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  *

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 package java.util;

 /**

  * An unbounded priority {@linkplain Queue queue} based on a priority heap.

  * The elements of the priority queue are ordered according to their

  * {@linkplain Comparable natural ordering}, or by a {@link Comparator}

  * provided at queue construction time, depending on which constructor is

  * used.  A priority queue does not permit {@code null} elements.

  * A priority queue relying on natural ordering also does not permit

  * insertion of non-comparable objects (doing so may result in

  * {@code ClassCastException}).

  *

  * <p>The <em>head</em> of this queue is the <em>least</em> element

  * with respect to the specified ordering.  If multiple elements are

  * tied for least value, the head is one of those elements -- ties are

  * broken arbitrarily.  The queue retrieval operations {@code poll},

  * {@code remove}, {@code peek}, and {@code element} access the

  * element at the head of the queue.

  *

  * <p>A priority queue is unbounded, but has an internal

  * <i>capacity</i> governing the size of an array used to store the

  * elements on the queue.  It is always at least as large as the queue

  * size.  As elements are added to a priority queue, its capacity

  * grows automatically.  The details of the growth policy are not

  * specified.

  *

  * <p>This class and its iterator implement all of the

  * <em>optional</em> methods of the {@link Collection} and {@link

  * Iterator} interfaces.  The Iterator provided in method {@link

  * #iterator()} is <em>not</em> guaranteed to traverse the elements of

  * the priority queue in any particular order. If you need ordered

  * traversal, consider using {@code Arrays.sort(pq.toArray())}.

  *

  * <p> <strong>Note that this implementation is not synchronized.</strong>

  * Multiple threads should not access a {@code PriorityQueue}

  * instance concurrently if any of the threads modifies the queue.

  * Instead, use the thread-safe {@link

  * java.util.concurrent.PriorityBlockingQueue} class.

  *

  * <p>Implementation note: this implementation provides

  * O(log(n)) time for the enqueing and dequeing methods

  * ({@code offer}, {@code poll}, {@code remove()} and {@code add});

  * linear time for the {@code remove(Object)} and {@code contains(Object)}

  * methods; and constant time for the retrieval methods

  * ({@code peek}, {@code element}, and {@code size}).

  *

  * <p>This class is a member of the

  * <a href="{@docRoot}/../technotes/guides/collections/index.html">

  * Java Collections Framework</a>.

  *

  * @since 1.5

  * @author Josh Bloch, Doug Lea

  * @param <E> the type of elements held in this collection

  */

 public class PriorityQueue<E> extends AbstractQueue<E>

     implements java.io.Serializable {

     private static final long serialVersionUID = -7720805057305804111L;

     private static final int DEFAULT_INITIAL_CAPACITY = 11;

     /**

      * Priority queue represented as a balanced binary heap: the two

      * children of queue[n] are queue[2*n+1] and queue[2*(n+1)].  The

      * priority queue is ordered by comparator, or by the elements'

      * natural ordering, if comparator is null: For each node n in the

      * heap and each descendant d of n, n <= d.  The element with the

      * lowest value is in queue[0], assuming the queue is nonempty.

      */

     private transient Object[] queue;

     /**

      * The number of elements in the priority queue.

      */

     private int size = 0;

     /**

      * The comparator, or null if priority queue uses elements'

      * natural ordering.

      */

     private final Comparator<? super E> comparator;

     /**

      * The number of times this priority queue has been

      * <i>structurally modified</i>.  See AbstractList for gory details.

      */

     private transient int modCount = 0;

     /**

      * Creates a {@code PriorityQueue} with the default initial

      * capacity (11) that orders its elements according to their

      * {@linkplain Comparable natural ordering}.

      */

     public PriorityQueue() {

         this(DEFAULT_INITIAL_CAPACITY, null);

     }

     /**

      * Creates a {@code PriorityQueue} with the specified initial

      * capacity that orders its elements according to their

      * {@linkplain Comparable natural ordering}.

      *

      * @param initialCapacity the initial capacity for this priority queue

      * @throws IllegalArgumentException if {@code initialCapacity} is less

      *         than 1

      */

     public PriorityQueue(int initialCapacity) {

         this(initialCapacity, null);

     }

     /**

      * Creates a {@code PriorityQueue} with the specified initial capacity

      * that orders its elements according to the specified comparator.

