add internal/app/transcode.go

internal/app/transcode.go

@@ -0,0 +1,37 @@
+package app
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+// ffmpegArgs builds the transcode command line: FLAC -> 128k 44.1kHz stereo
+// MP3 with ID3v2.3 + ID3v1 tags so even old stereo displays show the title.
+func ffmpegArgs(src, dst, bitrate, title, album string) []string {
+	return []string{
+		"-y", "-hide_banner", "-loglevel", "error",
+		"-i", src,
+		"-metadata", "title=" + title,
+		"-metadata", "album=" + album,
+		"-codec:a", "libmp3lame",
+		"-b:a", bitrate,
+		"-ar", "44100",
+		"-ac", "2",
+		"-id3v2_version", "3",
+		"-write_id3v1", "1",
+		dst,
+	}
+}
+
+// Transcode runs ffmpeg src -> dst. dst is written atomically via a temp file.
+func Transcode(src, dst, bitrate, title, album string) error {
+	tmp := dst + ".tmp.mp3" // keep .mp3 suffix so ffmpeg can detect the muxer
+	args := ffmpegArgs(src, tmp, bitrate, title, album)
+	cmd := exec.Command("ffmpeg", args...)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		os.Remove(tmp)
+		return fmt.Errorf("ffmpeg %s: %w: %s", src, err, out)
+	}
+	return os.Rename(tmp, dst)
+}