Inline & Value Classes in Kotlin

// Inline functions
inline fun measureTimeMillis(block: () -> Unit): Long {
    val start = System.currentTimeMillis()
    block()
    return System.currentTimeMillis() - start
}

// No function call overhead
val time = measureTimeMillis {
    // Code is inlined here
    Thread.sleep(100)
}

// Inline classes (value classes)
@JvmInline
value class UserId(val id: Long) {
    init {
        require(id > 0) { "Id must be positive" }
    }

    val displayId: String
        get() = "USER_$id"
}

// No boxing overhead at runtime
fun getUser(userId: UserId) {
    // userId is passed as primitive long
}

// Crossinline and noinline
inline fun inlineFunc(
    crossinline body: () -> Unit,  // Can't use return
    noinline callback: () -> Unit   // Not inlined
) {
    val runnable = object : Runnable {
        override fun run() {
            body()  // OK with crossinline
        }
    }
    callback()  // Regular function call
}