describe('ExpenseCalculator', () => {
  describe('calculateTotal', () => {
    test('sums expense amounts correctly', () => {
      // Arrange
      const expenses = [
        { amount: 100, category: 'food' },
        { amount: 50, category: 'transport' },
        { amount: 25, category: 'entertainment' }
      ];

      // Act
      const total = calculateTotal(expenses);

      // Assert
      expect(total).toBe(175);
    });

    test('handles empty expense list', () => {
      expect(calculateTotal([])).toBe(0);
    });

    test('handles negative amounts', () => {
      const expenses = [
        { amount: 100, category: 'food' },
        { amount: -50, category: 'refund' }
      ];
      expect(calculateTotal(expenses)).toBe(50);
    });
  });
});

describe('ExpenseAPI Integration', () => {
  beforeAll(async () => {
    await database.connect(TEST_DB_URL);
  });

  afterAll(async () => {
    await database.disconnect();
  });

  beforeEach(async () => {
    await database.clear();
    await seedTestData();
  });

  test('POST /expenses creates expense and updates total', async () => {
    const response = await request(app)
      .post('/api/expenses')
      .send({
        amount: 50,
        category: 'food',
        description: 'Lunch'
      })
      .expect(201);

    expect(response.body).toMatchObject({
      id: expect.any(Number),
      amount: 50,
      category: 'food'
    });

    // Verify database state
    const total = await getTotalExpenses();
    expect(total).toBe(50);
  });
});

test('user can track expense from start to finish', async ({ page }) => {
  // Navigate to app
  await page.goto('/');

  // Add new expense
  await page.click('[data-testid="add-expense-btn"]');
  await page.fill('[data-testid="amount"]', '50.00');
  await page.selectOption('[data-testid="category"]', 'food');
  await page.fill('[data-testid="description"]', 'Lunch');
  await page.click('[data-testid="submit"]');

  // Verify expense appears in list
  await expect(page.locator('[data-testid="expense-item"]')).toContainText('Lunch');
  await expect(page.locator('[data-testid="total"]')).toContainText('$50.00');
});

// Test concurrent operations
test('handles concurrent updates correctly', async () => {
  const promises = Array.from({ length: 100 }, () =>
    incrementExpenseCount()
  );

  await Promise.all(promises);
  expect(getExpenseCount()).toBe(100);
});

// Test null safety
test.each([null, undefined, '', 0, false])
  ('handles invalid input: %p', (input) => {
    expect(() => processExpense(input)).toThrow('Invalid expense');
  });

// Test boundaries explicitly
describe('pagination', () => {
  test('handles empty list', () => {
    expect(paginate([], 1, 10)).toEqual([]);
  });

  test('handles single item', () => {
    expect(paginate([item], 1, 10)).toEqual([item]);
  });

  test('handles last page with partial items', () => {
    const items = Array.from({ length: 25 }, (_, i) => i);
    expect(paginate(items, 3, 10)).toHaveLength(5);
  });
});

describe('security', () => {
  test('prevents SQL injection', async () => {
    const malicious = "'; DROP TABLE expenses; --";
    await expect(
      searchExpenses(malicious)
    ).resolves.not.toThrow();
  });

  test('sanitizes XSS in descriptions', () => {
    const xss = '<script>alert("xss")</script>';
    const expense = createExpense({ description: xss });
    expect(expense.description).not.toContain('<script>');
  });

  test('requires authentication for expense operations', async () => {
    await request(app)
      .post('/api/expenses')
      .send({ amount: 50 })
      .expect(401);
  });
});

test('processes large expense list efficiently', () => {
  const largeList = Array.from({ length: 10000 }, (_, i) => ({
    amount: i,
    category: 'test'
  }));

  const start = performance.now();
  const total = calculateTotal(largeList);
  const duration = performance.now() - start;

  expect(duration).toBeLessThan(100); // Should complete in <100ms
  expect(total).toBe(49995000);
});

test.each([
  // [input, expected, description]
  [[10, 20, 30], 60, 'normal positive values'],
  [[0, 0, 0], 0, 'all zeros'],
  [[-10, 20, -5], 5, 'mixed positive and negative'],
  [[0.1, 0.2], 0.3, 'decimal precision'],
  [[Number.MAX_SAFE_INTEGER], Number.MAX_SAFE_INTEGER, 'large numbers'],
])('calculateTotal(%p) = %p (%s)', (amounts, expected, description) => {
  const expenses = amounts.map(amount => ({ amount, category: 'test' }));
  expect(calculateTotal(expenses)).toBeCloseTo(expected);
});

python3 scripts/find_untested_code.py src

# Generate coverage (using Jest example)
npm test -- --coverage

# Analyze coverage gaps
python3 scripts/analyze_coverage.py coverage/coverage-final.json

// Extract common setup
function createTestExpense(overrides = {}) {
  return {
    amount: 50,
    category: 'food',
    description: 'Test expense',
    date: new Date('2024-01-01'),
    ...overrides
  };
}

test('filters by category', () => {
  const expenses = [
    createTestExpense({ category: 'food' }),
    createTestExpense({ category: 'transport' }),
  ];
  // ...
});

// Bad: Magic numbers
expect(calculateDiscount(100, 0.15)).toBe(85);

// Good: Named constants
const ORIGINAL_PRICE = 100;
const DISCOUNT_RATE = 0.15;
const EXPECTED_PRICE = 85;
expect(calculateDiscount(ORIGINAL_PRICE, DISCOUNT_RATE)).toBe(EXPECTED_PRICE);

// Bad: Tests depend on execution order
let sharedState;
test('test 1', () => {
  sharedState = { value: 1 };
});
test('test 2', () => {
  expect(sharedState.value).toBe(1); // Depends on test 1
});

// Good: Independent tests
test('test 1', () => {
  const state = { value: 1 };
  expect(state.value).toBe(1);
});
test('test 2', () => {
  const state = { value: 1 };
  expect(state.value).toBe(1);
});

# Run all tests
npm test

# Run with coverage
npm test -- --coverage

# Run specific test file
npm test -- ExpenseCalculator.test.ts

# Run in watch mode
npm test -- --watch

# Run E2E tests
npm run test:e2e

python3 scripts/analyze_coverage.py [coverage-file]

python3 scripts/find_untested_code.py [src-dir] [--pattern test|spec]