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Reactive Extensions for Java
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/**
 * 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.ArrayDeque;
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 FlowableTakeLast extends AbstractFlowableWithUpstream {
    final int count;

    public FlowableTakeLast(Flowable source, int count) {
        super(source);
        this.count = count;
    }

    @Override
    protected void subscribeActual(Subscriber s) {
        source.subscribe(new TakeLastSubscriber(s, count));
    }

    static final class TakeLastSubscriber extends ArrayDeque implements FlowableSubscriber, Subscription {

        private static final long serialVersionUID = 7240042530241604978L;
        final Subscriber downstream;
        final int count;

        Subscription upstream;
        volatile boolean done;
        volatile boolean cancelled;

        final AtomicLong requested = new AtomicLong();

        final AtomicInteger wip = new AtomicInteger();

        TakeLastSubscriber(Subscriber actual, int count) {
            this.downstream = actual;
            this.count = count;
        }

        @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 (count == size()) {
                poll();
            }
            offer(t);
        }

        @Override
        public void onError(Throwable t) {
            downstream.onError(t);
        }

        @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();
        }

        void drain() {
            if (wip.getAndIncrement() == 0) {
                Subscriber a = downstream;
                long r = requested.get();
                do {
                    if (cancelled) {
                        return;
                    }
                    if (done) {
                        long e = 0L;

                        while (e != r) {
                            if (cancelled) {
                                return;
                            }
                            T v = poll();
                            if (v == null) {
                                a.onComplete();
                                return;
                            }
                            a.onNext(v);
                            e++;
                        }
                        if (e != 0L && r != Long.MAX_VALUE) {
                            r = requested.addAndGet(-e);
                        }
                    }
                } while (wip.decrementAndGet() != 0);
            }
        }
    }
}