      *

      * @param  initialCapacity the initial capacity for this priority queue

      * @param  comparator the comparator that will be used to order this

      *         priority queue.  If {@code null}, the {@linkplain Comparable

      *         natural ordering} of the elements will be used.

      * @throws IllegalArgumentException if {@code initialCapacity} is

      *         less than 1

      */

     public PriorityQueue(int initialCapacity,

                          Comparator<? super E> comparator) {

         // Note: This restriction of at least one is not actually needed,

         // but continues for 1.5 compatibility

         if (initialCapacity < 1)

             throw new IllegalArgumentException();

         this.queue = new Object[initialCapacity];

         this.comparator = comparator;

     }

     /**

      * Creates a {@code PriorityQueue} containing the elements in the

      * specified collection.  If the specified collection is an instance of

      * a {@link SortedSet} or is another {@code PriorityQueue}, this

      * priority queue will be ordered according to the same ordering.

      * Otherwise, this priority queue will be ordered according to the

      * {@linkplain Comparable natural ordering} of its elements.

      *

      * @param  c the collection whose elements are to be placed

      *         into this priority queue

      * @throws ClassCastException if elements of the specified collection

      *         cannot be compared to one another according to the priority

      *         queue's ordering

      * @throws NullPointerException if the specified collection or any

      *         of its elements are null

      */

     @SuppressWarnings("unchecked")

     public PriorityQueue(Collection<? extends E> c) {

         if (c instanceof SortedSet<?>) {

             SortedSet<? extends E> ss = (SortedSet<? extends E>) c;

             this.comparator = (Comparator<? super E>) ss.comparator();

             initElementsFromCollection(ss);

         }

         else if (c instanceof PriorityQueue<?>) {

             PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;

             this.comparator = (Comparator<? super E>) pq.comparator();

             initFromPriorityQueue(pq);

         }

         else {

             this.comparator = null;

             initFromCollection(c);

         }

     }

     /**

      * Creates a {@code PriorityQueue} containing the elements in the

      * specified priority queue.  This priority queue will be

      * ordered according to the same ordering as the given priority

      * queue.

      *

      * @param  c the priority queue whose elements are to be placed

      *         into this priority queue

      * @throws ClassCastException if elements of {@code c} cannot be

      *         compared to one another according to {@code c}'s

      *         ordering

      * @throws NullPointerException if the specified priority queue or any

      *         of its elements are null

      */

     @SuppressWarnings("unchecked")

     public PriorityQueue(PriorityQueue<? extends E> c) {

         this.comparator = (Comparator<? super E>) c.comparator();

         initFromPriorityQueue(c);

     }

     /**

      * Creates a {@code PriorityQueue} containing the elements in the

      * specified sorted set.   This priority queue will be ordered

      * according to the same ordering as the given sorted set.

      *

      * @param  c the sorted set whose elements are to be placed

      *         into this priority queue

      * @throws ClassCastException if elements of the specified sorted

      *         set cannot be compared to one another according to the

      *         sorted set's ordering

      * @throws NullPointerException if the specified sorted set or any

      *         of its elements are null

      */

     @SuppressWarnings("unchecked")

     public PriorityQueue(SortedSet<? extends E> c) {

         this.comparator = (Comparator<? super E>) c.comparator();

         initElementsFromCollection(c);

     }

     private void initFromPriorityQueue(PriorityQueue<? extends E> c) {

         if (c.getClass() == PriorityQueue.class) {

             this.queue = c.toArray();

             this.size = c.size();

         } else {

             initFromCollection(c);

         }

     }

     private void initElementsFromCollection(Collection<? extends E> c) {

         Object[] a = c.toArray();

         // If c.toArray incorrectly doesn't return Object[], copy it.

         if (a.getClass() != Object[].class)

             a = Arrays.copyOf(a, a.length, Object[].class);

         int len = a.length;

         if (len == 1 || this.comparator != null)

             for (int i = 0; i < len; i++)

                 if (a[i] == null)

                     throw new NullPointerException();

         this.queue = a;

         this.size = a.length;

     }

     /**

      * Initializes queue array with elements from the given Collection.

      *

      * @param c the collection

      */

     private void initFromCollection(Collection<? extends E> c) {

         initElementsFromCollection(c);

         heapify();

     }

     /**

      * The maximum size of array to allocate.

      * Some VMs reserve some header words in an array.

