/*
    * Copyright (C) 2010 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.util.concurrent;
  
  import com.google.common.collect.ObjectArrays;
  
  import java.util.AbstractQueue;
  import java.util.Collection;
  import java.util.ConcurrentModificationException;
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.TimeUnit;
  
  import javax.annotation.Nullable;
  
  /**
   * A bounded {@linkplain BlockingQueue blocking queue} backed by an
   * array.  This queue orders elements FIFO (first-in-first-out).  The
   * <em>head</em> of the queue is that element that has been on the
   * queue the longest time.  The <em>tail</em> of the queue is that
   * element that has been on the queue the shortest time. New elements
   * are inserted at the tail of the queue, and the queue retrieval
   * operations obtain elements at the head of the queue.
   *
   * <p>This is a classic &quot;bounded buffer&quot;, in which a
   * fixed-sized array holds elements inserted by producers and
   * extracted by consumers.  Once created, the capacity cannot be
   * increased.  Attempts to <tt>put</tt> an element into a full queue
   * will result in the operation blocking; attempts to <tt>take</tt> an
   * element from an empty queue will similarly block.
   *
   * <p> This class supports an optional fairness policy for ordering
   * waiting producer and consumer threads.  By default, this ordering
   * is not guaranteed. However, a queue constructed with fairness set
   * to <tt>true</tt> grants threads access in FIFO order. Fairness
   * generally decreases throughput but reduces variability and avoids
   * starvation.
   *
   * <p>This class and its iterator implement all of the
   * <em>optional</em> methods of the {@link Collection} and {@link
   * Iterator} interfaces.
   *
   * @author Doug Lea
   * @author Justin T. Sampson
   * @param <E> the type of elements held in this collection
   */
  public class MonitorBasedArrayBlockingQueue<E> extends AbstractQueue<E>
          implements BlockingQueue<E> {
  
      // Based on revision 1.58 of ArrayBlockingQueue by Doug Lea, from
      // http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/
  
      /** The queued items  */
      final E[] items;
      /** items index for next take, poll or remove */
      int takeIndex;
      /** items index for next put, offer, or add. */
      int putIndex;
      /** Number of items in the queue */
      private int count;
  
      /*
       * Concurrency control uses the classic two-condition algorithm
       * found in any textbook.
       */
  
      /** Monitor guarding all access */
      final Monitor monitor;
  
      /** Guard for waiting takes */
      private final Monitor.Guard notEmpty;
  
      /** Guard for waiting puts */
      private final Monitor.Guard notFull;
  
      // Internal helper methods
  
      /**
       * Circularly increment i.
       */
      final int inc(int i) {
          return (++i == items.length) ? 0 : i;
      }
  
     /**
      * Inserts element at current put position, advances, and signals.
      * Call only when occupying monitor.
      */
     private void insert(E x) {
         items[putIndex] = x;
         putIndex = inc(putIndex);
         ++count;
     }
 
     /**
      * Extracts element at current take position, advances, and signals.
      * Call only when occupying monitor.
      */
     private E extract() {
         final E[] items = this.items;
         E x = items[takeIndex];
         items[takeIndex] = null;
         takeIndex = inc(takeIndex);
         --count;
         return x;
     }
 
     /**
      * Utility for remove and iterator.remove: Delete item at position i.
      * Call only when occupying monitor.
      */
     void removeAt(int i) {
         final E[] items = this.items;
         // if removing front item, just advance
         if (i == takeIndex) {
             items[takeIndex] = null;
             takeIndex = inc(takeIndex);
         } else {
             // slide over all others up through putIndex.
             for (;;) {
                 int nexti = inc(i);
                 if (nexti != putIndex) {
                     items[i] = items[nexti];
                     i = nexti;
                 } else {
                     items[i] = null;
                     putIndex = i;
                     break;
                 }
             }
         }
         --count;
     }
 
     /**
      * Creates an <tt>MonitorBasedArrayBlockingQueue</tt> with the given (fixed)
      * capacity and default access policy.
      *
      * @param capacity the capacity of this queue
      * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
      */
     public MonitorBasedArrayBlockingQueue(int capacity) {
         this(capacity, false);
     }
 
