/*
* @(#)PriorityQueue.java 1.6 04/06/11
*
* Copyright 2004 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.util;
/**
* An unbounded priority {@linkplain Queue queue} based on a priority
* heap. This queue orders elements according to an order specified
* at construction time, which is specified either according to their
* <i>natural order</i> (see {@link Comparable}), or according to a
* {@link java.util.Comparator}, depending on which constructor is
* used. A priority queue does not permit <tt>null</tt> elements.
* A priority queue relying on natural ordering also does not
* permit insertion of non-comparable objects (doing so may result
* in <tt>ClassCastException</tt>).
*
* <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 <tt>poll</tt>,
* <tt>remove</tt>, <tt>peek</tt>, and <tt>element</tt> 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 PriorityQueue in any
* particular order. If you need ordered traversal, consider using
* <tt>Arrays.sort(pq.toArray())</tt>.
*
* <p> <strong>Note that this implementation is not synchronized.</strong>
* Multiple threads should not access a <tt>PriorityQueue</tt>
* instance concurrently if any of the threads modifies the list
* structurally. Instead, use the thread-safe {@link
* java.util.concurrent.PriorityBlockingQueue} class.
*
*
* <p>Implementation note: this implementation provides O(log(n)) time
* for the insertion methods (<tt>offer</tt>, <tt>poll</tt>,
* <tt>remove()</tt> and <tt>add</tt>) methods; linear time for the
* <tt>remove(Object)</tt> and <tt>contains(Object)</tt> methods; and
* constant time for the retrieval methods (<tt>peek</tt>,
* <tt>element</tt>, and <tt>size</tt>).
*
* <p>This class is a member of the
* <a href="{@docRoot}/../guide/collections/index.html">
* Java Collections Framework</a>.
* @since 1.5
* @version 1.6, 06/11/04
* @author Josh Bloch
* @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] 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[1], assuming the queue is
* nonempty. (A one-based array is used in preference to the traditional
* zero-based array to simplify parent and child calculations.)
*
* queue.length must be >= 2, even if size == 0.
*/
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 <tt>PriorityQueue</tt> with the default initial capacity
* (11) that orders its elements according to their natural
* ordering (using <tt>Comparable</tt>).
*/
public PriorityQueue() {
this(DEFAULT_INITIAL_CAPACITY, null);
}
/**
* Creates a <tt>PriorityQueue</tt> with the specified initial capacity
* that orders its elements according to their natural ordering
* (using <tt>Comparable</tt>).
*
* @param initialCapacity the initial capacity for this priority queue.
* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
* than 1
*/
public PriorityQueue(int initialCapacity) {
this(initialCapacity, null);
}
/**
* Creates a <tt>PriorityQueue</tt> 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 used to order this priority queue.
* If <tt>null</tt> then the order depends on the elements' natural
* ordering.
* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
* than 1
*/
public PriorityQueue(int initialCapacity,
Comparator<? super E> comparator) {
if (initialCapacity < 1)
throw new IllegalArgumentException();
this.queue = new Object[initialCapacity + 1];
this.comparator = comparator;
}
/**
* Common code to initialize underlying queue array across
* constructors below.
*/
private void initializeArray(Collection<? extends E> c) {
int sz = c.size();
int initialCapacity = (int)Math.min((sz * 110L) / 100,
Integer.MAX_VALUE - 1);
if (initialCapacity < 1)
initialCapacity = 1;
this.queue = new Object[initialCapacity + 1];
}
/**
* Initially fill elements of the queue array under the
* knowledge that it is sorted or is another PQ, in which
* case we can just place the elements in the order presented.
*/
private void fillFromSorted(Collection<? extends E> c) {
for (Iterator<? extends E> i = c.iterator(); i.hasNext(); )
queue[++size] = i.next();
}
/**
* Initially fill elements of the queue array that is not to our knowledge
* sorted, so we must rearrange the elements to guarantee the heap
* invariant.
*/
private void fillFromUnsorted(Collection<? extends E> c) {
for (Iterator<? extends E> i = c.iterator(); i.hasNext(); )
queue[++size] = i.next();
heapify();
}
/**
* Creates a <tt>PriorityQueue</tt> containing the elements in the
* specified collection. The priority queue has an initial
* capacity of 110% of the size of the specified collection or 1
* if the collection is empty. If the specified collection is an
* instance of a {@link java.util.SortedSet} or is another
* <tt>PriorityQueue</tt>, the priority queue will be sorted
* according to the same comparator, or according to its elements'
* natural order if the collection is sorted according to its
* elements' natural order. Otherwise, the priority queue is
* ordered according to its elements' natural order.
