http://blog.jnito.com/entry/2014/06/06/104420

karaoke_machine.rb

# coding: utf-8

class KaraokeMachine
  NOTES = %w(C C# D D# E F F# G G# A A# B)

  def initialize(melody)
    @melody = parse(melody)
  end

  def transpose(amount)
    @melody.map do |note|
      if NOTES.include?(note)
        move(note, amount)
      else
        note
      end
    end.join('')
  end

  def parse(melody)
    melody.scan(/[\|A-G\ ](?:#)?/)
  end

  def move(note, amount)
    index = NOTES.find_index(note)
    NOTES[(index + amount) % NOTES.size]
  end
end

describe KaraokeMachine do
  it 'メロディ無し' do
    melody = ''
    answer = melody
    expect(KaraokeMachine.new(melody).transpose(0)).to eq answer
  end

  # ここから下のコメントを外していって、テストを全部パスさせてください！

  it '変更無し' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = melody
    expect(KaraokeMachine.new(melody).transpose(0)).to eq answer
  end

  it '2上げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = 'D E F# G |F# E D   |F# G A B |A G F#   |D   D   |D   D   |DDEEF#F#GG|F# E D   '
    expect(KaraokeMachine.new(melody).transpose(2)).to eq answer
  end

  it '7上げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = 'G A B C |B A G   |B C D E |D C B   |G   G   |G   G   |GGAABBCC|B A G   '
    expect(KaraokeMachine.new(melody).transpose(7)).to eq answer
  end

  it '1下げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = 'B C# D# E |D# C# B   |D# E F# G# |F# E D#   |B   B   |B   B   |BBC#C#D#D#EE|D# C# B   '
    expect(KaraokeMachine.new(melody).transpose(-1)).to eq answer
  end

  it '7下げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = 'F G A A# |A G F   |A A# C D |C A# A   |F   F   |F   F   |FFGGAAA#A#|A G F   '
    expect(KaraokeMachine.new(melody).transpose(-7)).to eq answer
  end

  it '1オクターブ上げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = melody
    expect(KaraokeMachine.new(melody).transpose(12)).to eq answer
  end

  it '1オクターブ下げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = melody
    expect(KaraokeMachine.new(melody).transpose(-12)).to eq answer
  end

  it '14上げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = 'D E F# G |F# E D   |F# G A B |A G F#   |D   D   |D   D   |DDEEF#F#GG|F# E D   '
    expect(KaraokeMachine.new(melody).transpose(14)).to eq answer
  end

  it '13下げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = 'B C# D# E |D# C# B   |D# E F# G# |F# E D#   |B   B   |B   B   |BBC#C#D#D#EE|D# C# B   '
    expect(KaraokeMachine.new(melody).transpose(-13)).to eq answer
  end

  it '2オクターブ上げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = melody
    expect(KaraokeMachine.new(melody).transpose(24)).to eq answer
  end

  it '2オクターブ下げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = melody
    expect(KaraokeMachine.new(melody).transpose(-24)).to eq answer
  end

  it '26上げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = 'D E F# G |F# E D   |F# G A B |A G F#   |D   D   |D   D   |DDEEF#F#GG|F# E D   '
    expect(KaraokeMachine.new(melody).transpose(26)).to eq answer
  end

  it '25上げる' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    answer = 'B C# D# E |D# C# B   |D# E F# G# |F# E D#   |B   B   |B   B   |BBC#C#D#D#EE|D# C# B   '
    expect(KaraokeMachine.new(melody).transpose(-25)).to eq answer
  end

  it '6上げる(メロディはF#から開始)' do
    melody = 'F# G# A# B |A# G# F#   |A# B C# D# |C# B A#   |F#   F#   |F#   F#   |F#F#G#G#A#A#BB|A# G# F#   '
    answer = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    expect(KaraokeMachine.new(melody).transpose(6)).to eq answer
  end

  it '6下げる(メロディはF#から開始)' do
    melody = 'F# G# A# B |A# G# F#   |A# B C# D# |C# B A#   |F#   F#   |F#   F#   |F#F#G#G#A#A#BB|A# G# F#   '
    answer = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    expect(KaraokeMachine.new(melody).transpose(-6)).to eq answer
  end

  it '連続してtransposeを呼び出す' do
    melody = 'C D E F |E D C   |E F G A |G F E   |C   C   |C   C   |CCDDEEFF|E D C   '
    karaoke = KaraokeMachine.new(melody)

    answer = melody
    expect(karaoke.transpose(0)).to eq answer

    answer = 'D E F# G |F# E D   |F# G A B |A G F#   |D   D   |D   D   |DDEEF#F#GG|F# E D   '
    expect(karaoke.transpose(2)).to eq answer

    answer = 'B C# D# E |D# C# B   |D# E F# G# |F# E D#   |B   B   |B   B   |BBC#C#D#D#EE|D# C# B   '
    expect(karaoke.transpose(-1)).to eq answer
  end
end