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/* * %W% %E% * * 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 %I%, %G% * @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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