[Rubyでじゃんけんアプリを作ってみた - Qiita](https://qiita.com/hiro-gen/items/85639c0a8f925e31fbbe) を見たので書いてみた

janken.rb

# frozen_string_literal: true

# じゃんけんゲームで出せる手の種類
module Hands
  HANDS = %i[rock scissors paper].freeze
  HANDS_LABEL = %w[グー チョキ パー].freeze

  private_constant :HANDS, :HANDS_LABEL
end

# じゃんけんゲームの開始・終了、状況を管理
class JankenGame
  include Hands

  def initialize(player:, computer:)
    @player = player
    @computer = computer
  end

  def start
    puts 'じゃんけんゲーム!!'
    puts ''

    loop do
      @player.hand   = ask_hand(@player)
      @computer.hand = ask_hand(@computer)

      game_result = judge_game

      break if game_end?(game_result)
    end
  end

  private

  def judge_game
    show_players_hand

    puts ''
    if @player.won?(@computer)
      puts 'あなたの勝利!!'
      :won
    elsif @player.draw?(@computer)
      puts '引き分け! もう一回!!'
      puts ''
      :draw
    elsif @player.lose?(@computer)
      puts 'あなたの負け...'
      :lose
    end
  end

  # 勝敗が決したらゲーム終了
  def game_end?(game_result)
    %i[won lose].include?(game_result)
  end

  def show_players_hand
    puts ''
    puts "プレイヤー  ： #{@player.hand_as_label}"
    puts "コンピュータ： #{@computer.hand_as_label}"
  end

  def ask_hand(player)
    return player.generated_hand_choose if player.is_computer?

    puts 'あなたの出す手は?'
    puts(HANDS_LABEL.map.with_index { |label, i| "[#{i}] #{label}" })
    hand_choose = gets.chomp

    raise_if_validate_error(hand_choose)

    HANDS.at(hand_choose.to_i)
  rescue TypeError, RuntimeError
    puts ''
    puts 'エラー!! 選択肢にある数字を入力してね！'
    puts ''

    retry
  end

  # 数字入力を期待し、それ以外は例外とする
  def raise_if_validate_error(hand_choose)
    raise TypeError unless hand_choose.match?(/\d+/)
    raise RuntimeError if HANDS.at(hand_choose.to_i).nil?
    raise RuntimeError if hand_choose.to_i.negative?
  end
end

# Rubocop のルールよりも見た目と位置揃えを優先したいので一時的に無効化
# rubocop:disable Layout/SpaceInsideArrayPercentLiteral, Layout/AlignHash

# じゃんけんゲームの出した手を勝敗パターンテーブルとして保持
module GameJudgeTable
  JUDGE_TABLE = {
    # 自分が勝つパターン
    %i[rock     scissors] => :me,
    %i[scissors paper]    => :me,
    %i[paper    rock]     => :me,
    # 相手が勝つパターン
    %i[rock     paper]    => :other,
    %i[scissors rock]     => :other,
    %i[paper    scissors] => :other,
    # 引き分けパターン
    %i[rock     rock]     => :draw,
    %i[scissors scissors] => :draw,
    %i[paper    paper]    => :draw
  }.freeze

  private_constant :JUDGE_TABLE
end
# rubocop:enable Layout/SpaceInsideArrayPercentLiteral, Layout/AlignHash

# じゃんけんゲームのプレイヤー
class Player
  include GameJudgeTable
  include Hands

  attr_reader :is_computer, :hand
  alias is_computer? is_computer

  def initialize(is_computer: false)
    @is_computer = is_computer
  end

  # @hand に保持される値は Hand::HANDS と同じ %i[rock scissors paper] の3種を期待
  def hand=(hand)
    raise TypeError unless HANDS.include?(hand)

    @hand = hand
  end

  # 勝敗パターンを「勝ち」「負け」「引き分け」の true/false 応答に変換
  [
    %i[won? me],
    %i[lose? other],
    %i[draw? draw]
  ].each do |method_name, refer_value|
    define_method(method_name) do |other_player|
      JUDGE_TABLE[[@hand, other_player.hand]].eql?(refer_value)
    end
  end

  # プレイヤーがコンピュータ側の場合があるので、選ぶ手を自動選択できるようにした
  def generated_hand_choose
    HANDS.sample
  end

  def hand_as_label
    index = HANDS.index(@hand)

    HANDS_LABEL.at(index)
  end
end

# ==============================

player = Player.new
computer = Player.new(is_computer: true)

janken_game = JankenGame.new(player: player, computer: computer)

janken_game.start