#!/bin/bash
IFS=$'\n\t'

# Author: http://crunchbang.org/forums/viewtopic.php?id=38748#p414992
# m4bronto
# https://gist.github.com/dcondrey/469e2850e7f88ac198e8c3ff111bda7c
# Modified to support a chapters.txt file with sequential list of chapter names
# Run as split-audio-by-chapters file.m4a [chapters.txt]
# Output files are sequential and generated


# https://stackoverflow.com/a/3352015
trim() {
    local var="$*"
    # remove leading whitespace characters
    var="${var#"${var%%[![:space:]]*}"}"
    # remove trailing whitespace characters
    var="${var%"${var##*[![:space:]]}"}"
    echo -n "$var"
}

CHAPTER_INDEX=0

if [ -f "$2" ]; then
  CHAPTERS_FILE_PASSED=1
  echo "Reading chapters from $2"
  readarray CHAPTERS < "$2"
  IFS=$'\n\r'
  IFS=$'\n\t'
else
  CHAPTERS_FILE_PASSED=0
fi

ffmpeg -i "$1" 2> /tmp/ffmpeg.txt

while read -r first _ _ start _ end; do
  unset IFS
  if [[ "$first" = "Chapter" ]]; then
    read -r
    read -r _ _ CHAPTER_TITLE

    REAL_CHAPTER_INDEX=$((CHAPTER_INDEX+1))
    CHAPTER_INDEX_PADDED=$(printf "%03d" "${REAL_CHAPTER_INDEX}")

    # If we are using a chapters.txt that is valid
    if ((CHAPTERS_FILE_PASSED=="1")) ; then
      CHAPTER_TITLE=${CHAPTERS[$CHAPTER_INDEX]}
    fi

    FILENAME="${CHAPTER_INDEX_PADDED} - $(trim "$CHAPTER_TITLE").m4a"

    # -vsync 2 = Frames are passed through with their timestamp or dropped so as to prevent 2 frames from having the same timestamp.
    # -sn = no subtitles
    # -dn = no data stream
    # -vn = no video stream
    # -metadata title= put in a chapter title
    # -acodec copy = copy the audio stream
    # +faststart = Relocating this moov atom to the beginning of the file can facilitate playback before the file is completely downloaded by the client. 
    ffmpeg -hide_banner -loglevel panic \
      -movflags +faststart \
      -vsync 2 -sn -dn -vn -i "$1" -ss "${start%?}" -to "$end" -metadata title="$CHAPTER_TITLE" -dn -vn -sn -acodec copy "$FILENAME" </dev/null

    ((CHAPTER_INDEX++))
  fi
done </tmp/ffmpeg.txt