#!/usr/bin/env bash

set -e

OUTPUT="audiobook.m4b"
FILES="files.txt"
CHAPTERS="chapters.txt"

# list of MP3 files in filename order
find . -maxdepth 1 -name '*.mp3' -print0 |
    sort -z |
    while IFS= read -r -d '' file; do
        file="${file#./}"
        printf "file '%s'\n" "$file"
    done > "$FILES"

# start chapter metadata file
{
    echo ";FFMETADATA1"
} > "$CHAPTERS"

START=0

while IFS= read -r line; do
    FILE="${line#file \'}"
    FILE="${FILE%\'}"

    # duration in milliseconds
    DURATION=$(ffprobe \
        -v error \
        -show_entries format=duration \
        -of csv=p=0 \
        "$FILE")

    DURATION_MS=$(awk "BEGIN {printf \"%.0f\", $DURATION * 1000}")
    END=$((START + DURATION_MS))

    # filename as the chapter title
    TITLE="${FILE%.mp3}"

    cat >> "$CHAPTERS" <<EOF
[CHAPTER]
TIMEBASE=1/1000
START=$START
END=$END
title=$TITLE
EOF

    START=$END

done < "$FILES"

read -p "Press Enter key after making any corrections to $CHAPTERS"

ffmpeg \
    -f concat \
    -safe 0 \
    -i "$FILES" \
    -i "$CHAPTERS" \
    -map 0:a \
    -map_metadata 1 \
    -c:a aac \
    -b:a 128k \
    -movflags +faststart \
    "$OUTPUT"