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Easy Redis Java client and Real-Time Data Platform. Valkey compatible. Sync/Async/RxJava3/Reactive API. Client side caching. Over 50 Redis based Java objects and services: JCache API, Apache Tomcat, Hibernate, Spring, Set, Multimap, SortedSet, Map, List, Queue, Deque, Semaphore, Lock, AtomicLong, Map Reduce, Bloom filter, Scheduler, RPC

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/*
 * Copyright (c) 2016-2021 VMware Inc. or its affiliates, All Rights Reserved.
 *
 * 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
 *
 *   https://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 reactor.core.publisher;

import java.util.NoSuchElementException;
import java.util.Objects;

import org.reactivestreams.Subscription;
import reactor.core.CoreSubscriber;
import reactor.util.annotation.Nullable;
import reactor.util.context.Context;

/**
 * Expects and emits a single item from the source or signals
 * NoSuchElementException (or a default generated value) for empty source,
 * IndexOutOfBoundsException for a multi-item source.
 *
 * @param  the value type
 * @see Reactive-Streams-Commons
 */
final class MonoSingle extends MonoFromFluxOperator {

	final T       defaultValue;
	final boolean completeOnEmpty;

	MonoSingle(Flux source) {
		super(source);
		this.defaultValue = null;
		this.completeOnEmpty = false;
	}

	MonoSingle(Flux source,
			@Nullable T defaultValue,
			boolean completeOnEmpty) {
		super(source);
		this.defaultValue = completeOnEmpty ? defaultValue :
				Objects.requireNonNull(defaultValue, "defaultValue");
		this.completeOnEmpty = completeOnEmpty;
	}

	@Override
	public CoreSubscriber subscribeOrReturn(CoreSubscriber actual) {
		return new SingleSubscriber<>(actual, defaultValue, completeOnEmpty);
	}

	@Override
	public Object scanUnsafe(Attr key) {
		if (key == Attr.RUN_STYLE) return Attr.RunStyle.SYNC;
		return super.scanUnsafe(key);
	}

	static final class SingleSubscriber extends Operators.MonoInnerProducerBase implements InnerConsumer {

		@Nullable
		final T       defaultValue;
		final boolean completeOnEmpty;

		Subscription s;

		int count;

		boolean done;

		@Override
		@Nullable
		public Object scanUnsafe(Attr key) {
			if (key == Attr.TERMINATED) return done;
			if (key == Attr.PARENT) return s;
			if (key == Attr.RUN_STYLE) return Attr.RunStyle.SYNC;

			return super.scanUnsafe(key);
		}

		@Override
		public Context currentContext() {
			return actual().currentContext();
		}

		SingleSubscriber(CoreSubscriber actual,
				@Nullable T defaultValue,
				boolean completeOnEmpty) {
			super(actual);
			this.defaultValue = defaultValue;
			this.completeOnEmpty = completeOnEmpty;
		}

		@Override
		public void doOnRequest(long n) {
			s.request(Long.MAX_VALUE);
		}

		@Override
		public void doOnCancel() {
			s.cancel();
		}

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

		@Override
		public void onNext(T t) {
			if (isCancelled()) {
				//this helps differentiating a duplicate malformed signal "done" from a count > 1 "done"
				discard(t);
				return;
			}
			if (done) {
				Operators.onNextDropped(t, actual().currentContext());
				return;
			}
			if (++count > 1) {
				discard(t);
				//mark as both cancelled and done
				cancel();
				onError(new IndexOutOfBoundsException("Source emitted more than one item"));
			}
			else {
				setValue(t);
			}
		}

		@Override
		public void onError(Throwable t) {
			if (done) {
				Operators.onErrorDropped(t, actual().currentContext());
				return;
			}
			done = true;
			discardTheValue();

			actual().onError(t);
		}

		@Override
		public void onComplete() {
			if (done) {
				return;
			}
			done = true;

			int c = count;
			if (c == 0) {

				if (completeOnEmpty) {
					actual().onComplete();
					return;
				}

				T t = defaultValue;

				if (t != null) {
					complete(t);
				}
				else {
					actual().onError(Operators.onOperatorError(this,
							new NoSuchElementException("Source was empty"),
							actual().currentContext()));
				}
			}
			else if (c == 1) {
				complete();
			}
		}

	}
}