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/**
 * Copyright 2014 Netflix, Inc.
 * 
 * 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 rx.internal.operators;

import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
import java.util.concurrent.atomic.AtomicLongFieldUpdater;

import rx.Observable.Operator;
import rx.Producer;
import rx.Scheduler;
import rx.Subscriber;
import rx.Subscription;
import rx.exceptions.MissingBackpressureException;
import rx.functions.Action0;
import rx.internal.util.RxRingBuffer;
import rx.internal.util.SynchronizedSubscription;
import rx.schedulers.ImmediateScheduler;
import rx.schedulers.TrampolineScheduler;

/**
 * Delivers events on the specified {@code Scheduler} asynchronously via an unbounded buffer.
 * 
 * 
 * 
 * @param 
 *            the transmitted value type
 */
public final class OperatorObserveOn implements Operator {

    private final Scheduler scheduler;

    /**
     * @param scheduler
     */
    public OperatorObserveOn(Scheduler scheduler) {
        this.scheduler = scheduler;
    }

    @Override
    public Subscriber call(Subscriber child) {
        if (scheduler instanceof ImmediateScheduler) {
            // avoid overhead, execute directly
            return child;
        } else if (scheduler instanceof TrampolineScheduler) {
            // avoid overhead, execute directly
            return child;
        } else {
            return new ObserveOnSubscriber(scheduler, child);
        }
    }

    /** Observe through individual queue per observer. */
    private static final class ObserveOnSubscriber extends Subscriber {
        final Subscriber child;
        private final Scheduler.Worker recursiveScheduler;
        private final ScheduledUnsubscribe scheduledUnsubscribe;
        final NotificationLite on = NotificationLite.instance();

        private final RxRingBuffer queue = RxRingBuffer.getSpscInstance();
        private boolean completed = false;

        private volatile long requested = 0;
        @SuppressWarnings("rawtypes")
        static final AtomicLongFieldUpdater REQUESTED = AtomicLongFieldUpdater.newUpdater(ObserveOnSubscriber.class, "requested");

        volatile long counter;
        @SuppressWarnings("rawtypes")
        static final AtomicLongFieldUpdater COUNTER_UPDATER = AtomicLongFieldUpdater.newUpdater(ObserveOnSubscriber.class, "counter");

        // do NOT pass the Subscriber through to couple the subscription chain ... unsubscribing on the parent should
        // not prevent anything downstream from consuming, which will happen if the Subscription is chained
        public ObserveOnSubscriber(Scheduler scheduler, Subscriber child) {
            this.child = child;
            this.recursiveScheduler = scheduler.createWorker();
            this.scheduledUnsubscribe = new ScheduledUnsubscribe(recursiveScheduler, queue);
            child.add(scheduledUnsubscribe);
            child.setProducer(new Producer() {

                @Override
                public void request(long n) {
                    REQUESTED.getAndAdd(ObserveOnSubscriber.this, n);
                    schedule();
                }

            });
            add(scheduledUnsubscribe);
            child.add(recursiveScheduler);
            child.add(this);
            
        }
        
        @Override
        public void onStart() {
            // signal that this is an async operator capable of receiving this many
            request(RxRingBuffer.SIZE);
        }

        @Override
        public void onNext(final T t) {
            if (isUnsubscribed() || completed) {
                return;
            }
            try {
                queue.onNext(t);
            } catch (MissingBackpressureException e) {
                onError(e);
                return;
            }
            schedule();
        }

        @Override
        public void onCompleted() {
            if (isUnsubscribed() || completed) {
                return;
            }
            completed = true;
            queue.onCompleted();
            schedule();
        }

        @Override
        public void onError(final Throwable e) {
            if (isUnsubscribed() || completed) {
                return;
            }
            completed = true;
            queue.onError(e);
            schedule();
        }

        protected void schedule() {
            if (COUNTER_UPDATER.getAndIncrement(this) == 0) {
                recursiveScheduler.schedule(new Action0() {

                    @Override
                    public void call() {
                        pollQueue();
                    }

                });
            }
        }

        private void pollQueue() {
            int emitted = 0;
            while (true) {
                while (!scheduledUnsubscribe.isUnsubscribed()) {
                    if (REQUESTED.getAndDecrement(this) != 0) {
                        Object o = queue.poll();
                        if (o == null) {
                            // nothing in queue
                            REQUESTED.incrementAndGet(this);
                            break;
                        } else {
                            if (!on.accept(child, o)) {
                                // non-terminal event so let's increment count
                                emitted++;
                            }
                        }
                    } else {
                        // we hit the end ... so increment back to 0 again
                        REQUESTED.incrementAndGet(this);
                        break;
                    }
                }
                long c = COUNTER_UPDATER.decrementAndGet(this);
                if (c <= 0) {
                    // request the number of items that we emitted in this poll loop
                    if (emitted > 0) {
                        request(emitted);
                    }
                    break;
                } else {
                    /*
                     * Set down to 1 and then iterate again.
                     * we lower it to 1 otherwise it could have grown very large while in the last poll loop
                     * and then we can end up looping all those times again here before existing even once we've drained
                     */
                    COUNTER_UPDATER.set(this, 1);
                    // we now loop again, and if anything tries scheduling again after this it will increment and cause us to loop again after
                }
            }
        }
    }

    static final class ScheduledUnsubscribe implements Subscription {
        final Scheduler.Worker worker;
        volatile int once;
        static final AtomicIntegerFieldUpdater ONCE_UPDATER = AtomicIntegerFieldUpdater.newUpdater(ScheduledUnsubscribe.class, "once");
        final RxRingBuffer queue;
        volatile boolean unsubscribed = false;

        public ScheduledUnsubscribe(Scheduler.Worker worker, RxRingBuffer queue) {
            this.worker = worker;
            this.queue = queue;
        }

        @Override
        public boolean isUnsubscribed() {
            return unsubscribed;
        }

        @Override
        public void unsubscribe() {
            if (ONCE_UPDATER.getAndSet(this, 1) == 0) {
                worker.schedule(new Action0() {
                    @Override
                    public void call() {
                        worker.unsubscribe();
                        unsubscribed = true;
                    }
                });
            }
        }

    }
}