Batch split multi-part MIDI files into single-part MIDI files.

split_midi_tracks.rb

require 'fileutils'
require 'midilib/sequence'
require 'midilib/io/midifile'

DEBUG = false
LOGGING = true
LOG_FILE = Dir.pwd + '/error.log'
INPUT_DIR = './vgm/'
OUTPUT_DIR = '../split'

def log_error error
  puts error_text(error)
  File.open(LOG_FILE, 'a') {|f| f.write(error) } if LOGGING
end

def fetch_instrument_name track
  program_change = track.events.select{ |event| event.class == MIDI::ProgramChange }.first
  if program_change.nil?
    if track.instrument.nil? || track.instrument.empty? || track.instrument == 'Unnamed'
      if track.name.nil? || track.name.empty? || track.name == 'Unnamed'
        return 'no name'
      else
        return track.name
      end
    else
      track.instrument
    end
  else
    MIDI::GM_PATCH_NAMES[program_change.program]
  end
end

def read_midi_to_sequence(filename)
  seq = MIDI::Sequence.new()

  File.open(filename, 'rb') do |file|
    begin
      seq.read(file)
    rescue
      log_error "[!] error reading file: #{filename}\n"

      if DEBUG
        require 'pry'; binding.pry
      end
    end
  end
  return seq
end

def success_text string
  "\033[32m#{string}\033[0m"
end

def warn_text string
  "\033[33m#{string}\033[0m"
end

def error_text string
  "\033[41m#{string}\033[0m"
end

def sanitize_filename(filename)
  fn = filename.downcase.split /(?<=.)\.(?=[^.])(?!.*\.[^.])/m
  fn.map! { |s| s.gsub /[^a-z0-9\-]+/i, '_' }
  return fn.join '.'
end

def write_dir_unless_exists(dir)
  unless Dir.exist?(dir)
    FileUtils.mkpath dir
    puts success_text("[+] mkdir #{dir}")
  end
end

def write_track_to_file track, filename, tempo
  # require 'pry'; binding.pry
  if track.nil? || track.events.count == 0 || !track.events.map(&:class).include?(MIDI::NoteOn)
    puts warn_text("[!] empty track found. skipping.")
  else
    # seq = MIDI::Sequence.new()
    seq = @seq.clone
    seq.tracks = []

    # add meta data
    # meta_track = MIDI::Track.new(seq)
    # seq.tracks << meta_track
    # meta_track.events << MIDI::Tempo.new(MIDI::Tempo.bpm_to_mpq(tempo.round))
    # meta_track.events << MIDI::MetaEvent.new(MIDI::META_SEQ_NAME, 'Sequence Name')

    notes_track = MIDI::Track.new(seq)
    seq.tracks << notes_track
    notes_track.events << MIDI::Tempo.new(MIDI::Tempo.bpm_to_mpq(tempo.round))
    notes_track.events << MIDI::MetaEvent.new(MIDI::META_SEQ_NAME, fetch_instrument_name(track))
    notes_track.merge(track.events)
    begin
      file = File.open(filename, 'wb')
      seq.write(file)
      # file.close
    rescue
      log_error "[!] error writing file: #{filename}\n"
      if DEBUG
        require 'pry'; binding.pry
      end
    end
    puts success_text("[+] wrote #{filename}")
  end
end

FileUtils.cd(INPUT_DIR, verbose: true)

Dir.glob('**/*.{mid,MID}') do |file|
  @seq = read_midi_to_sequence(file)
  write_dir_unless_exists(OUTPUT_DIR)
  dirname = "#{OUTPUT_DIR}/" + file.downcase.split('.mid').first + "-#{@seq.tempo.round}bpm"
  write_dir_unless_exists(dirname)

  @seq.tracks.each_with_index do |track, idx|
    instrument_name = fetch_instrument_name(track)
    filename = "#{idx}-#{instrument_name}"

    write_track_to_file(track, "#{dirname}/#{sanitize_filename(filename)}.mid", @seq.tempo)
  end
end