code: /*
   * 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.
   */
  
  /*
   * The code was inspired by the similarly named JCTools class:
   * https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/atomic
   */
  
  package io.reactivex.rxjava3.internal.queue;
  
  import java.util.concurrent.atomic.AtomicReference;
  
  import io.reactivex.rxjava3.annotations.Nullable;
  import io.reactivex.rxjava3.operators.SimplePlainQueue;
  
  /**
   * A multi-producer single consumer unbounded queue.
   * @param <T> the contained value type
   */
  public final class MpscLinkedQueue<T> implements SimplePlainQueue<T> {
      private final AtomicReference<LinkedQueueNode<T>> producerNode;
      private final AtomicReference<LinkedQueueNode<T>> consumerNode;
  
      public MpscLinkedQueue() {
          producerNode = new AtomicReference<>();
          consumerNode = new AtomicReference<>();
          LinkedQueueNode<T> node = new LinkedQueueNode<>();
          spConsumerNode(node);
          xchgProducerNode(node); // this ensures correct construction: StoreLoad
      }
  
      /**
       * {@inheritDoc} <br>
       * <p>
       * IMPLEMENTATION NOTES:<br>
       * Offer is allowed from multiple threads.<br>
       * Offer allocates a new node and:
       * <ol>
       * <li>Swaps it atomically with current producer node (only one producer 'wins')
       * <li>Sets the new node as the node following from the swapped producer node
       * </ol>
       * This works because each producer is guaranteed to 'plant' a new node and link the old node. No 2 producers can
       * get the same producer node as part of XCHG guarantee.
       *
       * @see java.util.Queue#offer(java.lang.Object)
       */
      @Override
      public boolean offer(final T e) {
          if (null == e) {
              throw new NullPointerException("Null is not a valid element");
          }
          final LinkedQueueNode<T> nextNode = new LinkedQueueNode<>(e);
          final LinkedQueueNode<T> prevProducerNode = xchgProducerNode(nextNode);
          // Should a producer thread get interrupted here the chain WILL be broken until that thread is resumed
          // and completes the store in prev.next.
          prevProducerNode.soNext(nextNode); // StoreStore
          return true;
      }
  
      /**
       * {@inheritDoc} <br>
       * <p>
       * IMPLEMENTATION NOTES:<br>
       * Poll is allowed from a SINGLE thread.<br>
       * Poll reads the next node from the consumerNode and:
       * <ol>
       * <li>If it is null, the queue is assumed empty (though it might not be).
       * <li>If it is not null set it as the consumer node and return it's now evacuated value.
       * </ol>
       * This means the consumerNode.value is always null, which is also the starting point for the queue. Because null
       * values are not allowed to be offered this is the only node with it's value set to null at any one time.
       *
       * @see java.util.Queue#poll()
       */
      @Nullable
      @Override
      public T poll() {
          LinkedQueueNode<T> currConsumerNode = lpConsumerNode(); // don't load twice, it's alright
          LinkedQueueNode<T> nextNode = currConsumerNode.lvNext();
          if (nextNode != null) {
              // we have to null out the value because we are going to hang on to the node
              final T nextValue = nextNode.getAndNullValue();
              spConsumerNode(nextNode);
              return nextValue;
          }
          else if (currConsumerNode != lvProducerNode()) {
              // spin, we are no longer wait free
              while ((nextNode = currConsumerNode.lvNext()) == null) { } // NOPMD
              // got the next node...
  
              // we have to null out the value because we are going to hang on to the node
              final T nextValue = nextNode.getAndNullValue();
              spConsumerNode(nextNode);
              return nextValue;
          }
          return null;
      }
  
      @Override
      public boolean offer(T v1, T v2) {
          offer(v1);
          offer(v2);
          return true;
      }
  
      @Override
      public void clear() {
          while (poll() != null && !isEmpty()) { } // NOPMD
      }
      LinkedQueueNode<T> lvProducerNode() {
          return producerNode.get();
      }
      LinkedQueueNode<T> xchgProducerNode(LinkedQueueNode<T> node) {
          return producerNode.getAndSet(node);
      }
      LinkedQueueNode<T> lvConsumerNode() {
          return consumerNode.get();
      }
  
      LinkedQueueNode<T> lpConsumerNode() {
          return consumerNode.get();
      }
      void spConsumerNode(LinkedQueueNode<T> node) {
          consumerNode.lazySet(node);
      }
  
      /**
       * {@inheritDoc} <br>
       * <p>
       * IMPLEMENTATION NOTES:<br>
       * Queue is empty when producerNode is the same as consumerNode. An alternative implementation would be to observe
       * the producerNode.value is null, which also means an empty queue because only the consumerNode.value is allowed to
       * be null.
       */
      @Override
      public boolean isEmpty() {
          return lvConsumerNode() == lvProducerNode();
      }
  
      static final class LinkedQueueNode<E> extends AtomicReference<LinkedQueueNode<E>> {
  
          private static final long serialVersionUID = 2404266111789071508L;
  
          private E value;
  
          LinkedQueueNode() {
          }
  
          LinkedQueueNode(E val) {
              spValue(val);
          }
          /**
           * Gets the current value and nulls out the reference to it from this node.
           *
           * @return value
           */
          public E getAndNullValue() {
              E temp = lpValue();
              spValue(null);
              return temp;
          }
  
          public E lpValue() {
              return value;
          }
  
          public void spValue(E newValue) {
              value = newValue;
          }
  
          public void soNext(LinkedQueueNode<E> n) {
              lazySet(n);
          }
  
          public LinkedQueueNode<E> lvNext() {
              return get();
          }
      }
  }
  