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

import org.reactivestreams.*;

import io.reactivex.*;
import io.reactivex.exceptions.*;
import io.reactivex.functions.Action;
import io.reactivex.internal.subscriptions.SubscriptionHelper;
import io.reactivex.internal.util.BackpressureHelper;
import io.reactivex.plugins.RxJavaPlugins;

Handle backpressure with a bounded buffer and custom strategy.
Type parameters:
<T> – the input and output value type/**
 * Handle backpressure with a bounded buffer and custom strategy.
 *
 * @param <T> the input and output value type
 */
public final class FlowableOnBackpressureBufferStrategy<T> extends AbstractFlowableWithUpstream<T, T> {

    final long bufferSize;

    final Action onOverflow;

    final BackpressureOverflowStrategy strategy;

    public FlowableOnBackpressureBufferStrategy(Flowable<T> source,
            long bufferSize, Action onOverflow, BackpressureOverflowStrategy strategy) {
        super(source);
        this.bufferSize = bufferSize;
        this.onOverflow = onOverflow;
        this.strategy = strategy;
    }

    @Override
    protected void subscribeActual(Subscriber<? super T> s) {
        source.subscribe(new OnBackpressureBufferStrategySubscriber<T>(s, onOverflow, strategy, bufferSize));
    }

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

        private static final long serialVersionUID = 3240706908776709697L;

        final Subscriber<? super T> downstream;

        final Action onOverflow;

        final BackpressureOverflowStrategy strategy;

        final long bufferSize;

        final AtomicLong requested;

        final Deque<T> deque;

        Subscription upstream;

        volatile boolean cancelled;

        volatile boolean done;
        Throwable error;

        OnBackpressureBufferStrategySubscriber(Subscriber<? super T> actual, Action onOverflow,
                BackpressureOverflowStrategy strategy, long bufferSize) {
            this.downstream = actual;
            this.onOverflow = onOverflow;
            this.strategy = strategy;
            this.bufferSize = bufferSize;
            this.requested = new AtomicLong();
            this.deque = new ArrayDeque<T>();
        }

        @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) {
            if (done) {
                return;
            }
            boolean callOnOverflow = false;
            boolean callError = false;
            Deque<T> dq = deque;
            synchronized (dq) {
               if (dq.size() == bufferSize) {
                   switch (strategy) {
                   case DROP_LATEST:
                       dq.pollLast();
                       dq.offer(t);
                       callOnOverflow = true;
                       break;
                   case DROP_OLDEST:
                       dq.poll();
                       dq.offer(t);
                       callOnOverflow = true;
                       break;
                   default:
                       // signal error
                       callError = true;
                       break;
                   }
               } else {
                   dq.offer(t);
               }
            }

            if (callOnOverflow) {
                if (onOverflow != null) {
                    try {
                        onOverflow.run();
                    } catch (Throwable ex) {
                        Exceptions.throwIfFatal(ex);
                        upstream.cancel();
                        onError(ex);
                    }
                }
            } else if (callError) {
                upstream.cancel();
                onError(new MissingBackpressureException());
            } else {
                drain();
            }
        }

        @Override
        public void onError(Throwable t) {
            if (done) {
                RxJavaPlugins.onError(t);
                return;
            }
            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() {
            cancelled = true;
            upstream.cancel();

            if (getAndIncrement() == 0) {
                clear(deque);
            }
        }

        void clear(Deque<T> dq) {
            synchronized (dq) {
                dq.clear();
            }
        }

        void drain() {
            if (getAndIncrement() != 0) {
                return;
            }

            int missed = 1;
            Deque<T> dq = deque;
            Subscriber<? super T> a = downstream;
            for (;;) {
                long r = requested.get();
                long e = 0L;
                while (e != r) {
                    if (cancelled) {
                        clear(dq);
                        return;
                    }

                    boolean d = done;

                    T v;

                    synchronized (dq) {
                        v = dq.poll();
                    }

                    boolean empty = v == null;

                    if (d) {
                        Throwable ex = error;
                        if (ex != null) {
                            clear(dq);
                            a.onError(ex);
                            return;
                        }
                        if (empty) {
                            a.onComplete();
                            return;
                        }
                    }

                    if (empty) {
                        break;
                    }

                    a.onNext(v);

                    e++;
                }

                if (e == r) {
                    if (cancelled) {
                        clear(dq);
                        return;
                    }

                    boolean d = done;

                    boolean empty;

                    synchronized (dq) {
                        empty = dq.isEmpty();
                    }

                    if (d) {
                        Throwable ex = error;
                        if (ex != null) {
                            clear(dq);
                            a.onError(ex);
                            return;
                        }
                        if (empty) {
                            a.onComplete();
                            return;
                        }
                    }
                }

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

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