*
* @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 <tt>c</tt> or any element within it
* is <tt>null</tt>
*/
public PriorityQueue(Collection<? extends E> c) {
initializeArray(c);
if (c instanceof SortedSet) {
SortedSet<? extends E> s = (SortedSet<? extends E>)c;
comparator = (Comparator<? super E>)s.comparator();
fillFromSorted(s);
} else if (c instanceof PriorityQueue) {
PriorityQueue<? extends E> s = (PriorityQueue<? extends E>) c;
comparator = (Comparator<? super E>)s.comparator();
fillFromSorted(s);
} else {
comparator = null;
fillFromUnsorted(c);
}
}
/**
* Creates a <tt>PriorityQueue</tt> containing the elements in the
* specified collection. The priority queue has an initial
* capacity of 110% of the size of the specified collection or 1
* if the collection is empty. This priority queue will be sorted
* according to the same comparator as the given collection, or
* according to its elements' natural order if the collection is
* sorted according to its elements' natural order.
*
* @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 <tt>c</tt> or any element within it
* is <tt>null</tt>
*/
public PriorityQueue(PriorityQueue<? extends E> c) {
initializeArray(c);
comparator = (Comparator<? super E>)c.comparator();
fillFromSorted(c);
}
/**
* Creates a <tt>PriorityQueue</tt> containing the elements in the
* specified collection. The priority queue has an initial
* capacity of 110% of the size of the specified collection or 1
* if the collection is empty. This priority queue will be sorted
* according to the same comparator as the given collection, or
* according to its elements' natural order if the collection is
* sorted according to its elements' natural order.
*
* @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 <tt>c</tt> or any element within it
* is <tt>null</tt>
*/
public PriorityQueue(SortedSet<? extends E> c) {
initializeArray(c);
comparator = (Comparator<? super E>)c.comparator();
fillFromSorted(c);
}
/**
* Resize array, if necessary, to be able to hold given index
*/
private void grow(int index) {
int newlen = queue.length;
if (index < newlen) // don't need to grow
return;
if (index == Integer.MAX_VALUE)
throw new OutOfMemoryError();
while (newlen <= index) {
if (newlen >= Integer.MAX_VALUE / 2) // avoid overflow
newlen = Integer.MAX_VALUE;
else
newlen <<= 2;
}
Object[] newQueue = new Object[newlen];
System.arraycopy(queue, 0, newQueue, 0, queue.length);
queue = newQueue;
}
/**
* Inserts the specified element into this priority queue.
*
* @return <tt>true</tt>
* @throws ClassCastException if the specified element cannot be compared
* with elements currently in the priority queue according
* to the priority queue's ordering.
* @throws NullPointerException if the specified element is <tt>null</tt>.
*/
public boolean offer(E o) {
if (o == null)
throw new NullPointerException();
modCount++;
++size;
// Grow backing store if necessary
if (size >= queue.length)
grow(size);
queue[size] = o;
fixUp(size);
return true;
}
public E peek() {
if (size == 0)
return null;
return (E) queue[1];
}
// Collection Methods - the first two override to update docs
/**
* Adds the specified element to this queue.
* @return <tt>true</tt> (as per the general contract of
* <tt>Collection.add</tt>).
*
* @throws NullPointerException if the specified element is <tt>null</tt>.
* @throws ClassCastException if the specified element cannot be compared
* with elements currently in the priority queue according
* to the priority queue's ordering.
*/
public boolean add(E o) {
return offer(o);
}
/**
* Removes a single instance of the specified element from this
* queue, if it is present.
*/
public boolean remove(Object o) {
if (o == null)
return false;
if (comparator == null) {
for (int i = 1; i <= size; i++) {
if (((Comparable<E>)queue[i]).compareTo((E)o) == 0) {
removeAt(i);
return true;
}
}
} else {
for (int i = 1; i <= size; i++) {
if (comparator.compare((E)queue[i], (E)o) == 0) {
removeAt(i);
return true;
}
}
}
return false;
}
/**
* 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 class Itr implements Iterator<E> {
/**
* Index (into queue array) of element to be returned by
* subsequent call to next.
*/
private int cursor = 1;
/**
* Index of element returned by most recent call to next,
* unless that element came from the forgetMeNot list.
* Reset to 0 if element is deleted by a call to remove.
*/
private int lastRet = 0;
/**
* The modCount value that the iterator believes that the backing
* List should have. If this expectation is violated, the iterator
* has detected concurrent modification.
*/
private int expectedModCount = modCount;
/**
* A list 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 fixup instead of a fixdown.) 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 use need to store elements in this field.
*/
private ArrayList<E> forgetMeNot = null;
/**
* Element returned by the most recent call to next iff that
* element was drawn from the forgetMeNot list.
