🔄 TDD (Test-Driven Development)

📖 What is TDD?

TDD is a development methodology where you write tests first and then write code that passes those tests. You can reduce bugs and improve design!

💡 TDD Cycle

Red-Green-Refactor

1. 🔴 Red: Write a failing test
2. 🟢 Green: Write minimal code to pass the test
3. 🔵 Refactor: Improve the code

🎯 First TDD Example

Creating a Calculator

import io.kotest.core.spec.style.StringSpec
import io.kotest.matchers.shouldBe

// 1️⃣ Red: Write test first
class CalculatorTest : StringSpec({
    "2와 3을 더하면 5가 된다" {
        val calculator = Calculator()
        calculator.add(2, 3) shouldBe 5
    }
})

// 컴파일 에러! Calculator가 없음

// 2️⃣ Green: Minimal code
class Calculator {
    fun add(a: Int, b: Int): Int {
        return a + b
    }
}

// 3️⃣ Refactor: 개선할 부분 없음 (간단하므로)

String Processing

// 1️⃣ Red: Write test
class StringProcessorTest : StringSpec({
    "빈 문자열은 빈 문자열을 반환한다" {
        val processor = StringProcessor()
        processor.reverse("") shouldBe ""
    }

    "hello를 뒤집으면 olleh가 된다" {
        val processor = StringProcessor()
        processor.reverse("hello") shouldBe "olleh"
    }
})

// 2️⃣ Green: Implementation
class StringProcessor {
    fun reverse(text: String): String {
        return text.reversed()
    }
}

// 3️⃣ Refactor: 더 나은 방법이 있을까?
// 이미 간결함!

🎨 Practical TDD

Shopping Cart Development

Step 1: Empty Cart

// Red
class ShoppingCartTest : StringSpec({
    "새 카트는 아이템이 0개다" {
        val cart = ShoppingCart()
        cart.getItemCount() shouldBe 0
    }
})

// Green
class ShoppingCart {
    private val items = mutableListOf<Item>()

    fun getItemCount(): Int = items.size
}

data class Item(val name: String, val price: Double)

Step 2: Adding Items

// Red
"아이템을 추가하면 개수가 증가한다" {
    val cart = ShoppingCart()
    cart.addItem(Item("사과", 1000.0))

    cart.getItemCount() shouldBe 1
}

// Green
class ShoppingCart {
    private val items = mutableListOf<Item>()

    fun addItem(item: Item) {
        items.add(item)
    }

    fun getItemCount(): Int = items.size
}

Step 3: Calculating Total

// Red
"총액을 계산한다" {
    val cart = ShoppingCart()
    cart.addItem(Item("사과", 1000.0))
    cart.addItem(Item("바나나", 1500.0))

    cart.getTotal() shouldBe 2500.0
}

// Green
class ShoppingCart {
    private val items = mutableListOf<Item>()

    fun addItem(item: Item) {
        items.add(item)
    }

    fun getItemCount(): Int = items.size

    fun getTotal(): Double {
        return items.sumOf { it.price }
    }
}

// Refactor: 깔끔함!

🔥 Business Logic TDD

Discount Calculator

// Red: Test first
class DiscountCalculatorTest : StringSpec({
    "10% 할인" {
        val calculator = DiscountCalculator()
        calculator.calculate(10000.0, 0.1) shouldBe 9000.0
    }

    "할인율이 0이면 원가" {
        val calculator = DiscountCalculator()
        calculator.calculate(10000.0, 0.0) shouldBe 10000.0
    }

    "할인율이 1이면 0원" {
        val calculator = DiscountCalculator()
        calculator.calculate(10000.0, 1.0) shouldBe 0.0
    }
})

// Green: Implementation
class DiscountCalculator {
    fun calculate(price: Double, rate: Double): Double {
        require(rate in 0.0..1.0) { "할인율은 0~1 사이여야 합니다" }
        return price * (1 - rate)
    }
}

// Refactor: 검증 로직 추가했으므로 테스트 추가
"잘못된 할인율은 예외 발생" {
    val calculator = DiscountCalculator()

    shouldThrow<IllegalArgumentException> {
        calculator.calculate(10000.0, 1.5)
    }
}

User Registration

// Red
class UserRegistrationTest : StringSpec({
    "유효한 사용자 등록" {
        val service = UserRegistration()
        val user = service.register("hong@example.com", "Pass123!")

        user.email shouldBe "hong@example.com"
    }

    "중복 이메일은 실패" {
        val service = UserRegistration()
        service.register("hong@example.com", "Pass123!")

        shouldThrow<DuplicateEmailException> {
            service.register("hong@example.com", "Pass456!")
        }
    }

    "약한 비밀번호는 실패" {
        val service = UserRegistration()

        shouldThrow<WeakPasswordException> {
            service.register("hong@example.com", "123")
        }
    }
})

