content: /*
   * 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.asterix.om.util.container;
  
  import java.util.ArrayList;
  import java.util.BitSet;
  import java.util.List;
  
  /**
   * Object pool backed by a list. The argument for creating E instances could be different. This class also considers
   * arguments in object reusing, e.g., it reuses an E instances ONLY when the construction argument is "equal".
   */
  public class ListObjectPool<E, T> implements IObjectPool<E, T> {
  
      private IObjectFactory<E, T> factory;
  
      /**
       * cache of objects
       */
      private List<E> pool = new ArrayList<>();
  
      /**
       * args that are used to create each element in the pool
       */
      private List<T> args = new ArrayList<>();
  
      /**
       * bits indicating which element is in use
       */
      private BitSet usedBits = new BitSet();
  
      public ListObjectPool(IObjectFactory<E, T> factory) {
          this.factory = factory;
      }
  
      @Override
      public E allocate(T arg) {
          int freeSlot = -1;
          while (freeSlot + 1 < pool.size()) {
              freeSlot = usedBits.nextClearBit(freeSlot + 1);
              if (freeSlot >= pool.size())
                  break;
  
              // the two cases where an element in the pool is a match
              if ((arg == null && args.get(freeSlot) == null)
                      || (arg != null && args.get(freeSlot) != null && arg.equals(args.get(freeSlot)))) {
                  // the element is not used and the arg is the same as
                  // input arg
                  usedBits.set(freeSlot);
                  return pool.get(freeSlot);
              }
          }
  
          // if we do not find a reusable object, allocate a new one
          E element = factory.create(arg);
          pool.add(element);
          args.add(arg);
          usedBits.set(pool.size() - 1);
          return element;
      }
  
      @Override
      public void reset() {
          usedBits.clear();
      }
  
      @Override
      public boolean free(E element) {
          for (int i = usedBits.length(); (i = usedBits.previousSetBit(i - 1)) >= 0;) {
              if (element == pool.get(i)) {
                  usedBits.clear(i);
                  return true;
              }
          }
          return false;
      }
  }
  