/  E2E  \          ~10% — Slow, expensive, high confidence
       /----------\   
      / Integration \       ~20% — Medium speed, tests boundaries
     /----------------\
    /    Unit Tests    \    ~70% — Fast, cheap, isolated
   /____________________\

// Pure function — ideal for unit testing
function calculateDiscount(price: number, tier: "basic" | "premium"): number {
  return tier === "premium" ? price * 0.2 : price * 0.1;
}
 
describe("calculateDiscount", () => {
  it("applies 20% discount for premium tier", () => {
    expect(calculateDiscount(100, "premium")).toBe(20);
  });
 
  it("applies 10% discount for basic tier", () => {
    expect(calculateDiscount(100, "basic")).toBe(10);
  });
});

describe("POST /api/users", () => {
  it("creates a user and returns 201", async () => {
    const res = await request(app)
      .post("/api/users")
      .send({ name: "Alice", email: "alice@test.com" });
 
    expect(res.status).toBe(201);
    expect(res.body.user.email).toBe("alice@test.com");
 
    // Verify it actually hit the database
    const user = await db.user.findUnique({ where: { email: "alice@test.com" } });
    expect(user).not.toBeNull();
  });
});

test("user can sign up and create a post", async ({ page }) => {
  await page.goto("/signup");
  await page.fill("[name=email]", "test@example.com");
  await page.fill("[name=password]", "secure123");
  await page.click("button[type=submit]");
 
  await expect(page).toHaveURL("/dashboard");
 
  await page.click("text=New Post");
  await page.fill("[name=title]", "My First Post");
  await page.click("text=Publish");
 
  await expect(page.locator(".success-toast")).toBeVisible();
});