Copyright (c) 2016-present, RxJava Contributors.
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.
/**
 * Copyright (c) 2016-present, RxJava Contributors.
 *
 * 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 io.reactivex.internal.observers;

import java.util.NoSuchElementException;
import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicReference;

import io.reactivex.Observer;
import io.reactivex.disposables.Disposable;
import io.reactivex.internal.disposables.DisposableHelper;
import io.reactivex.internal.util.BlockingHelper;
import io.reactivex.plugins.RxJavaPlugins;

import static io.reactivex.internal.util.ExceptionHelper.timeoutMessage;

An Observer + Future that expects exactly one upstream value and provides it
via the (blocking) Future API.
Type parameters:
<T> – the value type/**
 * An Observer + Future that expects exactly one upstream value and provides it
 * via the (blocking) Future API.
 *
 * @param <T> the value type
 */
public final class FutureObserver<T> extends CountDownLatch
implements Observer<T>, Future<T>, Disposable {

    T value;
    Throwable error;

    final AtomicReference<Disposable> upstream;

    public FutureObserver() {
        super(1);
        this.upstream = new AtomicReference<Disposable>();
    }

    @Override
    public boolean cancel(boolean mayInterruptIfRunning) {
        for (;;) {
            Disposable a = upstream.get();
            if (a == this || a == DisposableHelper.DISPOSED) {
                return false;
            }

            if (upstream.compareAndSet(a, DisposableHelper.DISPOSED)) {
                if (a != null) {
                    a.dispose();
                }
                countDown();
                return true;
            }
        }
    }

    @Override
    public boolean isCancelled() {
        return DisposableHelper.isDisposed(upstream.get());
    }

    @Override
    public boolean isDone() {
        return getCount() == 0;
    }

    @Override
    public T get() throws InterruptedException, ExecutionException {
        if (getCount() != 0) {
            BlockingHelper.verifyNonBlocking();
            await();
        }

        if (isCancelled()) {
            throw new CancellationException();
        }
        Throwable ex = error;
        if (ex != null) {
            throw new ExecutionException(ex);
        }
        return value;
    }

    @Override
    public T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
        if (getCount() != 0) {
            BlockingHelper.verifyNonBlocking();
            if (!await(timeout, unit)) {
                throw new TimeoutException(timeoutMessage(timeout, unit));
            }
        }

        if (isCancelled()) {
            throw new CancellationException();
        }

        Throwable ex = error;
        if (ex != null) {
            throw new ExecutionException(ex);
        }
        return value;
    }

    @Override
    public void onSubscribe(Disposable d) {
        DisposableHelper.setOnce(this.upstream, d);
    }

    @Override
    public void onNext(T t) {
        if (value != null) {
            upstream.get().dispose();
            onError(new IndexOutOfBoundsException("More than one element received"));
            return;
        }
        value = t;
    }

    @Override
    public void onError(Throwable t) {
        if (error == null) {
            error = t;

            for (;;) {
                Disposable a = upstream.get();
                if (a == this || a == DisposableHelper.DISPOSED) {
                    RxJavaPlugins.onError(t);
                    return;
                }
                if (upstream.compareAndSet(a, this)) {
                    countDown();
                    return;
                }
            }
        } else {
            RxJavaPlugins.onError(t);
        }
    }

    @Override
    public void onComplete() {
        if (value == null) {
            onError(new NoSuchElementException("The source is empty"));
            return;
        }
        for (;;) {
            Disposable a = upstream.get();
            if (a == this || a == DisposableHelper.DISPOSED) {
                return;
            }
            if (upstream.compareAndSet(a, this)) {
                countDown();
                return;
            }
        }
    }

    @Override
    public void dispose() {
        // ignoring as `this` means a finished Disposable only
    }

    @Override
    public boolean isDisposed() {
        return isDone();
    }
}