split-notes.rb

# Requirements:
# 1. Install Ruby
# 2. gem install midilib
#
# To run:
#
#     ruby split-notes.rb <file.mid> <number-of-parts>

require 'midilib'

name = ARGV[0]
raise "#{name} not exist" unless File.exist? name

parts_count = ARGV[1].to_i
raise "#{parts_count} must be more than 0" unless parts_count > 0

out_name = name.sub(/(?:_src)?(?:\.\w+)?$/, '')

seq = MIDI::Sequence.new()
File.open(name, 'rb') { |io| seq.read(io) }

parts = (1..parts_count).to_a
event_part = { }.compare_by_identity

seq.each do |track|
  usage = Hash.new(-1)
  using = Hash.new(false)
  note_part = { }
  track.each do |event|
    case event
    when MIDI::NoteOn
      usable_parts = parts.select { |part| !using[part] }
      raise "No usable part!" if usable_parts.empty?
      part = usable_parts.min_by { |id| usage[id] }
      usage[part] = event.time_from_start
      using[part] = true
      note_part[event.note] = part
      event_part[event] = part
    when MIDI::NoteOff
      part = note_part[event.note] or raise "Part not found!"
      using[part] = false
      event_part[event] = part
    end
  end
end

parts.each do |part|
  out = seq.dup
  out.tracks = out.tracks.map { |track|
    out_track = track.dup
    out_track.events = out_track.events.reject { |event| event_part.include?(event) && event_part[event] != part }
    out_track.recalc_delta_from_times
    out_track
  }
  out_part_name = "#{out_name}_part#{part}.mid"
  puts out_part_name
  File.open(out_part_name, 'wb') { |io| out.write io }
end