feat: detect actual audio format and resolve quality mismatch

sniff.go

@@ -0,0 +1,58 @@
+package main
+
+import (
+	"encoding/binary"
+	"errors"
+	"io"
+	"os"
+)
+
+func detectAudioFormat(path string) (string, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+	b := make([]byte, 64*1024)
+	n, err := f.Read(b)
+	if err != nil && err != io.EOF {
+		return "", err
+	}
+	b = b[:n]
+	if len(b) >= 4 && string(b[:4]) == "fLaC" {
+		return "flac", nil
+	}
+	if len(b) >= 12 && string(b[:4]) == "RIFF" && string(b[8:12]) == "WAVE" {
+		return "wav", nil
+	}
+	if len(b) >= 12 && string(b[:4]) == "FORM" && (string(b[8:12]) == "AIFF" || string(b[8:12]) == "AIFC") {
+		return "aiff", nil
+	}
+	if len(b) >= 4 && string(b[:4]) == "OggS" {
+		return "ogg", nil
+	}
+	if len(b) >= 12 && string(b[4:8]) == "ftyp" {
+		return "m4a", nil
+	}
+	start := 0
+	if len(b) >= 10 && string(b[:3]) == "ID3" {
+		size := int(b[6]&0x7f)<<21 | int(b[7]&0x7f)<<14 | int(b[8]&0x7f)<<7 | int(b[9]&0x7f)
+		start = 10 + size
+		if start > len(b) {
+			start = 10
+		}
+	}
+	for i := start; i+4 <= len(b) && i < start+4096; i++ {
+		if b[i] == 0xff && b[i+1]&0xe0 == 0xe0 {
+			h := binary.BigEndian.Uint32(b[i : i+4])
+			layer := (h >> 17) & 3
+			if layer == 1 {
+				return "mp3", nil
+			}
+			if b[i+1]&0x06 == 0x06 {
+				return "aac", nil
+			}
+		}
+	}
+	return "", errors.New("unknown audio format")
+}