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.operators.flowable;

import java.util.concurrent.atomic.*;

import org.reactivestreams.*;

import io.reactivex.*;
import io.reactivex.internal.subscriptions.SubscriptionHelper;
import io.reactivex.internal.util.BackpressureHelper;

public final class FlowableOnBackpressureLatest<T> extends AbstractFlowableWithUpstream<T, T> {

    public FlowableOnBackpressureLatest(Flowable<T> source) {
        super(source);
    }

    @Override
    protected void subscribeActual(Subscriber<? super T> s) {
        source.subscribe(new BackpressureLatestSubscriber<T>(s));
    }

    static final class BackpressureLatestSubscriber<T> extends AtomicInteger implements FlowableSubscriber<T>, Subscription {

        private static final long serialVersionUID = 163080509307634843L;

        final Subscriber<? super T> downstream;

        Subscription upstream;

        volatile boolean done;
        Throwable error;

        volatile boolean cancelled;

        final AtomicLong requested = new AtomicLong();

        final AtomicReference<T> current = new AtomicReference<T>();

        BackpressureLatestSubscriber(Subscriber<? super T> downstream) {
            this.downstream = downstream;
        }

        @Override
        public void onSubscribe(Subscription s) {
            if (SubscriptionHelper.validate(this.upstream, s)) {
                this.upstream = s;
                downstream.onSubscribe(this);
                s.request(Long.MAX_VALUE);
            }
        }

        @Override
        public void onNext(T t) {
            current.lazySet(t);
            drain();
        }

        @Override
        public void onError(Throwable t) {
            error = t;
            done = true;
            drain();
        }

        @Override
        public void onComplete() {
            done = true;
            drain();
        }

        @Override
        public void request(long n) {
            if (SubscriptionHelper.validate(n)) {
                BackpressureHelper.add(requested, n);
                drain();
            }
        }

        @Override
        public void cancel() {
            if (!cancelled) {
                cancelled = true;
                upstream.cancel();

                if (getAndIncrement() == 0) {
                    current.lazySet(null);
                }
            }
        }

        void drain() {
            if (getAndIncrement() != 0) {
                return;
            }
            final Subscriber<? super T> a = downstream;
            int missed = 1;
            final AtomicLong r = requested;
            final AtomicReference<T> q = current;

            for (;;) {
                long e = 0L;

                while (e != r.get()) {
                    boolean d = done;
                    T v = q.getAndSet(null);
                    boolean empty = v == null;

                    if (checkTerminated(d, empty, a, q)) {
                        return;
                    }

                    if (empty) {
                        break;
                    }

                    a.onNext(v);

                    e++;
                }

                if (e == r.get() && checkTerminated(done, q.get() == null, a, q)) {
                    return;
                }

                if (e != 0L) {
                    BackpressureHelper.produced(r, e);
                }

                missed = addAndGet(-missed);
                if (missed == 0) {
                    break;
                }
            }
        }

        boolean checkTerminated(boolean d, boolean empty, Subscriber<?> a, AtomicReference<T> q) {
            if (cancelled) {
                q.lazySet(null);
                return true;
            }

            if (d) {
                Throwable e = error;
                if (e != null) {
                    q.lazySet(null);
                    a.onError(e);
                    return true;
                } else
                if (empty) {
                    a.onComplete();
                    return true;
                }
            }

            return false;
        }
    }
}