refactor: move audio sniffer to internal package

internal/sniff/audio.go

@@ -0,0 +1,61 @@
+package sniff
+
+import "bytes"
+
+type Sniffer interface {
+	Sniff(header []byte) bool
+}
+
+var audioExtensions = map[string]Sniffer{
+	// ref: https://mimesniff.spec.whatwg.org
+	".mp3": prefixSniffer("ID3"),
+	".ogg": prefixSniffer("OggS"),
+	".wav": prefixSniffer("RIFF"),
+
+	// ref: https://www.loc.gov/preservation/digital/formats/fdd/fdd000027.shtml
+	".wma": prefixSniffer{
+		0x30, 0x26, 0xb2, 0x75, 0x8e, 0x66, 0xcf, 0x11,
+		0xa6, 0xd9, 0x00, 0xaa, 0x00, 0x62, 0xce, 0x6c,
+	},
+
+	// ref: https://www.garykessler.net/library/file_sigs.html
+	".m4a": mpeg4Sniffer{},            // MPEG-4 container, m4a treat as audio
+	".aac": prefixSniffer{0xFF, 0xF1}, // MPEG-4 AAC-LC
+
+	".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
+
+}
+
+// AudioExtension sniffs the known audio types, and returns the file extension.
+// header is recommended to at least 16 bytes.
+func AudioExtension(header []byte) (string, bool) {
+	for ext, sniffer := range audioExtensions {
+		if sniffer.Sniff(header) {
+			return ext, true
+		}
+	}
+	return "", false
+}
+
+// AudioExtensionWithFallback is equivalent to AudioExtension, but returns fallback
+// most likely to use .mp3 as fallback, because mp3 files may not have ID3v2 tag.
+func AudioExtensionWithFallback(header []byte, fallback string) string {
+	ext, ok := AudioExtension(header)
+	if !ok {
+		return fallback
+	}
+	return ext
+}
+
+type prefixSniffer []byte
+
+func (s prefixSniffer) Sniff(header []byte) bool {
+	return bytes.HasPrefix(header, s)
+}
+
+type mpeg4Sniffer struct{}
+
+func (mpeg4Sniffer) Sniff(header []byte) bool {
+	return len(header) >= 8 && bytes.Equal([]byte("ftyp"), header[4:8])
+}