/*
 * 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.rxjava3.internal.operators.observable;

import java.util.*;

import io.reactivex.rxjava3.core.ObservableSource;
import io.reactivex.rxjava3.internal.util.*;
import io.reactivex.rxjava3.observers.DefaultObserver;

/**
 * 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" height="490" src="https://github.com/ReactiveX/RxJava/wiki/images/rx-operators/B.mostRecent.v3.png" alt="">
 * 
 * @param <T> the value type
 */
public final class BlockingObservableMostRecent<T> implements Iterable<T> {

    final ObservableSource<T> source;

    final T initialValue;

    public BlockingObservableMostRecent(ObservableSource<T> source, T initialValue) {
        this.source = source;
        this.initialValue = initialValue;
    }

    @Override
    public Iterator<T> iterator() {
        MostRecentObserver<T> mostRecentObserver = new MostRecentObserver<>(initialValue);

        source.subscribe(mostRecentObserver);

        return mostRecentObserver.getIterable();
    }

    static final class MostRecentObserver<T> extends DefaultObserver<T> {
        volatile Object value;

        MostRecentObserver(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 MostRecentIterator} return is not thread safe. In other words don't call {@link MostRecentIterator#hasNext()} in one
         * thread expect {@link MostRecentIterator#next()} called from a different thread to work.
         * @return the Iterator
         */
        public MostRecentIterator getIterable() {
            return new MostRecentIterator();
        }

        final class MostRecentIterator implements java.util.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");
            }
        }
    }
}
