Cart Pricing: Design an OOD Cart System with Pluggable Discount and Tax Strategies

Question

## Problem

Design a shopping cart system using object-oriented design. The cart holds items with quantities and unit prices. It must support pluggable discount strategies (e.g., percentage off, buy-2-get-1-free) and a pluggable tax calculator. The `checkout()` method returns a detailed receipt.

```python
from abc import ABC, abstractmethod

class DiscountStrategy(ABC):
    @abstractmethod
    def apply(self, subtotal: float, items: list) -> float: ...

class TaxStrategy(ABC):
    @abstractmethod
    def compute(self, amount: float) -> float: ...

class Cart:
    def __init__(self, discount: DiscountStrategy, tax: TaxStrategy): ...
    def add_item(self, name: str, unit_price: float, qty: int) -> None: ...
    def checkout(self) -> dict: ...
    # receipt: {items, subtotal, discount, tax, total}
```

**Example:**
```
cart = Cart(PercentDiscount(10), FlatTax(0.08))
cart.add_item("Widget", 20.0, 3)  # subtotal=60
# discount: 10% off -> 54.00
# tax: 8% of 54 -> 4.32
# total: 58.32
```

## Follow-ups

1. Which design patterns are being used here, and what are the trade-offs vs. a simple `if/elif` discount chain?
2. How would you compose multiple discount strategies (e.g., apply percent discount AND a coupon code)?
3. How would you make the cart serializable to JSON so an in-progress cart can be saved and restored?
4. Where would you add logging and auditing so each discount applied is traceable per transaction?

Cart Pricing: Design an OOD Cart System with Pluggable Discount and Tax Strategies

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