BusyWaitSingleton.kt

@file:Suppress("UNCHECKED_CAST")

package circlet.platform.server.application

import java.util.concurrent.atomic.*

class BusyWaitSingleton<T>(private val factory: () -> T) {

    private val value = AtomicReference<State>(State.Empty)

    fun singleton(): T {
        while (true) {
            when (val state = value.get()) {
                State.Empty -> {
                    if (value.compareAndSet(State.Empty, State.Initializing)) {
                        val result = factory()
                        if (!value.compareAndSet(State.Initializing, State.Initialized(result))) {
                            error("Invalid state")
                        }
                        return result
                    }
                }

                is State.Initialized<*> -> return state.value as T

                State.Initializing -> Unit // continue loop: busy wait
            }
        }
    }

    private sealed class State {
        class Initialized<T>(val value: T) : State()

        object Initializing : State()

        object Empty : State()
    }
}