feat(qmc): improve m4a & mp4 sniff

algo/qmc/qmc.go

@@ -90,7 +90,7 @@ func (d *Decoder) validateDecode() error {
 		return fmt.Errorf("qmc seek to start: %w", err)
 	}
 
-	buf := make([]byte, 16)
+	buf := make([]byte, 64)
 	if _, err := io.ReadFull(d.raw, buf); err != nil {
 		return fmt.Errorf("qmc read header: %w", err)
 	}

cmd/um/main.go

@@ -186,7 +186,7 @@ func tryDecFile(inputFile string, outputDir string, allDec []common.NewDecoderFu
 	}
 
 	header := bytes.NewBuffer(nil)
-	_, err = io.CopyN(header, dec, 16)
+	_, err = io.CopyN(header, dec, 64)
 	if err != nil {
 		return fmt.Errorf("read header failed: %w", err)
 	}

go.sum

@@ -37,8 +37,6 @@ golang.org/x/crypto v0.3.0 h1:a06MkbcxBrEFc0w0QIZWXrH/9cCX6KJyWbBOIwAn+7A=
 golang.org/x/crypto v0.3.0/go.mod h1:hebNnKkNXi2UzZN1eVRvBB7co0a+JxK6XbPiWVs/3J4=
 golang.org/x/exp v0.0.0-20221204150635-6dcec336b2bb h1:QIsP/NmClBICkqnJ4rSIhnrGiGR7Yv9ZORGGnmmLTPk=
 golang.org/x/exp v0.0.0-20221204150635-6dcec336b2bb/go.mod h1:CxIveKay+FTh1D0yPZemJVgC/95VzuuOLq5Qi4xnoYc=
-golang.org/x/text v0.4.0 h1:BrVqGRd7+k1DiOgtnFvAkoQEWQvBc25ouMJM6429SFg=
-golang.org/x/text v0.4.0/go.mod h1:mrYo+phRRbMaCq/xk9113O4dZlRixOauAjOtrjsXDZ8=
 golang.org/x/text v0.5.0 h1:OLmvp0KP+FVG99Ct/qFiL/Fhk4zp4QQnZ7b2U+5piUM=
 golang.org/x/text v0.5.0/go.mod h1:mrYo+phRRbMaCq/xk9113O4dZlRixOauAjOtrjsXDZ8=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

internal/sniff/audio.go

@@ -1,6 +1,11 @@
 package sniff
 
-import "bytes"
+import (
+	"bytes"
+	"encoding/binary"
+
+	"golang.org/x/exp/slices"
+)
 
 type Sniffer interface {
 	Sniff(header []byte) bool
@@ -19,8 +24,8 @@ var audioExtensions = map[string]Sniffer{
 	},
 
 	// ref: https://www.garykessler.net/library/file_sigs.html
-	".m4a": mpeg4Sniffer{},            // MPEG-4 container, m4a treat as audio
+	".m4a": m4aSniffer{},    // MPEG-4 container, Apple Lossless Audio Codec
-	".aac": prefixSniffer{0xFF, 0xF1}, // MPEG-4 AAC-LC
+	".mp4": &mpeg4Sniffer{}, // MPEG-4 container, other fallback
 
 	".flac": prefixSniffer("fLaC"), // ref: https://xiph.org/flac/format.html
 	".dff":  prefixSniffer("FRM8"), // DSDIFF, ref: https://www.sonicstudio.com/pdf/dsd/DSDIFF_1.5_Spec.pdf
@@ -54,8 +59,48 @@ func (s prefixSniffer) Sniff(header []byte) bool {
 	return bytes.HasPrefix(header, s)
 }
 
+type m4aSniffer struct{}
+
+func (m4aSniffer) Sniff(header []byte) bool {
+	box := readMpeg4FtypBox(header)
+	if box == nil {
+		return false
+	}
+
+	return box.majorBrand == "M4A " || slices.Contains(box.compatibleBrands, "M4A ")
+}
+
 type mpeg4Sniffer struct{}
 
-func (mpeg4Sniffer) Sniff(header []byte) bool {
+func (s *mpeg4Sniffer) Sniff(header []byte) bool {
-	return len(header) >= 8 && bytes.Equal([]byte("ftyp"), header[4:8])
+	return readMpeg4FtypBox(header) != nil
+}
+
+type mpeg4FtpyBox struct {
+	majorBrand       string
+	minorVersion     uint32
+	compatibleBrands []string
+}
+
+func readMpeg4FtypBox(header []byte) *mpeg4FtpyBox {
+	if (len(header) < 8) || !bytes.Equal([]byte("ftyp"), header[4:8]) {
+		return nil // not a valid ftyp box
+	}
+
+	size := binary.BigEndian.Uint32(header[0:4]) // size
+	if size < 16 || size%4 != 0 {
+		return nil // invalid ftyp box
+	}
+
+	box := mpeg4FtpyBox{
+		majorBrand:   string(header[8:12]),
+		minorVersion: binary.BigEndian.Uint32(header[12:16]),
+	}
+
+	// compatible brands
+	for i := 16; i < int(size) && i+4 < len(header); i += 4 {
+		box.compatibleBrands = append(box.compatibleBrands, string(header[i:i+4]))
+	}
+
+	return &box
 }