     /**
      * Creates an <tt>MonitorBasedArrayBlockingQueue</tt> with the given (fixed)
      * capacity and the specified access policy.
      *
      * @param capacity the capacity of this queue
      * @param fair if <tt>true</tt> then queue accesses for threads blocked
      *        on insertion or removal, are processed in FIFO order;
      *        if <tt>false</tt> the access order is unspecified.
      * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
      */
     public MonitorBasedArrayBlockingQueue(int capacity, boolean fair) {
         if (capacity <= 0)
             throw new IllegalArgumentException();
         this.items = newEArray(capacity);
         monitor = new Monitor(fair);
         notEmpty = new Monitor.Guard(monitor) {
             @Override public boolean isSatisfied() {
                 return count > 0;
             }
         };
         notFull = new Monitor.Guard(monitor) {
             @Override public boolean isSatisfied() {
                 return count < items.length;
             }
         };
     }
 
     @SuppressWarnings("unchecked") // please don't try this home, kids
     private static <E> E[] newEArray(int capacity) {
         return (E[]) new Object[capacity];
     }
 
     /**
      * Creates an <tt>MonitorBasedArrayBlockingQueue</tt> with the given (fixed)
      * capacity, the specified access policy and initially containing the
      * elements of the given collection,
      * added in traversal order of the collection's iterator.
      *
      * @param capacity the capacity of this queue
      * @param fair if <tt>true</tt> then queue accesses for threads blocked
      *        on insertion or removal, are processed in FIFO order;
      *        if <tt>false</tt> the access order is unspecified.
      * @param c the collection of elements to initially contain
      * @throws IllegalArgumentException if <tt>capacity</tt> is less than
      *         <tt>c.size()</tt>, or less than 1.
      * @throws NullPointerException if the specified collection or any
      *         of its elements are null
      */
     public MonitorBasedArrayBlockingQueue(int capacity, boolean fair,
                               Collection<? extends E> c) {
         this(capacity, fair);
         if (capacity < c.size())
             throw new IllegalArgumentException();
 
         for (E e : c)
             add(e);
     }
 
     /**
      * Inserts the specified element at the tail of this queue if it is
      * possible to do so immediately without exceeding the queue's capacity,
      * returning <tt>true</tt> upon success and throwing an
      * <tt>IllegalStateException</tt> if this queue is full.
      *
      * @param e the element to add
      * @return <tt>true</tt> (as specified by {@link Collection#add})
      * @throws IllegalStateException if this queue is full
      * @throws NullPointerException if the specified element is null
      */
     @Override public boolean add(E e) {
         return super.add(e);
     }
 
     /**
      * Inserts the specified element at the tail of this queue if it is
      * possible to do so immediately without exceeding the queue's capacity,
      * returning <tt>true</tt> upon success and <tt>false</tt> if this queue
      * is full.  This method is generally preferable to method {@link #add},
      * which can fail to insert an element only by throwing an exception.
      *
      * @throws NullPointerException if the specified element is null
      */
     @Override
     public boolean offer(E e) {
         if (e == null) throw new NullPointerException();
         final Monitor monitor = this.monitor;
         if (monitor.enterIf(notFull)) {
             try {
                 insert(e);
                 return true;
             } finally {
                 monitor.leave();
             }
         } else {
           return false;
         }
     }
 
     /**
      * Inserts the specified element at the tail of this queue, waiting
      * for space to become available if the queue is full.
      *
      * @throws InterruptedException {@inheritDoc}
      * @throws NullPointerException {@inheritDoc}
      */
     @Override
     public void put(E e) throws InterruptedException {
         if (e == null) throw new NullPointerException();
         final Monitor monitor = this.monitor;
         monitor.enterWhen(notFull);
         try {
             insert(e);
         } finally {
             monitor.leave();
         }
     }
 
     /**
      * Inserts the specified element at the tail of this queue, waiting
      * up to the specified wait time for space to become available if
      * the queue is full.
      *
      * @throws InterruptedException {@inheritDoc}
      * @throws NullPointerException {@inheritDoc}
      */
     @Override
     public boolean offer(E e, long timeout, TimeUnit unit)
         throws InterruptedException {
 
         if (e == null) throw new NullPointerException();
         final Monitor monitor = this.monitor;
         if (monitor.enterWhen(notFull, timeout, unit)) {
             try {
                 insert(e);
                 return true;
             } finally {
                 monitor.leave();
             }
         } else {
           return false;
         }
     }
 
