/**
 * Copyright 2016 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 io.reactivex.internal.operators.flowable;

import java.util.*;

import org.reactivestreams.Publisher;

import io.reactivex.internal.util.*;
import io.reactivex.subscribers.DefaultSubscriber;

/**
 * Returns an Iterable that always returns the item most recently emitted by an Observable, or a
 * seed value if no item has yet been emitted.
 * <p>
 * <img width="640" src="https://github.com/ReactiveX/RxJava/wiki/images/rx-operators/B.mostRecent.png" alt="">
 *
 * @param <T> the value type
 */
public final class BlockingFlowableMostRecent<T> implements Iterable<T> {

    final Publisher<? extends T> source;

    final T initialValue;

    public BlockingFlowableMostRecent(Publisher<? extends T> source, T initialValue) {
        this.source = source;
        this.initialValue = initialValue;
    }

    @Override
    public Iterator<T> iterator() {
        MostRecentSubscriber<T> mostRecentSubscriber = new MostRecentSubscriber<T>(initialValue);

        /**
         * Subscribe instead of unsafeSubscribe since this is the final subscribe in the chain
         * since it is for BlockingObservable.
         */
        source.subscribe(mostRecentSubscriber);

        return mostRecentSubscriber.getIterable();
    }

    static final class MostRecentSubscriber<T> extends DefaultSubscriber<T> {
        volatile Object value;

        MostRecentSubscriber(T value) {
            this.value = NotificationLite.next(value);
        }

        @Override
        public void onComplete() {
            value = NotificationLite.complete();
        }

        @Override
        public void onError(Throwable e) {
            value = NotificationLite.error(e);
        }

        @Override
        public void onNext(T args) {
            value = NotificationLite.next(args);
        }

        /**
         * The {@link Iterator} return is not thread safe. In other words don't call {@link Iterator#hasNext()} in one
         * thread expect {@link Iterator#next()} called from a different thread to work.
         * @return the Iterator
         */
        public Iterator<T> getIterable() {
            return new Iterator<T>() {
                /**
                 * buffer to make sure that the state of the iterator doesn't change between calling hasNext() and next().
                 */
                private Object buf;

                @Override
                public boolean hasNext() {
                    buf = value;
                    return !NotificationLite.isComplete(buf);
                }

                @Override
                public T next() {
                    try {
                        // if hasNext wasn't called before calling next.
                        if (buf == null) {
                            buf = value;
                        }
                        if (NotificationLite.isComplete(buf)) {
                            throw new NoSuchElementException();
                        }
                        if (NotificationLite.isError(buf)) {
                            throw ExceptionHelper.wrapOrThrow(NotificationLite.getError(buf));
                        }
                        return NotificationLite.getValue(buf);
                    }
                    finally {
                        buf = null;
                    }
                }

                @Override
                public void remove() {
                    throw new UnsupportedOperationException("Read only iterator");
                }
            };
        }
    }
}
