#!/bin/sh

# mtag — rebuild MP3 ID3 tags from each file's path/name, then file the song
# under <music>/<artist>/<album>/<title>.mp3 (qndl's layout).
#
# ncmpcpp/MPD show the *embedded* ID3 tags, not the filename — so renaming a
# file or folder doesn't change what you see. mtag reads the tags back out of
# the on-disk layout (so a rename "sticks") and then moves the file to match.
#
# Resolution rules (is_known = non-empty and not "Unknown Artist"/"Unknown Album"):
#   title  = filename without extension
#   artist = grandparent folder, if known
#   album  = parent folder, if known
#   When the artist is NOT known from a folder, a "Artist - Title" filename is
#   parsed (split on the FIRST " - ") to recover the artist and title.
#   When no real album is known, the album falls back to the title (single style).
#   If the artist still can't be determined, its TAG is left untouched, and the
#   move target falls back to the existing tag, then "Unknown Artist".

set -u

MUSIC="${XDG_MUSIC_DIR:-$HOME/Music}"
PLAYLIST="${XDG_CONFIG_HOME:-$HOME/.config}/mpd/playlists/entire.m3u"
TITLES="${XDG_DOTFILES_DIR:-$HOME/.dotfiles}/global/Music/.music_titles.txt"
DRY=0

usage() {
  cat <<EOF
Usage: mtag [--dry-run|-n] [DIR]

By default mtag REWRITES tags AND MOVES each *.mp3 into
<music>/<artist>/<album>/<title>.mp3, prunes emptied folders, regenerates the
'entire.m3u' playlist (if present), syncs qndl's '.music_titles.txt' restore/
delete labels (if present), and refreshes MPD.

  (no args)      apply everything (tags + move + playlist + mpc update)
  --dry-run, -n  preview only — show planned tag and move changes, touch nothing
  DIR            scan DIR instead of the whole music dir ($MUSIC)
  -h, --help     this help

Tags are written as ID3v2.3 (UTF-16) so Korean/Japanese text is safe, and
ffmpeg -c copy preserves all other embedded data (e.g. the source URL that
dmenudelmusic relies on). Collisions during a move are skipped, never clobbered.
EOF
}

# --- argument parsing -------------------------------------------------------
SCAN=""
for arg in "$@"; do
  case "$arg" in
  --dry-run | -n) DRY=1 ;;
  -h | --help)
    usage
    exit 0
    ;;
  -*)
    printf 'mtag: unknown option: %s\n\n' "$arg" >&2
    usage >&2
    exit 2
    ;;
  *) SCAN="$arg" ;;
  esac
done
[ -n "$SCAN" ] || SCAN="$MUSIC"

# --- prerequisites ----------------------------------------------------------
for bin in ffmpeg ffprobe; do
  command -v "$bin" >/dev/null 2>&1 || {
    printf 'mtag: %s not found (required)\n' "$bin" >&2
    exit 1
  }
done
[ -d "$SCAN" ] || {
  printf 'mtag: not a directory: %s\n' "$SCAN" >&2
  exit 1
}

# --- helpers ----------------------------------------------------------------
is_known() {
  case "$1" in
  "" | "Unknown Artist" | "Unknown Album") return 1 ;;
  *) return 0 ;;
  esac
}

# Make a string safe as a single path component: '/' is the only byte Linux
# forbids in a name, so replace just that (keeps spaces, brackets, CJK as-is).
san() {
  printf '%s' "$1" | tr '/' '_'
}

# Show a path relative to the music root when possible (nicer output).
reldisp() {
  case "$1" in
  "$MUSIC"/*) printf '%s' "${1#"$MUSIC"/}" ;;
  *) printf '%s' "$1" ;;
  esac
}

get_tag() {
  # get_tag <file> <tag-name>
  ffprobe -v error -show_entries "format_tags=$2" \
    -of default=nw=1:nk=1 "$1" 2>/dev/null | head -n 1
}

get_vid() {
  # Recover the YouTube ID from the embedded source URL (same method as
  # dmenudelmusic), or empty if there is none.
  strings "$1" 2>/dev/null | grep 'watch?v=' |
    sed 's/.*watch?v=\([A-Za-z0-9_-]*\).*/\1/' | head -n 1
}