     @Override
     public E poll() {
         final Monitor monitor = this.monitor;
         if (monitor.enterIf(notEmpty)) {
             try {
                 return extract();
             } finally {
                 monitor.leave();
             }
         } else {
           return null;
         }
     }
 
     @Override
     public E take() throws InterruptedException {
         final Monitor monitor = this.monitor;
         monitor.enterWhen(notEmpty);
         try {
             return extract();
         } finally {
             monitor.leave();
         }
     }
 
     @Override
     public E poll(long timeout, TimeUnit unit) throws InterruptedException {
         final Monitor monitor = this.monitor;
         if (monitor.enterWhen(notEmpty, timeout, unit)) {
             try {
                 return extract();
             } finally {
                 monitor.leave();
             }
         } else {
           return null;
         }
     }
 
     @Override
     public E peek() {
         final Monitor monitor = this.monitor;
         if (monitor.enterIf(notEmpty)) {
             try {
                 return items[takeIndex];
             } finally {
                 monitor.leave();
             }
         } else {
             return null;
         }
     }
 
     // this doc comment is overridden to remove the reference to collections
     // greater in size than Integer.MAX_VALUE
     /**
      * Returns the number of elements in this queue.
      *
      * @return the number of elements in this queue
      */
     @Override public int size() {
         final Monitor monitor = this.monitor;
         monitor.enter();
         try {
             return count;
         } finally {
             monitor.leave();
         }
     }
 
     // this doc comment is a modified copy of the inherited doc comment,
     // without the reference to unlimited queues.
     /**
      * Returns the number of additional elements that this queue can ideally
      * (in the absence of memory or resource constraints) accept without
      * blocking. This is always equal to the initial capacity of this queue
      * less the current <tt>size</tt> of this queue.
      *
      * <p>Note that you <em>cannot</em> always tell if an attempt to insert
      * an element will succeed by inspecting <tt>remainingCapacity</tt>
      * because it may be the case that another thread is about to
      * insert or remove an element.
      */
     @Override
     public int remainingCapacity() {
         final Monitor monitor = this.monitor;
         monitor.enter();
         try {
             return items.length - count;
         } finally {
             monitor.leave();
         }
     }
 
     /**
      * Removes a single instance of the specified element from this queue,
      * if it is present.  More formally, removes an element <tt>e</tt> such
      * that <tt>o.equals(e)</tt>, if this queue contains one or more such
      * elements.
      * Returns <tt>true</tt> if this queue contained the specified element
      * (or equivalently, if this queue changed as a result of the call).
      *
      * @param o element to be removed from this queue, if present
      * @return <tt>true</tt> if this queue changed as a result of the call
      */
     @Override public boolean remove(@Nullable Object o) {
         if (o == null) return false;
         final E[] items = this.items;
         final Monitor monitor = this.monitor;
         monitor.enter();
         try {
             int i = takeIndex;
             int k = 0;
             for (;;) {
                 if (k++ >= count)
                     return false;
                 if (o.equals(items[i])) {
                     removeAt(i);
                     return true;
                 }
                 i = inc(i);
             }
         } finally {
             monitor.leave();
         }
     }
 
     /**
      * Returns <tt>true</tt> if this queue contains the specified element.
      * More formally, returns <tt>true</tt> if and only if this queue contains
      * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
      *
      * @param o object to be checked for containment in this queue
      * @return <tt>true</tt> if this queue contains the specified element
      */
     @Override public boolean contains(@Nullable Object o) {
         if (o == null) return false;
         final E[] items = this.items;
         final Monitor monitor = this.monitor;
         monitor.enter();
         try {
             int i = takeIndex;
             int k = 0;
             while (k++ < count) {
                 if (o.equals(items[i]))
                     return true;
                 i = inc(i);
             }
             return false;
         } finally {
             monitor.leave();
         }
     }
 
