kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/Deferred.kt

/*
 * Copyright 2016-2017 JetBrains s.r.o.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package kotlinx.coroutines.experimental

import kotlin.coroutines.experimental.CoroutineContext
import kotlin.coroutines.experimental.startCoroutine

/**
 * Deferred value is a non-blocking cancellable future.
 * It is created with [async] coroutine builder.
 * It is in [active][isActive] state while the value is being computed.
 *
 * Deferred value has four or five possible states.
 *
 * | **State**                        | [isActive] | [isCompleted] | [isCompletedExceptionally] | [isCancelled] |
 * | -------------------------------- | ---------- | ------------- | -------------------------- | ------------- |
 * | _New_ (optional initial state)   | `false`    | `false`       | `false`                    | `false`       |
 * | _Active_ (default initial state) | `true`     | `false`       | `false`                    | `false`       |
 * | _Resolved_  (final state)        | `false`    | `true`        | `false`                    | `false`       |
 * | _Failed_    (final state)        | `false`    | `true`        | `true`                     | `false`       |
 * | _Cancelled_ (final state)        | `false`    | `true`        | `true`                     | `true`        |
 *
 * Usually, a deferred value is created in _active_ state (it is created and started), so its only visible
 * states are _active_ and _completed_ (_resolved_, _failed_, or _cancelled_) state.
 * However, [async] coroutine builder has an optional `start` parameter that creates a deferred value in _new_ state
 * when this parameter is set to `false`.
 * Such a deferred can be be made _active_ by invoking [start], [join], or [await].
 */
public interface Deferred<out T> : Job {
    /**
     * Returns `true` if computation of this deferred value has _completed exceptionally_ -- it had
     * either _failed_ with exception during computation or was [cancelled][cancel].
     *
     * It implies that [isActive] is `false` and [isCompleted] is `true`.
     */
    val isCompletedExceptionally: Boolean

    /**
     * Returns `true` if computation of this deferred value was [cancelled][cancel].
     *
     * It implies that [isActive] is `false`, [isCompleted] is `true`, and [isCompletedExceptionally] is `true`.
     */
    val isCancelled: Boolean

    /**
     * Awaits for completion of this value without blocking a thread and resumes when deferred computation is complete,
     * returning the resulting value or throwing the corresponding exception if the deferred had completed exceptionally.
     *
     * This suspending function is cancellable.
     * If the [Job] of the current coroutine is completed while this suspending function is waiting, this function
     * immediately resumes with [CancellationException].
     */
    public suspend fun await(): T

    /**
     * Returns *completed* result or throws [IllegalStateException] if this deferred value has not
     * [completed][isCompleted] yet. It throws the corresponding exception if this deferred has
     * [completed exceptionally][isCompletedExceptionally].
     *
     * This function is designed to be used from [invokeOnCompletion] handlers, when there is an absolute certainty that
     * the value is already complete.
     */
    public fun getCompleted(): T

    /**
     * @suppress **Deprecated**: Use `isActive`.
     */
    @Deprecated(message = "Use `isActive`", replaceWith = ReplaceWith("isActive"))
    public val isComputing: Boolean get() = isActive
}

/**
 * Creates new coroutine and returns its future result as an implementation of [Deferred].
 *
 * The running coroutine is cancelled when the resulting object is [cancelled][Job.cancel].
 * The [context] for the new coroutine must be explicitly specified.
 * See [CoroutineDispatcher] for the standard [context] implementations that are provided by `kotlinx.coroutines`.
 * The [context][CoroutineScope.context] of the parent coroutine from its [scope][CoroutineScope] may be used,
 * in which case the [Job] of the resulting coroutine is a child of the job of the parent coroutine.
 *
 * An optional [start] parameter can be set to `false` to start coroutine _lazily_. When `start = false`,
 * the resulting [Deferred] is created in _new_ state. It can be explicitly started with [start][Job.start]
 * function and will be started implicitly on the first invocation of [join][Job.join] or [await][Deferred.await].
 *
 * By default, the coroutine is immediately started. Set an optional [start] parameters to `false`
 * to create coroutine without starting it. In this case it will be _lazy_ and will start
 */
public fun <T> async(context: CoroutineContext, start: Boolean = true, block: suspend CoroutineScope.() -> T) : Deferred<T> {
    val newContext = newCoroutineContext(context)
    val coroutine = if (start)
        DeferredCoroutine<T>(newContext, active = true) else
        LazyDeferredCoroutine(newContext, block)
    coroutine.initParentJob(context[Job])
    if (start) block.startCoroutine(coroutine, coroutine)
    return coroutine
}

/**
 * @suppress **Deprecated**: `defer` was renamed to `async`.
 */
@Deprecated(message = "`defer` was renamed to `async`", level = DeprecationLevel.WARNING,
        replaceWith = ReplaceWith("async(context, block = block)"))
public fun <T> defer(context: CoroutineContext, block: suspend CoroutineScope.() -> T) : Deferred<T> =
    async(context, block = block)

private open class DeferredCoroutine<T>(
    context: CoroutineContext,
    active: Boolean
) : AbstractCoroutine<T>(context, active), Deferred<T> {
    override val isCompletedExceptionally: Boolean get() = getState() is CompletedExceptionally
    override val isCancelled: Boolean get() = getState() is Cancelled

    @Suppress("UNCHECKED_CAST")
    suspend override fun await(): T {
        // fast-path -- check state (avoid extra object creation)
        while(true) { // lock-free loop on state
            val state = this.getState()
            if (state !is Incomplete) {
                // already complete -- just return result
                if (state is CompletedExceptionally) throw state.exception
                return state as T

            }
            if (startInternal(state) >= 0) break // break unless needs to retry
        }
        return awaitSuspend() // slow-path
    }

    @Suppress("UNCHECKED_CAST")
    private suspend fun awaitSuspend(): T = suspendCancellableCoroutine { cont ->
        cont.unregisterOnCompletion(invokeOnCompletion {
            val state = getState()
            check(state !is Incomplete)
            if (state is CompletedExceptionally)
                cont.resumeWithException(state.exception)
            else
                cont.resume(state as T)
        })
    }

    @Suppress("UNCHECKED_CAST")
    override fun getCompleted(): T {
        val state = getState()
        check(state !is Incomplete) { "This deferred value has not completed yet" }
        if (state is CompletedExceptionally) throw state.exception
        return state as T
    }
}

private class LazyDeferredCoroutine<T>(
        context: CoroutineContext,
        val block: suspend CoroutineScope.() -> T
) : DeferredCoroutine<T>(context, active = false) {
    override fun onStart() {
        block.startCoroutine(this, this)
    }
}