/*

  * Licensed to the Apache Software Foundation (ASF) under one

  * or more contributor license agreements.  See the NOTICE file

  * distributed with this work for additional information

  * regarding copyright ownership.  The ASF licenses this file

  * to you 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 org.apache.giraph.utils;

 import java.util.concurrent.Semaphore;

 import static com.google.common.base.Preconditions.checkState;

 /**

  * Implementation of a semaphore where number of available permits can change

  */

 public final class AdjustableSemaphore extends Semaphore {

   /** Maximum number of available permits */

   private int maxPermits;

   /**

    * Constructor

    * @param permits initial number of available permits

    */

   public AdjustableSemaphore(int permits) {

     super(permits);

     maxPermits = permits;

   }

   @edu.umd.cs.findbugs.annotations.SuppressWarnings(

       "UG_SYNC_SET_UNSYNC_GET")

   public int getMaxPermits() {

     return maxPermits;

   }

   /**

    * Adjusts the maximum number of available permits.

    *

    * @param newMax max number of permits

    */

   public synchronized void setMaxPermits(int newMax) {

     checkState(newMax >= 0, "setMaxPermits: number of permits cannot be " +

         "less than 0");

     int delta = newMax - this.maxPermits;

     if (delta > 0) {

       // Releasing semaphore to make room for 'delta' more permits

       release(delta);

     } else if (delta < 0) {

       // Reducing number of permits in the semaphore

       reducePermits(-delta);

     }

     this.maxPermits = newMax;

   }

 }