headerSize
+	if payloadSize <= 0 {
+		return 0, 0, false
+	}
+
+	payload := make([]byte, payloadSize)
+	if _, err := f.ReadAt(payload, configHeader.offset+configHeader.headerSize); err != nil {
+		return 0, 0, false
+	}
+
+	return parseALACSpecificConfig(payload)
+}
+
+func parseALACSpecificConfig(payload []byte) (int, int, bool) {
+	if len(payload) < 24 {
+		return 0, 0, false
+	}
+
+	bitDepth := int(payload[5])
+	sampleRate := int(binary.BigEndian.Uint32(payload[20:24]))
+	if bitDepth > 0 && sampleRate > 0 {
+		return bitDepth, sampleRate, true
+	}
+
+	// Some encoders prepend 4 bytes before the ALACSpecificConfig payload.
+	if len(payload) >= 28 {
+		bitDepth = int(payload[9])
+		sampleRate = int(binary.BigEndian.Uint32(payload[24:28]))
+		if bitDepth > 0 && sampleRate > 0 {
+			return bitDepth, sampleRate, true
+		}
+	}
+
+	return 0, 0, false
+}
+
 type atomHeader struct {
 	offset     int64
 	size       int64

go_backend/metadata_m4a_quality_test.go

@@ -0,0 +1,49 @@
+package gobackend
+
+import "testing"
+
+func TestParseALACSpecificConfigStandardPayload(t *testing.T) {
+	payload := make([]byte, 24)
+	payload[5] = 24
+	payload[20] = 0x00
+	payload[21] = 0x00
+	payload[22] = 0xac
+	payload[23] = 0x44
+
+	bitDepth, sampleRate, ok := parseALACSpecificConfig(payload)
+	if !ok {
+		t.Fatal("expected standard ALAC payload to parse")
+	}
+	if bitDepth != 24 {
+		t.Fatalf("bitDepth = %d, want 24", bitDepth)
+	}
+	if sampleRate != 44100 {
+		t.Fatalf("sampleRate = %d, want 44100", sampleRate)
+	}
+}
+
+func TestParseALACSpecificConfigPayloadWithLeadingFourBytes(t *testing.T) {
+	payload := make([]byte, 28)
+	payload[9] = 16
+	payload[24] = 0x00
+	payload[25] = 0x00
+	payload[26] = 0xbb
+	payload[27] = 0x80
+
+	bitDepth, sampleRate, ok := parseALACSpecificConfig(payload)
+	if !ok {
+		t.Fatal("expected offset ALAC payload to parse")
+	}
+	if bitDepth != 16 {
+		t.Fatalf("bitDepth = %d, want 16", bitDepth)
+	}
+	if sampleRate != 48000 {
+		t.Fatalf("sampleRate = %d, want 48000", sampleRate)
+	}
+}
+
+func TestParseALACSpecificConfigRejectsShortPayload(t *testing.T) {
+	if _, _, ok := parseALACSpecificConfig(make([]byte, 12)); ok {
+		t.Fatal("expected short ALAC payload to be rejected")
+	}
+}