class Array
  #
  # TODO 
  # active_supportに同様のメソッドが無いかどうかの確認
  #
  def split_by(num)
    return [] if self.size < 1
    ret = [[]]
    counter = 0
    self.each do |val|
      (counter = 0;ret << []) if counter >= num
      ret[-1] << val
      counter += 1
    end
    return ret
  end
end