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rxjava3-observable developed by David Karnok
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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.observable.internal.operators;

import java.util.ArrayDeque;

import io.reactivex.common.Disposable;
import io.reactivex.common.internal.disposables.DisposableHelper;
import io.reactivex.observable.*;

public final class ObservableTakeLast extends AbstractObservableWithUpstream {
    final int count;

    public ObservableTakeLast(ObservableSource source, int count) {
        super(source);
        this.count = count;
    }

    @Override
    public void subscribeActual(Observer t) {
        source.subscribe(new TakeLastObserver(t, count));
    }

    static final class TakeLastObserver extends ArrayDeque implements Observer, Disposable {

        private static final long serialVersionUID = 7240042530241604978L;
        final Observer actual;
        final int count;

        Disposable s;

        volatile boolean cancelled;

        TakeLastObserver(Observer actual, int count) {
            this.actual = actual;
            this.count = count;
        }

        @Override
        public void onSubscribe(Disposable s) {
            if (DisposableHelper.validate(this.s, s)) {
                this.s = s;
                actual.onSubscribe(this);
            }
        }

        @Override
        public void onNext(T t) {
            if (count == size()) {
                poll();
            }
            offer(t);
        }

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

        @Override
        public void onComplete() {
            Observer a = actual;
            for (;;) {
                if (cancelled) {
                    return;
                }
                T v = poll();
                if (v == null) {
                    if (!cancelled) {
                        a.onComplete();
                    }
                    return;
                }
                a.onNext(v);
            }
        }

        @Override
        public void dispose() {
            if (!cancelled) {
                cancelled = true;
                s.dispose();
            }
        }

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