# Feature B — Shopping Cart

## Your goal

Implement `ShoppingCart` using strict TDD. **Do not write any production code before you have a failing test.**

## Domain rules to implement (in order)

| Step | Feature | Notes |
|------|---------|-------|
| 1 | Empty cart has subtotal 0 and count 0 | The baseline |
| 2 | Add items — subtotal = price × quantity | Multiple items accumulate |
| 3 | Remove item by name | Removing unknown name does nothing |
| 4 | Apply % discount to subtotal | 10% off $50 → $45 |
| 5 | CartItem rejects zero/negative price & quantity | Value Object validation |
| 6 | Bonus: duplicate name / float precision | Design decision — your call |

## The rhythm — repeat for every rule

```
1. Uncomment the next test → run → watch it FAIL (Red)
2. Write the minimum code to make it pass → run → GREEN
3. Refactor if needed → run → still GREEN
4. Move to the next test
```

## Running the tests

```bash
dart pub get
dart test
```

## Files

- `lib/shopping_cart.dart` — your implementation goes here
- `test/shopping_cart_test.dart` — uncomment tests one at a time

## Design hints (don't peek until you're stuck!)

<details>
<summary>Hint for Step 3 — Remove by name</summary>

When you need to remove by name, a `List<CartItem>` forces you to search by name.
A `Map<String, CartItem>` makes lookup O(1) and removal trivial.
Let the test drive you toward the right data structure.
</details>

<details>
<summary>Hint for Step 5 — Value Object</summary>

Validation belongs in `CartItem`'s constructor, not in `ShoppingCart.addItem()`.
If the item can't be created, the cart never sees it.
This is the Value Object pattern: invalid state is unrepresentable.
</details>

<details>
<summary>Hint for the float precision bonus test</summary>

Use `closeTo(expected, delta)` instead of `equals()` for floating point comparisons.
Example: `expect(result, closeTo(0.30, 0.001))`
</details>