tdd-workshop/shopping_cart/test/shopping_cart_test.dart

import 'package:test/test.dart';
import '../lib/shopping_cart.dart';

// ============================================================
//  SHOPPING CART — Workshop Starter
//  Follow Red → Green → Refactor strictly.
//  Uncomment one test at a time. Make it pass. Then the next.
// ============================================================

void main() {
  late ShoppingCart cart;

  setUp(() {
    cart = ShoppingCart();
  });

  // ──────────────────────────────────────────────────────────
  // STEP 1 — Empty cart
  // Before adding anything, the cart should be empty.
  // Trivial, but sets the baseline. Always start here.
  // ──────────────────────────────────────────────────────────
  group('Empty cart', () {
    test('subtotal is zero when cart is empty', () {
      // TODO: uncomment when ready
      // expect(cart.subtotal(), equals(0.0));
    });

    test('item count is zero when cart is empty', () {
      // TODO: uncomment when ready
      // expect(cart.itemCount(), equals(0));
    });
  });

  // ──────────────────────────────────────────────────────────
  // STEP 2 — Adding items
  // A single item: subtotal = price × quantity.
  // ──────────────────────────────────────────────────────────
  group('Adding items', () {
    test('subtotal reflects a single item added', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Apple', price: 1.50, quantity: 3));
      // expect(cart.subtotal(), equals(4.50));
    });

    test('item count reflects quantity of a single item', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Apple', price: 1.50, quantity: 3));
      // expect(cart.itemCount(), equals(3));
    });

    test('subtotal accumulates across multiple different items', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Apple', price: 1.50, quantity: 2));  // 3.00
      // cart.addItem(CartItem(name: 'Banana', price: 0.75, quantity: 4)); // 3.00
      // expect(cart.subtotal(), equals(6.00));
    });

    test('item count sums quantities across multiple items', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Apple', price: 1.50, quantity: 2));
      // cart.addItem(CartItem(name: 'Banana', price: 0.75, quantity: 4));
      // expect(cart.itemCount(), equals(6));
    });
  });

  // ──────────────────────────────────────────────────────────
  // STEP 3 — Removing items
  // Removing an item by name should drop it from the subtotal.
  // ──────────────────────────────────────────────────────────
  group('Removing items', () {
    test('removing an item reduces the subtotal', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Apple', price: 1.50, quantity: 2));
      // cart.addItem(CartItem(name: 'Banana', price: 0.75, quantity: 4));
      // cart.removeItem('Apple');
      // expect(cart.subtotal(), equals(3.00));
    });

    test('removing an item reduces the item count', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Apple', price: 1.50, quantity: 2));
      // cart.addItem(CartItem(name: 'Banana', price: 0.75, quantity: 4));
      // cart.removeItem('Apple');
      // expect(cart.itemCount(), equals(4));
    });

    test('removing a non-existent item does nothing', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Apple', price: 1.50, quantity: 2));
      // cart.removeItem('Orange'); // not in cart
      // expect(cart.subtotal(), equals(3.00));
    });

    test('cart is empty after removing the only item', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Apple', price: 1.50, quantity: 2));
      // cart.removeItem('Apple');
      // expect(cart.subtotal(), equals(0.0));
      // expect(cart.itemCount(), equals(0));
    });
  });

  // ──────────────────────────────────────────────────────────
  // STEP 4 — Discount
  // A percentage discount reduces the subtotal proportionally.
  // Think: what is 10% off $50.00? → $45.00
  // ──────────────────────────────────────────────────────────
  group('Applying a percentage discount', () {
    test('10% discount reduces subtotal correctly', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Apple', price: 10.00, quantity: 5)); // 50.00
      // expect(cart.totalAfterDiscount(10), equals(45.00));
    });

    test('0% discount returns the full subtotal', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Apple', price: 10.00, quantity: 5));
      // expect(cart.totalAfterDiscount(0), equals(50.00));
    });

    test('100% discount results in zero total', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Apple', price: 10.00, quantity: 5));
      // expect(cart.totalAfterDiscount(100), equals(0.0));
    });

    test('discount on empty cart returns zero', () {
      // TODO: uncomment when ready
      // expect(cart.totalAfterDiscount(20), equals(0.0));
    });
  });

  // ──────────────────────────────────────────────────────────
  // STEP 5 — CartItem validation (Value Object)
  // Invalid items should be rejected at construction time.
  // This is the "Value Object" pattern: impossible to create bad state.
  // ──────────────────────────────────────────────────────────
  group('CartItem validation', () {
    test('rejects an item with a zero price', () {
      // TODO: uncomment when ready
      // expect(
      //   () => CartItem(name: 'Free thing', price: 0, quantity: 1),
      //   throwsArgumentError,
      // );
    });

    test('rejects an item with a negative price', () {
      // TODO: uncomment when ready
      // expect(
      //   () => CartItem(name: 'Broken item', price: -5.00, quantity: 1),
      //   throwsArgumentError,
      // );
    });

    test('rejects an item with zero quantity', () {
      // TODO: uncomment when ready
      // expect(
      //   () => CartItem(name: 'Apple', price: 1.50, quantity: 0),
      //   throwsArgumentError,
      // );
    });
  });

  // ──────────────────────────────────────────────────────────
  // BONUS — Edge cases (if you finish early)
  // ──────────────────────────────────────────────────────────
  group('Edge cases', () {
    test('adding the same item name twice replaces it (or accumulates — you decide!)', () {
      // TODO: uncomment and decide the behavior, then implement it
      // Hint: this is a design decision. TDD lets the test define the rule.
      //
      // Option A — replace:
      // cart.addItem(CartItem(name: 'Apple', price: 1.50, quantity: 2));
      // cart.addItem(CartItem(name: 'Apple', price: 1.50, quantity: 5));
      // expect(cart.itemCount(), equals(5)); // replaced
      //
      // Option B — accumulate:
      // expect(cart.itemCount(), equals(7)); // stacked
    });

    test('subtotal handles floating point prices accurately', () {
      // TODO: uncomment when ready
      // cart.addItem(CartItem(name: 'Item', price: 0.1, quantity: 3)); // 0.1 + 0.1 + 0.1
      // expect(cart.subtotal(), closeTo(0.30, 0.001)); // use closeTo for floats!
    });
  });
}