      * Attempts to allocate larger arrays may result in

      * OutOfMemoryError: Requested array size exceeds VM limit

      */

     private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

     /**

      * Increases the capacity of the array.

      *

      * @param minCapacity the desired minimum capacity

      */

     private void grow(int minCapacity) {

         int oldCapacity = queue.length;

         // Double size if small; else grow by 50%

         int newCapacity = oldCapacity + ((oldCapacity < 64) ?

                                          (oldCapacity + 2) :

                                          (oldCapacity >> 1));

         // overflow-conscious code

         if (newCapacity - MAX_ARRAY_SIZE > 0)

             newCapacity = hugeCapacity(minCapacity);

         queue = Arrays.copyOf(queue, newCapacity);

     }

     private static int hugeCapacity(int minCapacity) {

         if (minCapacity < 0) // overflow

             throw new OutOfMemoryError();

         return (minCapacity > MAX_ARRAY_SIZE) ?

             Integer.MAX_VALUE :

             MAX_ARRAY_SIZE;

     }

     /**

      * Inserts the specified element into this priority queue.

      *

      * @return {@code true} (as specified by {@link Collection#add})

      * @throws ClassCastException if the specified element cannot be

      *         compared with elements currently in this priority queue

      *         according to the priority queue's ordering

      * @throws NullPointerException if the specified element is null

      */

     public boolean add(E e) {

         return offer(e);

     }

     /**

      * Inserts the specified element into this priority queue.

      *

      * @return {@code true} (as specified by {@link Queue#offer})

      * @throws ClassCastException if the specified element cannot be

      *         compared with elements currently in this priority queue

      *         according to the priority queue's ordering

      * @throws NullPointerException if the specified element is null

      */

     public boolean offer(E e) {

         if (e == null)

             throw new NullPointerException();

         modCount++;

         int i = size;

         if (i >= queue.length)

             grow(i + 1);

         size = i + 1;

         if (i == 0)

             queue[0] = e;

         else

             siftUp(i, e);

         return true;

     }

     public E peek() {

         if (size == 0)

             return null;

         return (E) queue[0];

     }

     private int indexOf(Object o) {

         if (o != null) {

             for (int i = 0; i < size; i++)

                 if (o.equals(queue[i]))

                     return i;

         }

         return -1;

     }

     /**

      * Removes a single instance of the specified element from this queue,

      * if it is present.  More formally, removes an element {@code e} such

      * that {@code o.equals(e)}, if this queue contains one or more such

      * elements.  Returns {@code true} if and only 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 {@code true} if this queue changed as a result of the call

      */

     public boolean remove(Object o) {

         int i = indexOf(o);

         if (i == -1)

             return false;

         else {

             removeAt(i);

             return true;

         }

     }

     /**

      * Version of remove using reference equality, not equals.

      * Needed by iterator.remove.

      *

      * @param o element to be removed from this queue, if present

      * @return {@code true} if removed

      */

     boolean removeEq(Object o) {

         for (int i = 0; i < size; i++) {

             if (o == queue[i]) {

                 removeAt(i);

                 return true;

             }

         }

         return false;

     }

     /**

      * Returns {@code true} if this queue contains the specified element.

      * More formally, returns {@code true} if and only if this queue contains

      * at least one element {@code e} such that {@code o.equals(e)}.

      *

      * @param o object to be checked for containment in this queue

      * @return {@code true} if this queue contains the specified element

      */

     public boolean contains(Object o) {

         return indexOf(o) != -1;

     }

     /**

      * Returns an array containing all of the elements in this queue.

      * The elements are in no particular order.

      *

      * <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

      */

     public Object[] toArray() {

         return Arrays.copyOf(queue, size);

     }

     /**

      * Returns an array containing all of the elements in this queue; the

      * runtime type of the returned array is that of the specified array.

      * The returned array elements are in no particular order.

      * 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 the queue fits in the specified array with room to spare

      * (i.e., the array has more elements than the queue), the element in

      * the array immediately following the end of the collection is set to

      * {@code null}.

      *

      * <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>

      *

      * 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

      */

     public <T> T[] toArray(T[] a) {

         if (a.length < size)

             // Make a new array of a's runtime type, but my contents:

             return (T[]) Arrays.copyOf(queue, size, a.getClass());

         System.arraycopy(queue, 0, a, 0, size);

         if (a.length > size)

             a[size] = null;

         return a;

     }

     /**

      * Returns an iterator over the elements in this queue. The iterator

      * does not return the elements in any particular order.