*/
private Object lastRetElt = null;
public boolean hasNext() {
return cursor <= size || forgetMeNot != null;
}
public E next() {
checkForComodification();
E result;
if (cursor <= size) {
result = (E) queue[cursor];
lastRet = cursor++;
}
else if (forgetMeNot == null)
throw new NoSuchElementException();
else {
int remaining = forgetMeNot.size();
result = forgetMeNot.remove(remaining - 1);
if (remaining == 1)
forgetMeNot = null;
lastRet = 0;
lastRetElt = result;
}
return result;
}
public void remove() {
checkForComodification();
if (lastRet != 0) {
E moved = PriorityQueue.this.removeAt(lastRet);
lastRet = 0;
if (moved == null) {
cursor--;
} else {
if (forgetMeNot == null)
forgetMeNot = new ArrayList<E>();
forgetMeNot.add(moved);
}
} else if (lastRetElt != null) {
PriorityQueue.this.remove(lastRetElt);
lastRetElt = null;
} else {
throw new IllegalStateException();
}
expectedModCount = modCount;
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
public int size() {
return size;
}
/**
* Removes all elements from the priority queue.
* The queue will be empty after this call returns.
*/
public void clear() {
modCount++;
// Null out element references to prevent memory leak
for (int i=1; i<=size; i++)
queue[i] = null;
size = 0;
}
public E poll() {
if (size == 0)
return null;
modCount++;
E result = (E) queue[1];
queue[1] = queue[size];
queue[size--] = null; // Drop extra ref to prevent memory leak
if (size > 1)
fixDown(1);
return result;
}
/**
* Removes and returns the ith element from queue. (Recall that queue
* is one-based, so 1 <= i <= size.)
*
* Normally this method leaves the elements at positions from 1 up to i-1,
* inclusive, untouched. Under these circumstances, it returns null.
* Occasionally, in order to maintain the heap invariant, it must move
* the last element of the list to some index in the range [2, i-1],
* and move the element previously at position (i/2) to position i.
* Under these circumstances, this method returns the element that was
* previously at the end of the list and is now at some position between
* 2 and i-1 inclusive.
*/
private E removeAt(int i) {
assert i > 0 && i <= size;
modCount++;
E moved = (E) queue[size];
queue[i] = moved;
queue[size--] = null; // Drop extra ref to prevent memory leak
if (i <= size) {
fixDown(i);
if (queue[i] == moved) {
fixUp(i);
if (queue[i] != moved)
return moved;
}
}
return null;
}
/**
* Establishes the heap invariant (described above) assuming the heap
* satisfies the invariant except possibly for the leaf-node indexed by k
* (which may have a nextExecutionTime less than its parent's).
*
* This method functions by "promoting" queue[k] up the hierarchy
* (by swapping it with its parent) repeatedly until queue[k]
* is greater than or equal to its parent.
*/
private void fixUp(int k) {
if (comparator == null) {
while (k > 1) {
int j = k >> 1;
if (((Comparable<E>)queue[j]).compareTo((E)queue[k]) <= 0)
break;
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
k = j;
}
} else {
while (k > 1) {
int j = k >>> 1;
if (comparator.compare((E)queue[j], (E)queue[k]) <= 0)
break;
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
k = j;
}
}
}
/**
* Establishes the heap invariant (described above) in the subtree
* rooted at k, which is assumed to satisfy the heap invariant except
* possibly for node k itself (which may be greater than its children).
*
* This method functions by "demoting" queue[k] down the hierarchy
* (by swapping it with its smaller child) repeatedly until queue[k]
* is less than or equal to its children.
*/
private void fixDown(int k) {
int j;
if (comparator == null) {
while ((j = k << 1) <= size && (j > 0)) {
if (j<size &&
((Comparable<E>)queue[j]).compareTo((E)queue[j+1]) > 0)
j++; // j indexes smallest kid
if (((Comparable<E>)queue[k]).compareTo((E)queue[j]) <= 0)
break;
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
k = j;
}
} else {
while ((j = k << 1) <= size && (j > 0)) {
if (j<size &&
comparator.compare((E)queue[j], (E)queue[j+1]) > 0)
j++; // j indexes smallest kid
if (comparator.compare((E)queue[k], (E)queue[j]) <= 0)
break;
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
k = j;
}
}
}
/**
* 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/2; i >= 1; i--)
fixDown(i);
}
/**
* Returns the comparator used to order this collection, or <tt>null</tt>
* if this collection is sorted according to its elements natural ordering
* (using <tt>Comparable</tt>).
*
* @return the comparator used to order this collection, or <tt>null</tt>
* if this collection is sorted according to its elements natural ordering.
*/
public Comparator<? super E> comparator() {
return comparator;
}
/**
* Save the state of the instance to a stream (that
* is, serialize it).
*
* @serialData The length of the array backing the instance is
* emitted (int), followed by all of its elements (each an
* <tt>Object</tt>) 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
s.writeInt(queue.length);
// Write out all elements in the proper order.
for (int i=1; i<=size; i++)
s.writeObject(queue[i]);
}
/**
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
* deserialize 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 array length and allocate array
int arrayLength = s.readInt();
queue = new Object[arrayLength];
// Read in all elements in the proper order.
for (int i=1; i<=size; i++)
queue[i] = (E) s.readObject();
}
}
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