Scalaでデザインパターン ~strategyパターン~

import scala.math._
​
object Main extends App {
  val user = User(10000, 10000, 0.2, 0.5, 0.2)
  val products = new ProductRepository().findAll(20)
  val shop = Shop(products)
  //  今回は別の戦略を使うため
  //  val strategy = new UnlimitedGenreStrategy()　
  val strategy = new OneGenreStrategy("toy")
  User.shopping(user)(shop, strategy)
}
​
class ProductRepository() {
  private val genres = Seq("beauty", "food", "toy")
​
  def findAll(num: Int): Seq[Product] = for {
    i <- (1 to num)
  } yield {
    val genre = genres(floor(random() * genres.length).toInt)
    Product(i, s"$genre$i", floor(random() * 5000).toInt, genre)
  }
}
​
case class Product(id: Int, name: String, price: Int, genre: String)
​// 所持金,予算,美容用品の予算割合,食品の予算割合,おもちゃの予算割合
case class User(wallet: Int, budget: Int, maxBeautyPercentage: Double, maxFoodPercentage: Double, maxToyPercentage: Double) {
  private def notBuy(product: Product): Unit = println(s"${product.price}円の${product.name}を購入しませんでした")
}
​
object User {
  def shopping(user: User)(shop: Shop, strategy: ShoppingStrategy): Unit = {
    // Userをイミュータブルにするために、購入した時に作り直している
    shop.products.foldLeft(user) { (u, product) =>
      // strategyを呼び出して購入するかの判断をしている
      if (strategy.judgeBuy(u, product)) {
        buy(product, u)
      } else {
        u.notBuy(product)
        u
      }
    }
  }
​
  private def buy(product: Product, user: User): User = {
    val balance = user.wallet - product.price
    println(s"${product.price}円の${product.name} を購入した！　残り$balance 円")
    User(balance, user.budget, user.maxBeautyPercentage, user.maxFoodPercentage, user.maxToyPercentage)
  }
}
​
​
case class Shop(products: Seq[Product])
​
trait ShoppingStrategy {
  // 買い物の戦略
  def judgeBuy(user: User, product: Product): Boolean
}
​
// 商品カテゴリごとに予算割合を事前に決めておく戦略。
class UnlimitedGenreStrategy() extends ShoppingStrategy {
  override def judgeBuy(user: User, product: Product): Boolean = {
    //  商品の予算割合を計算
    val budgetPercentage = product.price.toDouble / user.budget
    product.genre match {
      case _ if user.wallet < product.price => false
      case "beauty" => budgetPercentage <= user.maxBeautyPercentage
      case "food" => budgetPercentage <= user.maxFoodPercentage
      case "toy" => budgetPercentage <= user.maxToyPercentage
    }
  }
}

// 特定カテゴリの商品だけを購入判断する戦略
class OneGenreStrategy(genre: String) extends ShoppingStrategy {
  override def judgeBuy(user: User, product: Product): Boolean = product.genre match {
    case _ if user.wallet < product.price => false
    case "beauty" if "beauty" == genre => true
    case "food" if "food" == genre => true
    case "toy" if "toy" == genre => true
    case _ => false
  }
}