# --- main loop --------------------------------------------------------------
_list="$(mktemp)"
_titlemap="$(mktemp)"
trap 'rm -f "$_list" "$_titlemap"' EXIT INT TERM
find "$SCAN" -type f -name '*.mp3' | sort >"$_list"

_total=0
_changed=0
_moved=0

while IFS= read -r f; do
  [ -n "$f" ] || continue
  _total=$((_total + 1))

  # Path relative to the music root, so artist/album come from the right depth
  # regardless of where SCAN points.
  case "$f" in
  "$MUSIC"/*) rel="${f#"$MUSIC"/}" ;;
  *) rel="$f" ;;
  esac

  fname="$(basename "$rel")"
  base="${fname%.*}" # title candidate (drop extension)

  # Determine artist/album folders by depth under the music root.
  dir="$(dirname "$rel")"
  artistdir=""
  albumdir=""
  if [ "$dir" != "." ]; then
    albumdir="$(basename "$dir")" # immediate parent
    pdir="$(dirname "$dir")"
    [ "$pdir" != "." ] && artistdir="$(basename "$pdir")"
    # Two-level case (artist/title.mp3): the single folder is the ARTIST,
    # not the album.
    if [ -z "$artistdir" ]; then
      artistdir="$albumdir"
      albumdir=""
    fi
  fi

  title="$base"
  artist=""
  album=""
  is_known "$artistdir" && artist="$artistdir"
  is_known "$albumdir" && album="$albumdir"

  # Recover artist from a "Artist - Title" filename only when we couldn't get
  # one from a folder — avoids mangling titles that legitimately contain " - ".
  if [ -z "$artist" ]; then
    case "$base" in
    *" - "*)
      artist="${base%%" - "*}"
      title="${base#*" - "}"
      ;;
    esac
  fi

  # Single style: no real album -> album is the title.
  [ -n "$album" ] || album="$title"

  old_title="$(get_tag "$f" title)"
  old_artist="$(get_tag "$f" artist)"
  old_album="$(get_tag "$f" album)"

  # What would actually change? (artist only counts when we have a value)
  diff=0
  [ "$old_title" != "$title" ] && diff=1
  [ "$old_album" != "$album" ] && diff=1
  [ -n "$artist" ] && [ "$old_artist" != "$artist" ] && diff=1
  [ "$diff" -eq 1 ] && _changed=$((_changed + 1))

  # Move target: <music>/<artist>/<album>/<title>.mp3. The path needs a
  # concrete artist even when we won't write the tag, so fall back to the
  # existing tag, then "Unknown Artist".
  path_artist="$artist"
  [ -n "$path_artist" ] || path_artist="$old_artist"
  [ -n "$path_artist" ] || path_artist="Unknown Artist"
  target="$MUSIC/$(san "$path_artist")/$(san "$album")/$(san "$title").mp3"
  needmove=0
  [ "$target" != "$f" ] && needmove=1
  [ "$needmove" -eq 1 ] && _moved=$((_moved + 1))

  # Display
  mark="  "
  { [ "$diff" -eq 1 ] || [ "$needmove" -eq 1 ]; } && mark="* "
  printf '%s%s\n' "$mark" "$rel"
  printf '    title : %s\n' "$title"
  if [ -n "$artist" ]; then
    printf '    artist: %s\n' "$artist"
  else
    printf '    artist: (tag left as-is: %s)\n' "${old_artist:-<empty>}"
  fi
  printf '    album : %s\n' "$album"
  [ "$needmove" -eq 1 ] && printf '    move  : -> %s\n' "$(reldisp "$target")"

  [ "$DRY" -eq 1 ] && continue

  # Record the restore/delete label for qndl's .music_titles.txt, keyed by the
  # embedded YouTube ID, so 'qndl -r'/'-d' show the corrected "artist - title"
  # instead of the stale download-time label. Done before any move (the file is
  # still at "$f"). Files without an embedded ID are skipped.
  vid="$(get_vid "$f")"
  [ -n "$vid" ] && printf '%s\t%s - %s\n' "$vid" "$path_artist" "$title" >>"$_titlemap"

