001/*
002 * Copyright (C) 2012 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.base.Preconditions.checkArgument;
020import static com.google.common.base.Preconditions.checkNotNull;
021
022import com.google.common.annotations.Beta;
023import com.google.common.annotations.GwtCompatible;
024import com.google.common.annotations.VisibleForTesting;
025import com.google.errorprone.annotations.CanIgnoreReturnValue;
026import java.io.Serializable;
027import java.util.ArrayDeque;
028import java.util.Collection;
029import java.util.Queue;
030
031/**
032 * A non-blocking queue which automatically evicts elements from the head of the queue when
033 * attempting to add new elements onto the queue and it is full. This data structure is logically
034 * equivalent to a circular buffer (i.e., cyclic buffer or ring buffer).
035 *
036 * <p>An evicting queue must be configured with a maximum size. Each time an element is added
037 * to a full queue, the queue automatically removes its head element. This is different from
038 * conventional bounded queues, which either block or reject new elements when full.
039 *
040 * <p>This class is not thread-safe, and does not accept null elements.
041 *
042 * @author Kurt Alfred Kluever
043 * @since 15.0
044 */
045@Beta
046@GwtCompatible
047public final class EvictingQueue<E> extends ForwardingQueue<E> implements Serializable {
048
049  private final Queue<E> delegate;
050
051  @VisibleForTesting final int maxSize;
052
053  private EvictingQueue(int maxSize) {
054    checkArgument(maxSize >= 0, "maxSize (%s) must >= 0", maxSize);
055    this.delegate = new ArrayDeque<E>(maxSize);
056    this.maxSize = maxSize;
057  }
058
059  /**
060   * Creates and returns a new evicting queue that will hold up to {@code maxSize} elements.
061   *
062   * <p>When {@code maxSize} is zero, elements will be evicted immediately after being added to the
063   * queue.
064   */
065  public static <E> EvictingQueue<E> create(int maxSize) {
066    return new EvictingQueue<E>(maxSize);
067  }
068
069  /**
070   * Returns the number of additional elements that this queue can accept without evicting;
071   * zero if the queue is currently full.
072   *
073   * @since 16.0
074   */
075  public int remainingCapacity() {
076    return maxSize - size();
077  }
078
079  @Override
080  protected Queue<E> delegate() {
081    return delegate;
082  }
083
084  /**
085   * Adds the given element to this queue. If the queue is currently full, the element at the head
086   * of the queue is evicted to make room.
087   *
088   * @return {@code true} always
089   */
090  @Override
091  @CanIgnoreReturnValue
092  public boolean offer(E e) {
093    return add(e);
094  }
095
096  /**
097   * Adds the given element to this queue. If the queue is currently full, the element at the head
098   * of the queue is evicted to make room.
099   *
100   * @return {@code true} always
101   */
102  @Override
103  @CanIgnoreReturnValue
104  public boolean add(E e) {
105    checkNotNull(e); // check before removing
106    if (maxSize == 0) {
107      return true;
108    }
109    if (size() == maxSize) {
110      delegate.remove();
111    }
112    delegate.add(e);
113    return true;
114  }
115
116  @Override
117  @CanIgnoreReturnValue
118  public boolean addAll(Collection<? extends E> collection) {
119    int size = collection.size();
120    if (size >= maxSize) {
121      clear();
122      return Iterables.addAll(this, Iterables.skip(collection, size - maxSize));
123    }
124    return standardAddAll(collection);
125  }
126
127  @Override
128  public boolean contains(Object object) {
129    return delegate().contains(checkNotNull(object));
130  }
131
132  @Override
133  @CanIgnoreReturnValue
134  public boolean remove(Object object) {
135    return delegate().remove(checkNotNull(object));
136  }
137
138  // TODO(kak): Do we want to checkNotNull each element in containsAll, removeAll, and retainAll?
139
140  private static final long serialVersionUID = 0L;
141}