Creates a MIDI file with chords, declared by a cryptic chord syntax.

harmony-pump.rb

#!/usr/bin/env ruby

# self(44aa 78ea 455d), /03.09.2013./
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.


require 'bundler'
Bundler.require

class Chord
  attr_accessor :base, :name, :minor, :position, :nths, :octave, :shifts

  def notes
    @@notes ||= %w"i ii iii iv v vi vii".zip([0, 2, 4, 5, 7, 9, 11])
  end

  def hnotes
    @@hnotes ||= Hash[notes]
  end

  def parse
    syntax = /([b#])?(#{ hnotes.keys.join('|') })(\d+)?(o?)([<>]+)?$/i
    _, @offset, @position, @nths, @octave, @shifts = name.match(syntax).to_a

    @octave = @octave == "" ? nil : @octave
    @minor  = name == name.downcase
  end

  def initialize(base, name)
    @name = name
    parse
    @base = base
    @base += hnotes[@position.downcase]
    @base += {'b' => -1, '#' => 1}[@offset].to_i
  end

  def rotate(harmonies)
    return harmonies unless shifts

    shifts.each_char do |shift|
      # > == up, < == down
      settings = (shift == '>') ? [1, -1, 12] : [-1, 0, -12] # direction, index, offset

      harmonies.rotate!(settings[0])
      harmonies[settings[1]] += settings[2]
    end

    harmonies
  end

  def oddities(harmonies)
    additional = {7 => 1, 9 => 2, 11 => 3}[nths.to_i].to_i
    octaves    = octave ? 1 : 0
    harmonies.first(3 + additional + octaves)
  end

  def to_a
    level rotate oddities harmonies
  end

  private

  def harmonies
    if minor
      [octave ? 12 : nil, 0, 3, 7, 10, 14, 16]
    else
      [octave ? 12 : nil, 0, 4, 7, 11, 14, 16]
    end.compact
  end

  def level(harmonies) # as a verb
    harmonies.map(&base.method(:+))
  end
end

class HarmonyPump
  attr_accessor :name, :base, :chords, :track, :seq

  include MIDI

  def initialize(base, *notes)
    @base   = base
    @name   = notes.join('-')
    @chords = notes.map(&method(:chord))
    @seq    = Sequence.new

    populate_tracks
  end

  def chord(name)
    Chord.new(base, name)
  end

  def populate_tracks
    strack = Track.new(seq) # Settings track.
    seq.tracks << strack
    strack.events << Tempo.new(Tempo.bpm_to_mpq(120))
    strack.events << MetaEvent.new(META_SEQ_NAME, name)

    @track = Track.new(seq)
    seq.tracks << track

    track.name       = name
    track.instrument = GM_PATCH_NAMES[7]
    track.events    << Controller.new(0, CC_VOLUME, 255)
  end

  def length(chords, nth)
    len = seq.note_to_delta('half')
    if chords.count == nth + 1
      len * 3
    else
      len
    end
  end

  def notes
    # note.new(chan, note, velocity, delta_time)
    track.events << ProgramChange.new(0, 1, 0)

    chords.each_with_index do |chord, chord_i|
      notes = chord.to_a
      notes.each do |note|
        track.events << NoteOn .new(0, note, 127, 0)
      end
      notes.each_with_index do |note, i|
        track.events << NoteOff.new(0, note, 127, i == 0 ? length(chords, chord_i) : 0)
      end
    end
  end

  def >>(io)
    notes
    seq.write(io)
  end

  def debug
    chords.each do |chord|
      $stderr.write "Interpreted #{ chord.name } as:\t#{ chord.to_a }\n"
    end
  end
end

HarmonyPump.new(45, *%w"I bIII< bVIo<< bII7 I").tap(&:debug) >> $>

"midilib" gem is required.