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)
}