     /**
      * Returns an array containing all of the elements in this queue, in
      * proper sequence.
      *
      * <p>The returned array will be "safe" in that no references to it are
      * maintained by this queue.  (In other words, this method must allocate
      * a new array).  The caller is thus free to modify the returned array.
      *
      * <p>This method acts as bridge between array-based and collection-based
      * APIs.
      *
      * @return an array containing all of the elements in this queue
      */
     @Override public Object[] toArray() {
         final E[] items = this.items;
         final Monitor monitor = this.monitor;
         monitor.enter();
         try {
             Object[] a = new Object[count];
             int k = 0;
             int i = takeIndex;
             while (k < count) {
                 a[k++] = items[i];
                 i = inc(i);
             }
             return a;
         } finally {
             monitor.leave();
         }
     }
 
     /**
      * Returns an array containing all of the elements in this queue, in
      * proper sequence; the runtime type of the returned array is that of
      * the specified array.  If the queue fits in the specified array, it
      * is returned therein.  Otherwise, a new array is allocated with the
      * runtime type of the specified array and the size of this queue.
      *
      * <p>If this queue fits in the specified array with room to spare
      * (i.e., the array has more elements than this queue), the element in
      * the array immediately following the end of the queue is set to
      * <tt>null</tt>.
      *
      * <p>Like the {@link #toArray()} method, this method acts as bridge between
      * array-based and collection-based APIs.  Further, this method allows
      * precise control over the runtime type of the output array, and may,
      * under certain circumstances, be used to save allocation costs.
      *
      * <p>Suppose <tt>x</tt> is a queue known to contain only strings.
      * The following code can be used to dump the queue into a newly
      * allocated array of <tt>String</tt>:
      *
      * <pre>
      *     String[] y = x.toArray(new String[0]);</pre>
      *
      * <p>Note that <tt>toArray(new Object[0])</tt> is identical in function to
      * <tt>toArray()</tt>.
      *
      * @param a the array into which the elements of the queue are to
      *          be stored, if it is big enough; otherwise, a new array of the
      *          same runtime type is allocated for this purpose
      * @return an array containing all of the elements in this queue
      * @throws ArrayStoreException if the runtime type of the specified array
      *         is not a supertype of the runtime type of every element in
      *         this queue
      * @throws NullPointerException if the specified array is null
      */
     @Override public <T> T[] toArray(T[] a) {
         final E[] items = this.items;
         final Monitor monitor = this.monitor;
         monitor.enter();
         try {
             if (a.length < count)
                 a = ObjectArrays.newArray(a, count);
 
             int k = 0;
             int i = takeIndex;
             while (k < count) {
                 // This cast is not itself safe, but the following statement
                 // will fail if the runtime type of items[i] is not assignable
                 // to the runtime type of a[k++], which is all that the method
                 // contract requires (see @throws ArrayStoreException above).
                 @SuppressWarnings("unchecked")
                 T t = (T) items[i];
                 a[k++] = t;
                 i = inc(i);
             }
             if (a.length > count)
                 a[count] = null;
             return a;
         } finally {
             monitor.leave();
         }
     }
 
     @Override public String toString() {
         final Monitor monitor = this.monitor;
         monitor.enter();
         try {
             return super.toString();
         } finally {
             monitor.leave();
         }
     }
 
     /**
      * Atomically removes all of the elements from this queue.
      * The queue will be empty after this call returns.
      */
     @Override public void clear() {
         final E[] items = this.items;
         final Monitor monitor = this.monitor;
         monitor.enter();
         try {
             int i = takeIndex;
             int k = count;
             while (k-- > 0) {
                 items[i] = null;
                 i = inc(i);
             }
             count = 0;
             putIndex = 0;
             takeIndex = 0;
         } finally {
             monitor.leave();
         }
     }
 
     /**
      * @throws UnsupportedOperationException {@inheritDoc}
      * @throws ClassCastException            {@inheritDoc}
      * @throws NullPointerException          {@inheritDoc}
      * @throws IllegalArgumentException      {@inheritDoc}
      */
     @Override
     public int drainTo(Collection<? super E> c) {
         if (c == null)
             throw new NullPointerException();
         if (c == this)
             throw new IllegalArgumentException();
         final E[] items = this.items;
         final Monitor monitor = this.monitor;
         monitor.enter();
         try {
             int i = takeIndex;
             int n = 0;
             int max = count;
             while (n < max) {
                 c.add(items[i]);
                 items[i] = null;
                 i = inc(i);
                 ++n;
             }
             if (n > 0) {
                 count = 0;
                 putIndex = 0;
                 takeIndex = 0;
             }
             return n;
         } finally {
             monitor.leave();
         }
     }
 
