src/
├── commonMain/kotlin/          # 공통 코드 (모든 플랫폼에서 실행)
│   └── com/example/
│       ├── domain/             # 도메인 모델
│       └── repository/         # 추상 Repository
├── commonTest/kotlin/          # 공통 테스트
├── jvmMain/kotlin/             # JVM 전용 구현
├── androidMain/kotlin/         # Android 전용 구현
├── iosMain/kotlin/             # iOS 전용 구현 (Kotlin/Native)
└── jsMain/kotlin/              # JavaScript 전용 구현

// commonMain/kotlin/com/example/Platform.kt

// 플랫폼 이름을 반환하는 함수 — 구현은 각 플랫폼에서 제공
expect fun currentPlatform(): String

// 플랫폼별 UUID 생성기
expect class UuidGenerator() {
    fun generate(): String
}

// 플랫폼별 로거
expect object Logger {
    fun log(message: String)
    fun error(message: String, throwable: Throwable? = null)
}

// jvmMain/kotlin/com/example/Platform.jvm.kt

actual fun currentPlatform(): String = "JVM ${System.getProperty("java.version")}"

actual class UuidGenerator actual constructor() {
    actual fun generate(): String = java.util.UUID.randomUUID().toString()
}

actual object Logger {
    actual fun log(message: String) {
        println("[INFO] $message")
    }
    actual fun error(message: String, throwable: Throwable?) {
        System.err.println("[ERROR] $message")
        throwable?.printStackTrace()
    }
}

// iosMain/kotlin/com/example/Platform.ios.kt
import platform.UIKit.UIDevice
import platform.Foundation.NSUUID

actual fun currentPlatform(): String =
    UIDevice.currentDevice.systemName() + " " + UIDevice.currentDevice.systemVersion

actual class UuidGenerator actual constructor() {
    actual fun generate(): String = NSUUID().UUIDString
}

actual object Logger {
    actual fun log(message: String) {
        println("[iOS INFO] $message")
    }
    actual fun error(message: String, throwable: Throwable?) {
        println("[iOS ERROR] $message ${throwable?.message ?: ""}")
    }
}

// commonMain/kotlin/com/example/domain/User.kt
data class User(
    val id: String,
    val name: String,
    val email: String
)

// commonMain/kotlin/com/example/domain/UserRepository.kt
interface UserRepository {
    suspend fun findById(id: String): User?
    suspend fun save(user: User): User
    suspend fun findAll(): List<User>
}

// commonMain/kotlin/com/example/usecase/UserUseCase.kt
class UserUseCase(private val repository: UserRepository) {

    suspend fun getUser(id: String): Result<User> = runCatching {
        repository.findById(id) ?: throw NoSuchElementException("사용자 없음: $id")
    }

    suspend fun createUser(name: String, email: String): Result<User> = runCatching {
        val user = User(
            id = UuidGenerator().generate(),  // expect/actual
            name = name,
            email = email
        )
        repository.save(user)
    }
}

// build.gradle.kts
plugins {
    kotlin("multiplatform") version "2.0.0"
    kotlin("plugin.serialization") version "2.0.0"
}

kotlin {
    // 타깃 플랫폼 지정
    jvm()
    androidTarget()
    iosArm64()
    iosSimulatorArm64()
    js(IR) { browser() }

    sourceSets {
        commonMain.dependencies {
            // 멀티플랫폼 지원 라이브러리
            implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.8.1")
            implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.3")
            implementation("org.jetbrains.kotlinx:kotlinx-datetime:0.6.0")
        }
        jvmMain.dependencies {
            implementation("io.ktor:ktor-client-okhttp:2.3.10")
        }
        iosMain.dependencies {
            implementation("io.ktor:ktor-client-darwin:2.3.10")
        }
        jsMain.dependencies {
            implementation("io.ktor:ktor-client-js:2.3.10")
        }
    }
}

// commonMain에서 사용 가능
import kotlinx.serialization.*
import kotlinx.serialization.json.*

@Serializable
data class ApiResponse<T>(
    val data: T,
    val success: Boolean,
    val message: String = ""
)

@Serializable
data class UserDto(
    val id: String,
    val name: String,
    val email: String
)

// 직렬화/역직렬화 (모든 플랫폼 공통)
val json = Json { prettyPrint = true }

val response = ApiResponse(
    data = UserDto("1", "Alice", "alice@example.com"),
    success = true
)
val jsonStr = json.encodeToString(response)
println(jsonStr)

val decoded = json.decodeFromString<ApiResponse<UserDto>>(jsonStr)
println(decoded.data.name)  // Alice