module IkaConjugations
  U_VOWEL_CHANGES = {
    "る" => { "a" => "ら", "i" => "り", "u" => "る", "e" => "れ", "o" => "ろ" },
    "う" => { "a" => "あ", "i" => "い", "u" => "う", "e" => "え", "o" => "お" },
    "つ" => { "a" => "た", "i" => "ち", "u" => "つ", "e" => "て", "o" => "と" },
    "す" => { "a" => "さ", "i" => "し", "u" => "す", "e" => "せ", "o" => "そ" },
    "く" => { "a" => "か", "i" => "き", "u" => "く", "e" => "け", "o" => "こ" },
    "ぐ" => { "a" => "が", "i" => "ぎ", "u" => "ぐ", "e" => "げ", "o" => "ご" },
    "ぶ" => { "a" => "ば", "i" => "び", "u" => "ぶ", "e" => "べ", "o" => "ぼ" },
    "む" => { "a" => "ま", "i" => "み", "u" => "む", "e" => "め", "o" => "も" },
    "ぬ" => { "a" => "な", "i" => "に", "u" => "ぬ", "e" => "ね", "o" => "の" },
  }

  class KanaKanji
    attr_reader :kana, :kanji
    def initialize(kana, kanji=nil)
      @kana = kana
      @kanji = kanji || kana
    end

    def [](index)
      self.class.new kana[index], kanji[index]
    end

    def +(other)
      case other
      when String
        self.class.new kana + other, kanji + other
      when KanaKanji
        self.class.new kana + other.kana, kanji + other.kanji
      end
    end

    def last_character(new_form=nil)
      return kana[-1] unless new_form
      U_VOWEL_CHANGES[kana[-1]][new_form]
    end

    def all_but_last_character
      self.class.new kana[0...-1], kanji[0...-1]
    end
  end
end