/*
    * 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.GwtIncompatible;
  import com.google.common.annotations.VisibleForTesting;
  
  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.
   *
   * <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
  @GwtIncompatible("java.util.ArrayDeque")
  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 {@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
     */
    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.
     *
     * @return {@code true} always
     */
    @Override 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.
     *
     * @return {@code true} always
     */
    @Override 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 public boolean addAll(Collection<? extends E> collection) {
     return standardAddAll(collection);
   }
 
   @Override
   public boolean contains(Object object) {
     return delegate().contains(checkNotNull(object));
   }
 
   @Override
   public boolean remove(Object object) {
     return delegate().remove(checkNotNull(object));
   }
 
   // TODO(user): Do we want to checkNotNull each element in containsAll, removeAll, and retainAll?
 
   private static final long serialVersionUID = 0L;
 }