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SnappyData store based off Pivotal GemFireXD
There is a newer version: 2.0-BETA
/*
 * Copyright (c) 2010-2015 Pivotal Software, Inc. All rights reserved.
 *
 * 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. See accompanying
 * LICENSE file.
 */
package com.gemstone.gemfire.internal;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.Set;

import io.snappydata.collection.OpenHashSet;

/**
 * A Hash set where every modification makes an internal copy 
 * of a HashSet. Similar to java.util.concurrent.CopyOnWriteArrayList,
 * except methods provide the access time characteristics of HashSet, instead
 * of ArrayList, for example contains is O(1) instead of O(n).
 * 
 * Also, this class provides a getSnapshot method, which should
 * be used for any thing that needs an unchanging snapshot of this
 * this (For example, any serialization of this class should use getSnapshot).
 * @author dsmith
 *
 */
public class CopyOnWriteHashSet implements Set, Serializable  {
  
  private static final long serialVersionUID = 8591978652141659932L;

  private static final OpenHashSet EMPTY_SET = new OpenHashSet(1);

  @SuppressWarnings("unchecked")
  private volatile transient OpenHashSet snapshot = EMPTY_SET;

  public CopyOnWriteHashSet() {
  }

  public CopyOnWriteHashSet(Set copy) {
    this.snapshot = new OpenHashSet<>(copy);
  }

  /**
   * Because I'm lazy, this iterator does not support modification
   * of this set. If you need it, it shouldn't be too hard to implement.
   */
  public Iterator iterator() {
    return Collections.unmodifiableSet(snapshot).iterator();
  }

  public int size() {
    return snapshot.size();
  }

  public boolean add(T e) {
    synchronized(this) {
      OpenHashSet set = new OpenHashSet<>(snapshot);
      boolean result = set.add(e);
      snapshot = set;
      return result;
    }
  }

  public boolean addAll(Collection c) {
    synchronized(this) {
      OpenHashSet set = new OpenHashSet<>(snapshot);
      boolean result = set.addAll(c);
      snapshot = set;
      return result;
    }
  }

  public void clear() {
    synchronized(this) {
      snapshot = EMPTY_SET;
    }
  }

  public boolean contains(Object o) {
    return snapshot.contains(o);
  }

  public boolean containsAll(Collection c) {
    return snapshot.containsAll(c);
  }

  public boolean isEmpty() {
    return snapshot.isEmpty();
  }

  public boolean remove(Object o) {
    synchronized(this) {
      OpenHashSet set = new OpenHashSet<>(snapshot);
      boolean result = set.remove(o);
      snapshot = set;
      return result;
    }
  }

  public boolean retainAll(Collection c) {
    synchronized(this) {
      OpenHashSet set = new OpenHashSet<>(snapshot);
      boolean result = set.retainAll(c);
      snapshot = set;
      return result;
    }
  }

  public Object[] toArray() {
    return snapshot.toArray();
  }

  public  T[] toArray(T[] a) {
    return snapshot.toArray(a);
  }

  @Override
  public boolean equals(Object o) {
    return snapshot.equals(o);
  }

  @Override
  public int hashCode() {
    return snapshot.hashCode();
  }

  public boolean removeAll(Collection c) {
    synchronized(this) {
      OpenHashSet set = new OpenHashSet<>(snapshot);
      boolean result = set.removeAll(c);
      snapshot = set;
      return result;
    }
  }

  @Override
  public String toString() {
    return snapshot.toString();
  }

  /**
   * Return a snapshot of the set at this point in time.
   * The snapshot is guaranteed not to change. It is therefore
   * unmodifiable.
   * This will likely be more efficient than copying this
   * set.
   * @return A snapshot of this set.
   */
  public Set getSnapshot() {
    return Collections.unmodifiableSet(snapshot);
  }
  
  private void writeObject(ObjectOutputStream s) throws IOException {
    s.defaultWriteObject();
    s.writeObject(snapshot);
  }

  @SuppressWarnings("unchecked")
  private void readObject(ObjectInputStream s)
      throws java.io.IOException, ClassNotFoundException {
    s.defaultReadObject();
    this.snapshot = (OpenHashSet)s.readObject();
  }
}