code: /*
   * Copyright (c) 2019 the Eclipse Milo Authors
   *
   * This program and the accompanying materials are made
   * available under the terms of the Eclipse Public License 2.0
   * which is available at https://www.eclipse.org/legal/epl-2.0/
   *
   * SPDX-License-Identifier: EPL-2.0
   */
  
  package org.eclipse.milo.opcua.stack.core.util;
  
  import java.util.ArrayDeque;
  import java.util.concurrent.CompletableFuture;
  import java.util.concurrent.atomic.AtomicInteger;
  
  public class AsyncSemaphore {
  
      private final ArrayDeque<CompletableFuture<SemaphorePermit>> waitQueue = new ArrayDeque<>();
  
      private final AtomicInteger availablePermits;
  
      public AsyncSemaphore(int initialPermits) {
          availablePermits = new AtomicInteger(initialPermits);
      }
  
      public CompletableFuture<SemaphorePermit> acquire() {
          CompletableFuture<SemaphorePermit> f = new CompletableFuture<>();
  
          boolean permitAvailable = false;
  
          synchronized (this) {
              if (availablePermits.get() > 0) {
                  availablePermits.decrementAndGet();
  
                  permitAvailable = true;
              } else {
                  waitQueue.addLast(f);
              }
          }
  
          if (permitAvailable) {
              f.complete(new PermitImpl());
          }
  
          return f;
      }
  
      public interface SemaphorePermit {
  
          /**
           * Releases this semaphore permit.
           */
          void release();
  
      }
  
      private final class PermitImpl implements SemaphorePermit {
          @Override
          public void release() {
              CompletableFuture<SemaphorePermit> next;
  
              synchronized (AsyncSemaphore.this) {
                  next = waitQueue.pollFirst();
                  if (next == null) availablePermits.incrementAndGet();
              }
  
              if (next != null) next.complete(new PermitImpl());
          }
      }
  
  }
  