      *

      * @return an iterator over the elements in this queue

      */

     public Iterator<E> iterator() {

         return new Itr();

     }

     private final class Itr implements Iterator<E> {

         /**

          * Index (into queue array) of element to be returned by

          * subsequent call to next.

          */

         private int cursor = 0;

         /**

          * Index of element returned by most recent call to next,

          * unless that element came from the forgetMeNot list.

          * Set to -1 if element is deleted by a call to remove.

          */

         private int lastRet = -1;

         /**

          * A queue of elements that were moved from the unvisited portion of

          * the heap into the visited portion as a result of "unlucky" element

          * removals during the iteration.  (Unlucky element removals are those

          * that require a siftup instead of a siftdown.)  We must visit all of

          * the elements in this list to complete the iteration.  We do this

          * after we've completed the "normal" iteration.

          *

          * We expect that most iterations, even those involving removals,

          * will not need to store elements in this field.

          */

         private ArrayDeque<E> forgetMeNot = null;

         /**

          * Element returned by the most recent call to next iff that

          * element was drawn from the forgetMeNot list.

          */

         private E lastRetElt = null;

         /**

          * The modCount value that the iterator believes that the backing

          * Queue should have.  If this expectation is violated, the iterator

          * has detected concurrent modification.

          */

         private int expectedModCount = modCount;

         public boolean hasNext() {

             return cursor < size ||

                 (forgetMeNot != null && !forgetMeNot.isEmpty());

         }

         public E next() {

             if (expectedModCount != modCount)

                 throw new ConcurrentModificationException();

             if (cursor < size)

                 return (E) queue[lastRet = cursor++];

             if (forgetMeNot != null) {

                 lastRet = -1;

                 lastRetElt = forgetMeNot.poll();

                 if (lastRetElt != null)

                     return lastRetElt;

             }

             throw new NoSuchElementException();

         }

         public void remove() {

             if (expectedModCount != modCount)

                 throw new ConcurrentModificationException();

             if (lastRet != -1) {

                 E moved = PriorityQueue.this.removeAt(lastRet);

                 lastRet = -1;

                 if (moved == null)

                     cursor--;

                 else {

                     if (forgetMeNot == null)

                         forgetMeNot = new ArrayDeque<>();

                     forgetMeNot.add(moved);

                 }

             } else if (lastRetElt != null) {

                 PriorityQueue.this.removeEq(lastRetElt);

                 lastRetElt = null;

             } else {

                 throw new IllegalStateException();

             }

             expectedModCount = modCount;

         }

     }

     public int size() {

         return size;

     }

     /**

      * Removes all of the elements from this priority queue.

      * The queue will be empty after this call returns.

      */

     public void clear() {

         modCount++;

         for (int i = 0; i < size; i++)

             queue[i] = null;

         size = 0;

     }

     public E poll() {

         if (size == 0)

             return null;

         int s = --size;

         modCount++;

         E result = (E) queue[0];

         E x = (E) queue[s];

         queue[s] = null;

         if (s != 0)

             siftDown(0, x);

         return result;

     }

     /**

      * Removes the ith element from queue.

      *

      * Normally this method leaves the elements at up to i-1,

      * inclusive, untouched.  Under these circumstances, it returns

      * null.  Occasionally, in order to maintain the heap invariant,

      * it must swap a later element of the list with one earlier than

      * i.  Under these circumstances, this method returns the element

      * that was previously at the end of the list and is now at some

      * position before i. This fact is used by iterator.remove so as to

      * avoid missing traversing elements.

      */

     private E removeAt(int i) {

         assert i >= 0 && i < size;

         modCount++;

         int s = --size;

         if (s == i) // removed last element

             queue[i] = null;

         else {

             E moved = (E) queue[s];

             queue[s] = null;

             siftDown(i, moved);

             if (queue[i] == moved) {

                 siftUp(i, moved);

                 if (queue[i] != moved)

                     return moved;

             }

         }

         return null;

     }

     /**

      * Inserts item x at position k, maintaining heap invariant by

      * promoting x up the tree until it is greater than or equal to

      * its parent, or is the root.

