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.GwtCompatible;
023import com.google.common.annotations.J2ktIncompatible;
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 queue orders elements FIFO
034 * (first-in-first-out). This data structure is logically equivalent to a circular buffer (i.e.,
035 * cyclic buffer or ring buffer).
036 *
037 * <p>An evicting queue must be configured with a maximum size. Each time an element is added to a
038 * full queue, the queue automatically removes its head element. This is different from conventional
039 * bounded queues, which either block or reject new elements when full.
040 *
041 * <p>This class is not thread-safe, and does not accept null elements.
042 *
043 * @author Kurt Alfred Kluever
044 * @since 15.0
045 */
046@GwtCompatible
047@ElementTypesAreNonnullByDefault
048public final class EvictingQueue<E> extends ForwardingQueue<E> implements Serializable {
049
050  private final Queue<E> delegate;
051
052  @VisibleForTesting final int maxSize;
053
054  private EvictingQueue(int maxSize) {
055    checkArgument(maxSize >= 0, "maxSize (%s) must >= 0", maxSize);
056    this.delegate = new ArrayDeque<>(maxSize);
057    this.maxSize = maxSize;
058  }
059
060  /**
061   * Creates and returns a new evicting queue that will hold up to {@code maxSize} elements.
062   *
063   * <p>When {@code maxSize} is zero, elements will be evicted immediately after being added to the
064   * queue.
065   */
066  public static <E> EvictingQueue<E> create(int maxSize) {
067    return new EvictingQueue<>(maxSize);
068  }
069
070  /**
071   * Returns the number of additional elements that this queue can accept without evicting; zero if
072   * the queue is currently full.
073   *
074   * @since 16.0
075   */
076  public int remainingCapacity() {
077    return maxSize - size();
078  }
079
080  @Override
081  protected Queue<E> delegate() {
082    return delegate;
083  }
084
085  /**
086   * Adds the given element to this queue. If the queue is currently full, the element at the head
087   * of the queue is evicted to make room.
088   *
089   * @return {@code true} always
090   */
091  @Override
092  @CanIgnoreReturnValue
093  public boolean offer(E e) {
094    return add(e);
095  }
096
097  /**
098   * Adds the given element to this queue. If the queue is currently full, the element at the head
099   * of the queue is evicted to make room.
100   *
101   * @return {@code true} always
102   */
103  @Override
104  @CanIgnoreReturnValue
105  public boolean add(E e) {
106    checkNotNull(e); // check before removing
107    if (maxSize == 0) {
108      return true;
109    }
110    if (size() == maxSize) {
111      delegate.remove();
112    }
113    delegate.add(e);
114    return true;
115  }
116
117  @Override
118  @CanIgnoreReturnValue
119  public boolean addAll(Collection<? extends E> collection) {
120    int size = collection.size();
121    if (size >= maxSize) {
122      clear();
123      return Iterables.addAll(this, Iterables.skip(collection, size - maxSize));
124    }
125    return standardAddAll(collection);
126  }
127
128  @Override
129  @J2ktIncompatible // Incompatible return type change. Use inherited implementation
130  public Object[] toArray() {
131    /*
132     * If we could, we'd declare the no-arg `Collection.toArray()` to return "Object[] but elements
133     * have the same nullness as E." Since we can't, we declare it to return nullable elements, and
134     * we can override it in our non-null-guaranteeing subtypes to present a better signature to
135     * their users.
136     *
137     * However, the checker *we* use has this special knowledge about `Collection.toArray()` anyway,
138     * so in our implementation code, we can rely on that. That's why the expression below
139     * type-checks.
140     */
141    return super.toArray();
142  }
143
144  private static final long serialVersionUID = 0L;
145}