make_string.rb

  def make_string(length, type = :alphabet, random: false)
    char_set = nil
    case type
    when :alphabet
      char_set = ('a'..'z').to_a
    when :zenkaku
      char_set = ('あ'..'ん').to_a
    when :hankaku
      char_set = ('ｱ'..'ﾝ').to_a
    end
    char_set_len = char_set.length
    (0..Float::INFINITY).lazy.map do |num|
      nn = random ? rand(char_set_len) : num % char_set_len
      char_set.to_a[nn]
    end.take(length).to_a.join('')
  end