ケント・ベックに学ぶ良いコードの書き方🗒️

# Before
# 商品の金額を計算する
class PriceCalculator
  def discounted_total_price(order)
    total = 0

    order.items.each do |item|
      total += item.price * item.quantity
    end

    if total > 100
      total -= (total * 0.1)
    end

    if order.customer.premium_member?
      total -= (total * 0.1)
    end

    return total.round()
  end
end

# After
class PriceCalculator
  def discounted_total_price(order)
    total = total_price(order) 
    total = basic_discounted_price(total) 
    total = member_discounted_price(total)
    
    return total.round()
  end

  private

  def total_price(order)
    return order.items.reduce(0) do |sum, item|
      sum + item.price * item.quantity
    end
  end

  def basic_discounted_price(total)
    if total > 100
      return total - (total * 0.1)
    else
      return total
    end
  end

  def member_discounted_price(total, customer)
    if order.customer.premium_member?
      return total - (total * 0.1)
    else
      return total
    end
  end
end

# Before
class Member
  attr_reader :rank
  
  def initialize(rank)
    @rank = rank
  end
  
  def rent_video(type)
    if @rank == :premium
      if type == :new_release
        puts "Premium member renting a new release."
      elsif type == :regular
        puts "Premium member renting a regular video."
      end
    else
      if type == :new_release
        puts "Standard member cannot rent new releases."
      elsif type == :regular
        puts "Standard member renting a regular video."
      end
    end
  end
end

member = Member.new(:premium)
member.rent_video(:new_release)  # "Premium member renting a new release."

member2 = Member.new(:standard)
member2.rent_video(:new_release)  # "Standard member cannot rent new releases."

# After
class Member
  def initialize(rank)
    @rank = rank
  end
  
  def rent_video(video_type)
    rank_handler = case @rank
                   when :premium then PremiumRank.new
                   when :standard then StandardRank.new
                   else raise "Unknown member rank"
                   end
                    
    video_type_handler = case video_type
                         when :new_release then NewRelease.new
                         when :regular then RegularVideo.new
                         else raise "Unknown video type"
                         end
                          
    rank_handler.rent(video_type_handler)
  end
end

class Rank
  def rent(video_type)
    raise NotImplementedError, "This method should be overridden by subclasses"
  end
end

class VideoType
  def rent_by_premium
    raise NotImplementedError, "This method should be overridden by subclasses"
  end
  
  def rent_by_standard
    raise NotImplementedError, "This method should be overridden by subclasses"
  end
end

# After
class PremiumRank < Rank
  def rent(video_type)
    video_type.rent_by_premium
  end
end

class StandardRank < Rank
  def rent(video_type)
    video_type.rent_by_standard
  end
end

class NewRelease < VideoType
  def rent_by_premium
    puts "Premium member renting a new release."
  end
  
  def rent_by_standard
    puts "Standard member cannot rent new releases."
  end
end

class RegularVideo < VideoType
  def rent_by_premium
    puts "Premium member renting a regular video."
  end
  
  def rent_by_standard
    puts "Standard member renting a regular video."
  end
end

class Member
  attr_reader :rank
  
  def initialize(rank)
    @rank = rank
  end
  
  def rent_video(type)
    @rank.rent(type)
  end
end

PremiumRank.new.rent(NewRelease.new) #  "Premium member renting a new release."
StandardRank.new.rent(NewRelease.new) # "Standard member cannot rent new releases."
PremiumRank.new.rent(RegularVideo.new) # "Premium member renting a regular video."
StandardRank.new.rent(RegularVideo.new) # "Standard member renting a regular video."

# Before
class Order
  def calculate_total_price(customer, items, discount,
    tax_rate, shipping_cost, coupon, loyalty_points)

    # ベースの合計金額を計算
    total = items.sum(&:price)

    # アイテムが10個以上なら追加割引
    total *= 0.95 if items.size >= 10

    # その他すごく複雑な処理が続く...

    total
  end

  def other_method
    # 他の処理
  end

  # その他メソッドが続く...
end

order = Order.new
puts order.calculate_total_price(customer, items, 10, 0.08, 5, coupon, loyalty_points)


# After
class Calculator
  attr_reader :customer, :items, :discount, :tax_rate,
              :shipping_cost, :coupon, :loyalty_points

  def initialize(customer, items, discount, tax_rate,
                 shipping_cost, coupon, loyalty_points)
    @customer = customer
    @items = items
    @discount = discount
    @tax_rate = tax_rate
    @shipping_cost = shipping_cost
    @coupon = coupon
    @loyalty_points = loyalty_points
  end

  def calculate_total_price
    # ベースの合計金額を計算
    total = items.sum(&:price)

    # アイテムが10個以上なら追加割引
    total *= 0.95 if items.size >= 10

    # その他すごく複雑な処理が続く...

    total
  end
end

class Order
  attr_reader :calculator

  def calculate_total_price
    calculator.calculate_total_price
  end

  def other_method
    # 他の処理
  end

  # その他メソッドが続く...
end

order = Order.new
Order.calculator = Calculator.new(customer, items, 10, 0.08, 5, coupon, loyalty_points)
puts order.calculate_total_price