Discount - OOD Discount Engine with Multiple Pricing Rules

Question

## Problem

Design a discount engine for an e-commerce checkout system. Discounts can be:
- `PercentOff(percent)`: reduces price by a percentage
- `FlatOff(amount)`: reduces price by a fixed amount
- `BuyXGetYFree(x, y)`: for every `x` units purchased, `y` units are free
- `CapDiscount(max_discount)`: wraps another discount and caps its maximum saving

Discounts can be stacked. The final price must never go below zero.

```python
class Discount(ABC):
    @abstractmethod
    def apply(self, price: float, quantity: int) -> float: ...

# Implement: PercentOff, FlatOff, BuyXGetYFree, CapDiscount, StackedDiscount
```

## Example

```
price=100, qty=3
StackedDiscount([PercentOff(10), FlatOff(5)]).apply(100, 3)
-> 100 * 0.90 = 90, then 90 - 5 = 85.0

CapDiscount(PercentOff(50), max_discount=30).apply(100, 1)
-> would give 50 off, but capped at 30 -> 70.0
```

## Follow-ups

1. How do you handle order of application -- does `PercentOff` before `FlatOff` give a different result than the reverse?
2. How would you serialize and deserialize a stacked discount configuration to/from JSON?
3. Add a `BestOf(discounts)` rule that applies the single most beneficial discount from a set.
4. How do you unit test `BuyXGetYFree` with fractional quantities?

Discount - OOD Discount Engine with Multiple Pricing Rules

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