From c89600e6b2228ebdb287314e269cc60c2dbcabc6 Mon Sep 17 00:00:00 2001
From: hermes <hermes@obahan.xyz>
Date: Wed, 30 Sep 2026 21:59:32 +0200
Subject: [PATCH] add internal/app/transcode.go

---
 internal/app/transcode.go | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)
 create mode 100644 internal/app/transcode.go

diff --git a/internal/app/transcode.go b/internal/app/transcode.go
new file mode 100644
index 0000000..42fa97e
--- /dev/null
+++ b/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)
+}