Convert a bunch of mp3 files into an m4b file

mp3tom4b.rb

#!/usr/bin/env ruby

require 'open3'
require 'tempfile'

##
# Create a single m4b file from multiple mp3 files.
# Finds all *.mp3 files, merges them, in lexigraphical order, creates chapters based on the mp3 files, adds cover.jpg as a cover

## Todo
# Add copy metadata from the mp3s - book title / author etc. 
# Can those ffmpeg commands be merged into few commands? 

# Function to get the duration of an MP3 file in seconds
def get_duration(file)
  puts "Starting with #{file}"
  stdout, stderr, status = Open3.capture3("ffmpeg -i \"#{file}\" 2>&1")
  duration_line = stdout.match(/Duration: (\d+):(\d+):(\d+\.\d+)/)
  hours, minutes, seconds = duration_line.captures.map(&:to_f)
  (hours * 3600) + (minutes * 60) + seconds
end

# Function to format time in HH:MM:SS.mmm
def format_time(seconds)
  hours = (seconds / 3600).to_i
  minutes = ((seconds % 3600) / 60).to_i
  seconds = (seconds % 60).to_i
  milliseconds = ((seconds % 1) * 1000).to_i
  format("%02d:%02d:%02d.%03d", hours, minutes, seconds, milliseconds)
end


def generate_filelist_and_chapters
  # Initialize variables
  start_time = 0
  chapter_number = 1

  # Open filelist.txt for writing
  File.open("filelist.txt", "w") do |filelist|
    # Open chapters.txt for writing
    File.open("chapters.txt", "w") do |chapters|
      # Loop through each MP3 file
      chapters.puts ";FFMETADATA1"
      Dir.glob("*.mp3").sort.each do |mp3_file|
        # Get the duration of the current file
        duration = get_duration(mp3_file)

        # Format start time
        formatted_start_time = format_time(start_time)

        chapters.puts "[CHAPTER]"
        chapters.puts "TIMEBASE=1/1000"
        chapters.puts "START=#{(start_time * 1000).to_i}"
        chapters.puts "END=#{((start_time + duration) * 1000).to_i}"
        chapters.puts "title=Chapter #{chapter_number}"


        # Update start time and chapter number
        start_time += duration
        chapter_number += 1

        filelist.puts "file '#{mp3_file}'"
      end
    end
  end
end

def with_tempfile(ext1, ext2, ext3)
  Tempfile.create(['tmp1', ext1]) do |tempfile1|
    Tempfile.create(['tmp2', ext2]) do |tempfile2|
      Tempfile.create(['tmp3', ext3]) do |tempfile3|
        yield(tempfile1.path, tempfile2.path, tempfile3.path)
      end
    end
  end
end

generate_filelist_and_chapters

with_tempfile('.m4a', '.m4b', '.m4b') do |tmpfile1, tmpfile2, tmpfile3|
  system("ffmpeg -y -f concat -safe 0 -i filelist.txt  -map 0:a #{tmpfile1}")
  system("ffmpeg -y -i #{tmpfile1} -codec copy #{tmpfile2}")
  system("ffmpeg -y -i #{tmpfile2} -i chapters.txt -map_metadata 1 -map_chapters 1 -codec copy #{tmpfile3}")
  system("ffmpeg -y -i #{tmpfile3} -i cover.jpg -c copy -disposition:v attached_pic book.m4b")
end