      *

      * To simplify and speed up coercions and comparisons. the

      * Comparable and Comparator versions are separated into different

      * methods that are otherwise identical. (Similarly for siftDown.)

      *

      * @param k the position to fill

      * @param x the item to insert

      */

     private void siftUp(int k, E x) {

         if (comparator != null)

             siftUpUsingComparator(k, x);

         else

             siftUpComparable(k, x);

     }

     private void siftUpComparable(int k, E x) {

         Comparable<? super E> key = (Comparable<? super E>) x;

         while (k > 0) {

             int parent = (k - 1) >>> 1;

             Object e = queue[parent];

             if (key.compareTo((E) e) >= 0)

                 break;

             queue[k] = e;

             k = parent;

         }

         queue[k] = key;

     }

     private void siftUpUsingComparator(int k, E x) {

         while (k > 0) {

             int parent = (k - 1) >>> 1;

             Object e = queue[parent];

             if (comparator.compare(x, (E) e) >= 0)

                 break;

             queue[k] = e;

             k = parent;

         }

         queue[k] = x;

     }

     /**

      * Inserts item x at position k, maintaining heap invariant by

      * demoting x down the tree repeatedly until it is less than or

      * equal to its children or is a leaf.

      *

      * @param k the position to fill

      * @param x the item to insert

      */

     private void siftDown(int k, E x) {

         if (comparator != null)

             siftDownUsingComparator(k, x);

         else

             siftDownComparable(k, x);

     }

     private void siftDownComparable(int k, E x) {

         Comparable<? super E> key = (Comparable<? super E>)x;

         int half = size >>> 1;        // loop while a non-leaf

         while (k < half) {

             int child = (k << 1) + 1; // assume left child is least

             Object c = queue[child];

             int right = child + 1;

             if (right < size &&

                 ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)

                 c = queue[child = right];

             if (key.compareTo((E) c) <= 0)

                 break;

             queue[k] = c;

             k = child;

         }

         queue[k] = key;

     }

     private void siftDownUsingComparator(int k, E x) {

         int half = size >>> 1;

         while (k < half) {

             int child = (k << 1) + 1;

             Object c = queue[child];

             int right = child + 1;

             if (right < size &&

                 comparator.compare((E) c, (E) queue[right]) > 0)

                 c = queue[child = right];

             if (comparator.compare(x, (E) c) <= 0)

                 break;

             queue[k] = c;

             k = child;

         }

         queue[k] = x;

     }

     /**

      * Establishes the heap invariant (described above) in the entire tree,

      * assuming nothing about the order of the elements prior to the call.

      */

     private void heapify() {

         for (int i = (size >>> 1) - 1; i >= 0; i--)

             siftDown(i, (E) queue[i]);

     }

     /**

      * Returns the comparator used to order the elements in this

      * queue, or {@code null} if this queue is sorted according to

      * the {@linkplain Comparable natural ordering} of its elements.

      *

      * @return the comparator used to order this queue, or

      *         {@code null} if this queue is sorted according to the

      *         natural ordering of its elements

      */

     public Comparator<? super E> comparator() {

         return comparator;

     }

     /**

      * Saves the state of the instance to a stream (that

      * is, serializes it).

      *

      * @serialData The length of the array backing the instance is

      *             emitted (int), followed by all of its elements

      *             (each an {@code Object}) in the proper order.

      * @param s the stream

      */

     private void writeObject(java.io.ObjectOutputStream s)

         throws java.io.IOException{

         // Write out element count, and any hidden stuff

         s.defaultWriteObject();

         // Write out array length, for compatibility with 1.5 version

         s.writeInt(Math.max(2, size + 1));

         // Write out all elements in the "proper order".

         for (int i = 0; i < size; i++)

             s.writeObject(queue[i]);

     }

     /**

      * Reconstitutes the {@code PriorityQueue} instance from a stream

      * (that is, deserializes it).

      *

      * @param s the stream

      */

     private void readObject(java.io.ObjectInputStream s)

         throws java.io.IOException, ClassNotFoundException {

         // Read in size, and any hidden stuff

         s.defaultReadObject();

         // Read in (and discard) array length

         s.readInt();

         queue = new Object[size];

         // Read in all elements.

         for (int i = 0; i < size; i++)

             queue[i] = s.readObject();

         // Elements are guaranteed to be in "proper order", but the

         // spec has never explained what that might be.

         heapify();

     }

 }