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.
   */
  
  package io.reactivex.rxjava3.internal.disposables;
  
  import java.util.Objects;
  import java.util.concurrent.atomic.AtomicReference;
  
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.exceptions.ProtocolViolationException;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * Utility methods for working with Disposables atomically.
   */
  public enum DisposableHelper implements Disposable {
      /**
       * The singleton instance representing a terminal, disposed state, don't leak it.
       */
      DISPOSED
      ;
  
      /**
       * Checks if the given Disposable is the common {@link #DISPOSED} enum value.
       * @param d the disposable to check
       * @return true if d is {@link #DISPOSED}
       */
      public static boolean isDisposed(Disposable d) {
          return d == DISPOSED;
      }
  
      /**
       * Atomically sets the field and disposes the old contents.
       * @param field the target field
       * @param d the new Disposable to set
       * @return true if successful, false if the field contains the {@link #DISPOSED} instance.
       */
      public static boolean set(AtomicReference<Disposable> field, Disposable d) {
          for (;;) {
              Disposable current = field.get();
              if (current == DISPOSED) {
                  if (d != null) {
                      d.dispose();
                  }
                  return false;
              }
              if (field.compareAndSet(current, d)) {
                  if (current != null) {
                      current.dispose();
                  }
                  return true;
              }
          }
      }
  
      /**
       * Atomically sets the field to the given non-null Disposable and returns true
       * or returns false if the field is non-null.
       * If the target field contains the common DISPOSED instance, the supplied disposable
       * is disposed. If the field contains other non-null Disposable, an IllegalStateException
       * is signalled to the RxJavaPlugins.onError hook.
       * 
       * @param field the target field
       * @param d the disposable to set, not null
       * @return true if the operation succeeded, false
       */
      public static boolean setOnce(AtomicReference<Disposable> field, Disposable d) {
          Objects.requireNonNull(d, "d is null");
          if (!field.compareAndSet(null, d)) {
              d.dispose();
              if (field.get() != DISPOSED) {
                  reportDisposableSet();
              }
              return false;
          }
          return true;
      }
  
      /**
       * Atomically replaces the Disposable in the field with the given new Disposable
       * but does not dispose the old one.
       * @param field the target field to change
       * @param d the new disposable, null allowed
       * @return true if the operation succeeded, false if the target field contained
       * the common DISPOSED instance and the given disposable (if not null) is disposed.
       */
      public static boolean replace(AtomicReference<Disposable> field, Disposable d) {
          for (;;) {
              Disposable current = field.get();
              if (current == DISPOSED) {
                  if (d != null) {
                      d.dispose();
                  }
                  return false;
              }
              if (field.compareAndSet(current, d)) {
                  return true;
              }
          }
      }
  
      /**
       * Atomically disposes the Disposable in the field if not already disposed.
       * @param field the target field
       * @return true if the current thread managed to dispose the Disposable
       */
      public static boolean dispose(AtomicReference<Disposable> field) {
          Disposable current = field.get();
          Disposable d = DISPOSED;
          if (current != d) {
              current = field.getAndSet(d);
              if (current != d) {
                  if (current != null) {
                      current.dispose();
                  }
                  return true;
              }
          }
          return false;
      }
  
      /**
       * Verifies that current is null, next is not null, otherwise signals errors
       * to the RxJavaPlugins and returns false.
       * @param current the current Disposable, expected to be null
       * @param next the next Disposable, expected to be non-null
       * @return true if the validation succeeded
       */
      public static boolean validate(Disposable current, Disposable next) {
          if (next == null) {
              RxJavaPlugins.onError(new NullPointerException("next is null"));
              return false;
          }
          if (current != null) {
              next.dispose();
              reportDisposableSet();
              return false;
          }
          return true;
      }
  
      /**
       * Reports that the disposable is already set to the RxJavaPlugins error handler.
       */
      public static void reportDisposableSet() {
          RxJavaPlugins.onError(new ProtocolViolationException("Disposable already set!"));
      }
  
      /**
       * Atomically tries to set the given Disposable on the field if it is null or disposes it if
       * the field contains {@link #DISPOSED}.
       * @param field the target field
       * @param d the disposable to set
       * @return true if successful, false otherwise
       */
      public static boolean trySet(AtomicReference<Disposable> field, Disposable d) {
          if (!field.compareAndSet(null, d)) {
              if (field.get() == DISPOSED) {
                  d.dispose();
              }
              return false;
          }
          return true;
      }
  
      @Override
      public void dispose() {
          // deliberately no-op
      }
  
      @Override
      public boolean isDisposed() {
          return true;
      }
  }
  