     /**
      * @throws UnsupportedOperationException {@inheritDoc}
      * @throws ClassCastException            {@inheritDoc}
      * @throws NullPointerException          {@inheritDoc}
      * @throws IllegalArgumentException      {@inheritDoc}
      */
     @Override
     public int drainTo(Collection<? super E> c, int maxElements) {
         if (c == null)
             throw new NullPointerException();
         if (c == this)
             throw new IllegalArgumentException();
         if (maxElements <= 0)
             return 0;
         final E[] items = this.items;
         final Monitor monitor = this.monitor;
         monitor.enter();
         try {
             int i = takeIndex;
             int n = 0;
             int max = (maxElements < count) ? maxElements : count;
             while (n < max) {
                 c.add(items[i]);
                 items[i] = null;
                 i = inc(i);
                 ++n;
             }
             if (n > 0) {
                 count -= n;
                 takeIndex = i;
             }
             return n;
         } finally {
             monitor.leave();
         }
     }
 
     /**
      * Returns an iterator over the elements in this queue in proper sequence.
      * The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
      * will never throw {@link ConcurrentModificationException},
      * and guarantees to traverse elements as they existed upon
      * construction of the iterator, and may (but is not guaranteed to)
      * reflect any modifications subsequent to construction.
      *
      * @return an iterator over the elements in this queue in proper sequence
      */
     @Override public Iterator<E> iterator() {
         final Monitor monitor = this.monitor;
         monitor.enter();
         try {
             return new Itr();
         } finally {
             monitor.leave();
         }
     }
 
     /**
      * Iterator for MonitorBasedArrayBlockingQueue
      */
     private class Itr implements Iterator<E> {
         /**
          * Index of element to be returned by next,
          * or a negative number if no such.
          */
         private int nextIndex;
 
         /**
          * nextItem holds on to item fields because once we claim
          * that an element exists in hasNext(), we must return it in
          * the following next() call even if it was in the process of
          * being removed when hasNext() was called.
          */
         private E nextItem;
 
         /**
          * Index of element returned by most recent call to next.
          * Reset to -1 if this element is deleted by a call to remove.
          */
         private int lastRet;
 
         Itr() {
             lastRet = -1;
             if (count == 0)
                 nextIndex = -1;
             else {
                 nextIndex = takeIndex;
                 nextItem = items[takeIndex];
             }
         }
 
         @Override
         public boolean hasNext() {
             /*
              * No sync. We can return true by mistake here
              * only if this iterator passed across threads,
              * which we don't support anyway.
              */
             return nextIndex >= 0;
         }
 
         /**
          * Checks whether nextIndex is valid; if so setting nextItem.
          * Stops iterator when either hits putIndex or sees null item.
          */
         private void checkNext() {
             if (nextIndex == putIndex) {
                 nextIndex = -1;
                 nextItem = null;
             } else {
                 nextItem = items[nextIndex];
                 if (nextItem == null)
                     nextIndex = -1;
             }
         }
 
         @Override
         public E next() {
             final Monitor monitor = MonitorBasedArrayBlockingQueue.this.monitor;
             monitor.enter();
             try {
                 if (nextIndex < 0)
                     throw new NoSuchElementException();
                 lastRet = nextIndex;
                 E x = nextItem;
                 nextIndex = inc(nextIndex);
                 checkNext();
                 return x;
             } finally {
                 monitor.leave();
             }
         }
 
         @Override
         public void remove() {
             final Monitor monitor = MonitorBasedArrayBlockingQueue.this.monitor;
             monitor.enter();
             try {
                 int i = lastRet;
                 if (i == -1)
                     throw new IllegalStateException();
                 lastRet = -1;
 
                 int ti = takeIndex;
                 removeAt(i);
                 // back up cursor (reset to front if was first element)
                 nextIndex = (i == ti) ? takeIndex : i;
                 checkNext();
             } finally {
                 monitor.leave();
             }
         }
     }
 }