https://qiita.com/aaaasahi_17/items/dacc41d5a3584a23ee3f#comment-14ecccb0dbfbb13cb874

blackjack.rb

class Blackjack

  def play
    puts <<~END
    =======================

    Welcome to Blackjack!!!

    =======================
    END
    user = User.new "あなた"
    dealer = Dealer.new "ディーラー"
    dealer.deal user
    judge(user, dealer)
  end

  def judge(user, dealer)
    user.show_hand
    dealer.show_hand
    user_score = user.score
    dealer_score = dealer.score
    if  user_score == dealer_score
      puts "同点なので引き分けです。"
    elsif user_score == 21
      puts "ブラックジャック！！ あなたの勝ちです！"
    elsif user_score > 21
      puts "バースト！！ 合計点数が21を超えたのであなたの負けです。"
    elsif dealer_score > 21
      puts "ディーラーがバーストしました。あなたの勝ちです！"
    elsif user_score < dealer_score
      puts "ディーラーの勝ちです！"
    else
      puts "あなたの勝ちです！"
    end
  end
end

class Deck
  @@SUITS = ["ハート", "クローバー", "ダイヤ", "スペード"]
  @@INDEXES = ["A", "2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "K"]

  def initialize
    @cards = []
    refill
  end

  def refill
    @@SUITS.each do |suit|
      @@INDEXES.each do |number|
        @cards << Card.new(suit, number)
      end
    end
    @cards.shuffle!
  end

  def draw
    if @cards.empty?
      refill
    end
    @cards.pop
  end
end

class Card
  attr_reader :suit, :number

  def initialize(suit, number)
    @suit = suit
    @number = number
  end

  def info
    "#{@suit}の#{@number}"
  end

  def score
    if @number == "J" || @number == "Q" || @number == "K"
      10
    elsif @number == "A"
      1
    else
      @number.to_i
    end
  end
end

class Player
  def initialize(name)
    @name = name
    @hand = []
  end

  def hold(card)
    @hand << card
  end

  def score
    total = @hand.map{|card| card.score}.sum
    total + (total <= 11 && @hand.any?{|card| card.number == "A"} ? 10 : 0)
  end

  def show_hand
    puts ""
    puts "====#{@name}の手札===="
    @hand.each.with_index(1) do |card, i|
      puts "#{i}枚目 : #{card.info}"
    end
    puts "#{@name}の合計点数は#{score}です"
    puts "========================"
    puts ""
  end
end

class User < Player

  def hit?
    show_hand
    if score > 21
      false
    else
      puts <<~END
        どうしますか？

        1.Hit(カードを引く)
        2.Stand(今の手札で勝負)

      END
      gets.to_i == 1
    end
  end
end

class Dealer < Player

  def initialize(name)
    super(name)
    @deck = Deck.new
  end

  def deal(user)
    user.hold @deck.draw  # 最初にユーザと自分にカードを2枚ずつ配る
    user.hold @deck.draw
    hold @deck.draw
    hold @deck.draw

    show_hand_first

    while user.hit?         # ユーザがカードを求める間
      user.hold @deck.draw  # ユーザにカードを配る
    end
    while hit?              # 自分がカードを求める間
      hold @deck.draw       # 自分にカードを配る
    end
  end

  def hit?
    show_hand
    score < 17
  end

  def show_hand_first
    puts <<~END
    ====#{@name}ーの手札====
    1枚目：#{@hand.first.info}
    2枚目：伏せられている
    ========================
    END
  end
end

Blackjack.new.play