我正在做一个练习，设计一个超市，为产品提供各种折扣。折扣的例子有：

• 购买x数量的物品可免费获得y
• 以$y购买y数量的物品
• 用$z购买物品x和物品y

我已经设法创建了前两个折扣。但由于折扣是根据 * 购物篮项目 * 计算的，我很难看到我如何才能使折扣适用于多个产品。我可以创建一个全新的项目，并标记为“x和y”，但这感觉有点像作弊。请提供建议

import java.math.BigDecimal;

public class Item {

    private final String sku;
    private final BigDecimal unitPrice;

    public Item(String sku, BigDecimal unitPrice) {
        this.sku = sku;
        this.unitPrice = unitPrice;
    }

    public BigDecimal getUnitPrice() {
        return unitPrice;
    }
}

import java.math.BigDecimal;

public class BasketItem {

    private final Item item;
    private int quantity;
    private final Discount discount;

    public BasketItem(Item item, Discount discount) {
        this.item = item;
        this.discount = discount;
        this.quantity = 1;
    }

    public BasketItem(Item item, Discount discount, int quantity) {
        this.item = item;
        this.discount = discount;
        this.quantity = quantity;
    }

    public void increment() {
        this.quantity++;
    }

    public BigDecimal calculateSubTotal() {
        return item.getUnitPrice().multiply(new BigDecimal(quantity));
    }

    public BigDecimal calculateDiscount() {
        return discount.apply(this);
    }

    public Item getItem() {
        return item;
    }

    public int getQuantity() {
        return quantity;
    }

}

import java.math.BigDecimal;

public interface Discount {

    BigDecimal apply(BasketItem item);

}

import java.math.BigDecimal;
import java.security.InvalidParameterException;

public class BuyXGetYFree implements Discount {

    private final int buy;
    private final int free;

    public BuyXGetYFree(int buy, int free) {
        if (buy < 1) {
            throw new InvalidParameterException("Can't buy less than 1 item");
        }

        if (free < 1) {
            throw new InvalidParameterException("Can't free less than 1 item for free");
        }

        if (free > buy) {
            throw new InvalidParameterException("The free amount must not exceed the purchased amount");
        }

        this.buy = buy;
        this.free = free;
    }

    @Override
    public BigDecimal apply(BasketItem basketItem) {

        int quantity = basketItem.getQuantity();
        BigDecimal unitPrice = basketItem.getItem().getUnitPrice();

        if (buy + free > quantity && quantity > buy) {
            return unitPrice.multiply(BigDecimal.valueOf(quantity - buy));
        }

        int multiplier = quantity / (buy + free);
        int forFree = multiplier * free;
        return unitPrice.multiply(BigDecimal.valueOf(forFree));
    }
    
}

import java.math.BigDecimal;

public class NoDiscount implements Discount {

    @Override
    public BigDecimal apply(BasketItem item) {
        return BigDecimal.ZERO;
    }
}

import java.math.BigDecimal;
import java.security.InvalidParameterException;

public class BuyXForY implements Discount{

    int buy;
    BigDecimal pay;

    public BuyXForY(int buy, BigDecimal pay) {
        if (buy < 1) {
            throw new InvalidParameterException("Buy must be at least 1");
        }
        this.buy = buy;
        this.pay = pay;
    }

    @Override
    public BigDecimal apply(BasketItem item) {
        int quantity = item.getQuantity();

        if (quantity < buy) {
            return BigDecimal.ZERO;
        }

        int multiplier = quantity / buy;
        int remainder = quantity % buy;

        BigDecimal buyAtDiscountedPrice = new BigDecimal(multiplier).multiply(pay);
        BigDecimal buyAtNormalPrice = new BigDecimal(multiplier).multiply(new BigDecimal(remainder));
        BigDecimal normalPrice = new BigDecimal(quantity).multiply(item.getItem().getUnitPrice());

        return normalPrice.subtract(buyAtDiscountedPrice.add(buyAtNormalPrice));
    }
}

public class DiscountAssignment {

    private final Discount discount;
    private final Item item;

    public DiscountAssignment(Discount discount, Item item) {
        this.discount = discount;
        this.item = item;
    }

    public Discount getDiscount() {
        return discount;
    }

    public Item getItem() {
        return item;
    }

}

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;
import java.util.Optional;

public class Checkout {

    private final List<BasketItem> basketItems;
    private final List<DiscountAssignment> discountAssignments;

    public Checkout(List<DiscountAssignment> discountAssignments) {
        this.basketItems = new ArrayList<>();
        this.discountAssignments = discountAssignments;
    }

    public void scan(Item item) {
        Optional<BasketItem> basketItemOptional = basketItems.stream().filter(basketItem -> basketItem.getItem().equals(item)).findFirst();

        if (basketItemOptional.isPresent()) {
            basketItemOptional.get().increment();
            return;
        }

        Optional<DiscountAssignment> discountsToApply = discountAssignments.stream().filter(discountAssignment -> discountAssignment.getItem().equals(item)).findFirst();

        if (discountsToApply.isPresent()) {
            basketItems.add(new BasketItem(item, discountsToApply.get().getDiscount()));
        } else {
            basketItems.add(new BasketItem(item, new NoDiscount()));
        }
    }

    public BigDecimal total() {
        System.out.println(basketItems);

        return basketItems.stream()
                .map(BasketItem::calculateSubTotal)
                .reduce(BigDecimal.ZERO, BigDecimal::add)
                .subtract(basketItems.stream()
                        .map(BasketItem::calculateDiscount)
                        .reduce(BigDecimal.ZERO, BigDecimal::add));
    }

}

快速测试输出的总成本为4

import java.math.BigDecimal;
import java.util.List;

public class Test {

    public static void main(String[] args) {
        Item drink = new Item("Drink", new BigDecimal(1));
        Item candy = new Item("Candy", new BigDecimal(1));

        DiscountAssignment buyOneGetOneFreeDiscount = new DiscountAssignment(new BuyXGetYFree(1, 1), drink);
        DiscountAssignment buyTwoForOneDollar = new DiscountAssignment(new BuyXForY(2, BigDecimal.ONE), candy);

        Checkout checkout = new Checkout(List.of(buyOneGetOneFreeDiscount, buyTwoForOneDollar));
        checkout.scan(drink);
        checkout.scan(drink);
        checkout.scan(drink);
        checkout.scan(drink);
        checkout.scan(candy);
        checkout.scan(candy);

        System.out.println("Total cost: " + checkout.total());
    }

}

我尝试创建一个全新的捆绑项目。但每个项目必须单独扫描。