// Green
class UserRegistration {
    private val users = mutableMapOf<String, User>()

    fun register(email: String, password: String): User {
        if (users.containsKey(email)) {
            throw DuplicateEmailException()
        }

        if (password.length < 8) {
            throw WeakPasswordException()
        }

        val user = User(email, password)
        users[email] = user
        return user
    }
}

data class User(val email: String, val password: String)

class DuplicateEmailException : Exception()
class WeakPasswordException : Exception()

// Refactor: 검증 로직 분리
class UserRegistration {
    private val users = mutableMapOf<String, User>()
    private val validator = UserValidator()

    fun register(email: String, password: String): User {
        validator.validateEmail(email, users.keys)
        validator.validatePassword(password)

        val user = User(email, password)
        users[email] = user
        return user
    }
}

class UserValidator {
    fun validateEmail(email: String, existingEmails: Set<String>) {
        if (existingEmails.contains(email)) {
            throw DuplicateEmailException()
        }
    }

    fun validatePassword(password: String) {
        if (password.length < 8) {
            throw WeakPasswordException()
        }
    }
}

🎯 TDD Best Practices

Small Steps

// ❌ Too much at once
class ComplexTest : StringSpec({
    "사용자 등록, 로그인, 프로필 수정까지" {
        // 너무 복잡!
    }
})

// ✅ One at a time
class SimpleTest : StringSpec({
    "사용자 등록" {
        // 하나만 테스트
    }

    "로그인" {
        // 별도로 테스트
    }

    "프로필 수정" {
        // 또 별도로
    }
})

Clear Test Names

class GoodNaming : StringSpec({
    // ❌ Bad name
    "test1" { }

    // ✅ Good name
    "빈 장바구니의 총액은 0원이다" { }
    "할인율 10%를 적용하면 10% 할인된다" { }
    "중복 이메일로 가입하면 예외가 발생한다" { }
})

Given-When-Then Pattern

class GWTTest : StringSpec({
    "주문 생성 후 결제하면 상태가 PAID가 된다" {
        // Given
        val order = Order(items = listOf(Item("상품", 10000.0)))

        // When
        order.pay()

        // Then
        order.status shouldBe OrderStatus.PAID
    }
})

enum class OrderStatus { PENDING, PAID, SHIPPED }

data class Order(
    val items: List<Item>,
    var status: OrderStatus = OrderStatus.PENDING
) {
    fun pay() {
        status = OrderStatus.PAID
    }
}

🛡️ Benefits of TDD

1. Early Bug Detection

// Writing tests first clarifies requirements
"음수 금액은 허용하지 않는다" {
    shouldThrow<IllegalArgumentException> {
        Order(amount = -1000.0)
    }
}

// Validation logic is naturally added during implementation
data class Order(val amount: Double) {
    init {
        require(amount >= 0) { "금액은 0 이상이어야 합니다" }
    }
}

2. Safety Net for Refactoring

// With tests, you can refactor with confidence
class BeforeRefactor {
    fun calculate(price: Double, quantity: Int): Double {
        var total = price * quantity
        if (quantity > 10) {
            total = total * 0.9  // 10개 이상 10% 할인
        }
        return total
    }
}

// Refactoring
class AfterRefactor {
    fun calculate(price: Double, quantity: Int): Double {
        val subtotal = price * quantity
        val discount = getDiscount(quantity)
        return subtotal * (1 - discount)
    }

    private fun getDiscount(quantity: Int): Double {
        return if (quantity > 10) 0.1 else 0.0
    }
}

// 테스트가 통과하면 리팩토링 성공!

🤔 Frequently Asked Questions

Q1. Do I always have to write tests first?

A: Apply it first to complex logic or important features!

// ✅ TDD required
// - Business logic
// - Money calculations
// - Validation logic

// ❌ TDD optional
// - Simple getters/setters
// - UI layout
// - Configuration files

Q2. How many tests should I write?

A: Include boundary values and exception cases!

class ComprehensiveTest : StringSpec({
    // Normal case
    "정상적인 입력" { }

    // Boundary values
    "최소값" { }
    "최대값" { }

    // Exceptions
    "null 입력" { }
    "빈 문자열" { }
    "음수" { }
})

Q3. Can I apply TDD to legacy code?

A: Start with the parts you're modifying!

// 1. 기존 기능에 테스트 추가
// 2. 테스트가 통과하는지 확인
// 3. 리팩토링
// 4. 새 기능은 TDD로

🎬 Conclusion

Write robust code with TDD!

Key Takeaways:
✅ Red-Green-Refactor cycle
✅ Write tests first
✅ Proceed in small steps
✅ Safety net for refactoring
✅ Design improvement effect

Congratulations! You've completed the Testing series! 🎉

Next Step: Start backend development with Ktor Introduction!