  # Write tags (in place) first, so the moved file carries the new tags.
  # -map 0 + -c copy keep the audio stream and ALL other metadata (cover art,
  # the embedded source URL, etc.) untouched — only the listed tags change.
  if [ "$diff" -eq 1 ]; then
    tmp="${f%.mp3}.mtag.$$.mp3"
    set -- ffmpeg -v error -y -i "$f" -map 0 -c copy -id3v2_version 3 \
      -metadata "title=$title" -metadata "album=$album"
    [ -n "$artist" ] && set -- "$@" -metadata "artist=$artist"
    set -- "$@" "$tmp"
    if "$@" && [ -s "$tmp" ]; then
      mv -f "$tmp" "$f"
    else
      rm -f "$tmp"
      printf '    !! ffmpeg failed, left unchanged\n' >&2
    fi
  fi

  # Then relocate, skipping collisions and pruning emptied source folders.
  if [ "$needmove" -eq 1 ]; then
    if [ -e "$target" ]; then
      printf '    !! target exists, not moved: %s\n' "$(reldisp "$target")" >&2
      _moved=$((_moved - 1))
    else
      mkdir -p "$(dirname "$target")"
      if mv -n "$f" "$target"; then
        d="$(dirname "$f")"
        while [ "$d" != "$MUSIC" ] && [ "$d" != "/" ] && [ "$d" != "." ]; do
          rmdir "$d" 2>/dev/null || break
          d="$(dirname "$d")"
        done
      else
        printf '    !! move failed, left in place\n' >&2
        _moved=$((_moved - 1))
      fi
    fi
  fi
done <"$_list"

# --- finish -----------------------------------------------------------------
printf '\n'
if [ "$DRY" -eq 1 ]; then
  printf 'mtag: dry-run — %d tag change(s), %d move(s), of %d file(s).\n' \
    "$_changed" "$_moved" "$_total"
  printf 'mtag: (apply would also sync entire.m3u and qndl .music_titles.txt labels.)\n'
  printf 'mtag: run without --dry-run to apply.\n'
  exit 0
fi

printf 'mtag: updated %d tag(s), moved %d file(s), of %d total.\n' \
  "$_changed" "$_moved" "$_total"

# Keep qndl's playlist consistent: it lists music-relative *.mp3 paths, which
# moves invalidate. Regenerate it the same way qndl does, but only if it
# already exists (don't create the artifact on setups that don't use it).
if [ "$_moved" -gt 0 ] && [ -f "$PLAYLIST" ]; then
  find "$MUSIC" -name '*.mp3' | sed "s|$MUSIC/||" | sort >"$PLAYLIST"
  printf 'mtag: regenerated %s\n' "$(reldisp "$PLAYLIST")"
fi

# Keep qndl's restore/delete labels (.music_titles.txt) in sync with the new
# tags. Upsert by YouTube ID: update existing lines, append IDs not yet listed.
# Only rewrite the file when its content actually changes (avoids dotfiles noise).
if [ -s "$_titlemap" ] && [ -f "$TITLES" ]; then
  _tnew="$(mktemp)"
  awk '
    FNR==NR {
      t = index($0, "\t"); if (t == 0) next
      id = substr($0, 1, t - 1); lab = substr($0, t + 1)
      map[id] = lab; if (!(id in known)) { order[++n] = id; known[id] = 1 }
      next
    }
    {
      t = index($0, "\t"); lid = (t ? substr($0, 1, t - 1) : $0)
      if (lid in map) { print lid "\t" map[lid]; seen[lid] = 1 }
      else print $0
    }
    END { for (i = 1; i <= n; i++) { id = order[i]; if (!(id in seen)) print id "\t" map[id] } }
  ' "$_titlemap" "$TITLES" >"$_tnew"
  if cmp -s "$_tnew" "$TITLES"; then
    rm -f "$_tnew"
  else
    mv "$_tnew" "$TITLES"
    printf 'mtag: synced labels in %s\n' "$(reldisp "$TITLES")"
  fi
fi

if command -v mpc >/dev/null 2>&1; then
  printf 'mtag: running mpc update...\n'
  mpc update --wait >/dev/null 2>&1 && printf 'mtag: MPD database refreshed.\n'
else
  printf 'mtag: mpc not found — skipped database refresh.\n'
fi