/*
 * Copyright (C) 2012 The Guava Authors
 *
 * 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.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.Serializable;
import java.util.ArrayDeque;
import java.util.Collection;
import java.util.Queue;

A non-blocking queue which automatically evicts elements from the head of the queue when
attempting to add new elements onto the queue and it is full. This queue orders elements FIFO
(first-in-first-out). This data structure is logically equivalent to a circular buffer (i.e.,
cyclic buffer or ring buffer).
An evicting queue must be configured with a maximum size. Each time an element is added to a
full queue, the queue automatically removes its head element. This is different from conventional
bounded queues, which either block or reject new elements when full.
This class is not thread-safe, and does not accept null elements.
Author:
Kurt Alfred Kluever
Since:
15.0/**
 * A non-blocking queue which automatically evicts elements from the head of the queue when
 * attempting to add new elements onto the queue and it is full. This queue orders elements FIFO
 * (first-in-first-out). This data structure is logically equivalent to a circular buffer (i.e.,
 * cyclic buffer or ring buffer).
 *
 * <p>An evicting queue must be configured with a maximum size. Each time an element is added to a
 * full queue, the queue automatically removes its head element. This is different from conventional
 * bounded queues, which either block or reject new elements when full.
 *
 * <p>This class is not thread-safe, and does not accept null elements.
 *
 * @author Kurt Alfred Kluever
 * @since 15.0
 */
@Beta
@GwtCompatible
public final class EvictingQueue<E> extends ForwardingQueue<E> implements Serializable {

  private final Queue<E> delegate;

  @VisibleForTesting final int maxSize;

  private EvictingQueue(int maxSize) {
    checkArgument(maxSize >= 0, "maxSize (%s) must >= 0", maxSize);
    this.delegate = new ArrayDeque<E>(maxSize);
    this.maxSize = maxSize;
  }

  
Creates and returns a new evicting queue that will hold up to maxSize elements. 
When maxSize is zero, elements will be evicted immediately after being added to the queue. /**
   * Creates and returns a new evicting queue that will hold up to {@code maxSize} elements.
   *
   * <p>When {@code maxSize} is zero, elements will be evicted immediately after being added to the
   * queue.
   */
  public static <E> EvictingQueue<E> create(int maxSize) {
    return new EvictingQueue<E>(maxSize);
  }

  
Returns the number of additional elements that this queue can accept without evicting; zero if
the queue is currently full.
Since:
16.0/**
   * Returns the number of additional elements that this queue can accept without evicting; zero if
   * the queue is currently full.
   *
   * @since 16.0
   */
  public int remainingCapacity() {
    return maxSize - size();
  }

  @Override
  protected Queue<E> delegate() {
    return delegate;
  }

  
Adds the given element to this queue. If the queue is currently full, the element at the head
of the queue is evicted to make room.
Returns:
true always/**
   * Adds the given element to this queue. If the queue is currently full, the element at the head
   * of the queue is evicted to make room.
   *
   * @return {@code true} always
   */
  @Override
  @CanIgnoreReturnValue
  public boolean offer(E e) {
    return add(e);
  }

  
Adds the given element to this queue. If the queue is currently full, the element at the head
of the queue is evicted to make room.
Returns:
true always/**
   * Adds the given element to this queue. If the queue is currently full, the element at the head
   * of the queue is evicted to make room.
   *
   * @return {@code true} always
   */
  @Override
  @CanIgnoreReturnValue
  public boolean add(E e) {
    checkNotNull(e); // check before removing
    if (maxSize == 0) {
      return true;
    }
    if (size() == maxSize) {
      delegate.remove();
    }
    delegate.add(e);
    return true;
  }

  @Override
  @CanIgnoreReturnValue
  public boolean addAll(Collection<? extends E> collection) {
    int size = collection.size();
    if (size >= maxSize) {
      clear();
      return Iterables.addAll(this, Iterables.skip(collection, size - maxSize));
    }
    return standardAddAll(collection);
  }

  @Override
  public boolean contains(Object object) {
    return delegate().contains(checkNotNull(object));
  }

  @Override
  @CanIgnoreReturnValue
  public boolean remove(Object object) {
    return delegate().remove(checkNotNull(object));
  }

  // TODO(kak): Do we want to checkNotNull each element in containsAll, removeAll, and retainAll?

  private static final long serialVersionUID = 0L;
}