#!/usr/bin/env bash

root="${1:-.}"
find "$root" \( -iname '*.mp3' -o -iname '*.m4a' -o -iname '*.m4b' \) -print0 |
while IFS= read -r -d '' f; do

    comment=$(ffprobe -v error \
        -show_entries format_tags=comment \
        -of default=nw=1:nk=1 \
        -- "$f")

    [[ $comment =~ ^[Rr]ead[[:space:]]+[Bb]y[[:space:]]+(.+) ]] || continue

    composer="${BASH_REMATCH[1]}"
    echo "Updating: $f"
    echo "  Composer: $composer"

    dir=$(dirname -- "$f")
    base=$(basename -- "$f")
    ext="${base##*.}"

    case "${ext,,}" in
        mp3) format=mp3 ;;
        m4a|m4b) format=ipod ;;
        *) continue ;;
    esac

    tmp=$(mktemp -p "$dir" ".${base}.XXXXXX") || {
        echo "Couldn't create temporary file in $dir" >&2
        continue
    }

    if ffmpeg -nostdin -v error -y \
        -i "$f" \
        -map 0 \
        -map_metadata 0 \
        -c copy \
        -metadata composer="$composer" \
        -metadata comment= \
        -f "$format" \
        "$tmp"
    then
        mv -f -- "$tmp" "$f"
    else
        echo "FAILED: $f" >&2
        rm -f -- "$tmp"
    fi
done