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/* * @(#)PriorityQueue.java 1.8 05/08/27 * * Copyright 2005 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.util; import java.util.*; // for javadoc (till 6280605 is fixed) /** * 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 * <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 priority queue 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 1.8, 08/27/05 * @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 * {@linkplain Comparable natural ordering}. */ public PriorityQueue() { this(DEFAULT_INITIAL_CAPACITY, null); } /** * Creates a <tt>PriorityQueue</tt> 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 <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 that will be used to order * this priority queue. If <tt>null</tt>, the <i>natural * ordering</i> of the elements will be used. * @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(); ) { int k = ++size; if (k >= queue.length) grow(k); queue[k] = 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(); ) { int k = ++size; if (k >= queue.length) grow(k); queue[k] = 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 ordered * according to the same ordering. Otherwise, this priority queue * will be ordered according to the 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 */ 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 priority queue. The priority queue has an initial * capacity of 110% of the size of the specified priority queue or * 1 if the priority queue is empty. 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 <tt>c</tt> cannot be * compared to one another according to <tt>c</tt>'s * ordering * @throws NullPointerException if the specified priority queue or any * of its elements are null */ 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 sorted set. The priority queue has an initial * capacity of 110% of the size of the specified sorted set or 1 * if the sorted set is empty. 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 */ 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; } queue = Arrays.copyOf(queue, newlen); } /** * Inserts the specified element into this priority queue. * * @return <tt>true</tt> (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 <tt>true</tt> (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++; ++size; // Grow backing store if necessary if (size >= queue.length) grow(size); queue[size] = e; fixUp(size); return true; } public E peek() { if (size == 0) return null; return (E) queue[1]; } private int indexOf(Object o) { if (o == null) return -1; for (int i = 1; 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 <tt>e</tt> such * that <tt>o.equals(e)</tt>, if this queue contains one or more such * elements. Returns true 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 */ public boolean remove(Object o) { int i = indexOf(o); if (i == -1) return false; else { removeAt(i); return true; } } /** * 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 */ 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 list. (In other words, this method must allocate * a new array). The caller is thus free to modify the returned array. * * @return an array containing all of the elements in this queue. */ public Object[] toArray() { return Arrays.copyOfRange(queue, 1, size+1); } /** * Returns an array containing all of the elements in this queue. * The elements are in no particular order. 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 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 * <tt>null</tt>. (This is useful in determining the length of the * queue <i>only</i> if the caller knows that the queue does not contain * any null elements.) * * @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 the elements of the 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.copyOfRange(queue, 1, size+1, a.getClass()); System.arraycopy(queue, 1, 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 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 of the elements from this 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<? super 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<? super E>)queue[j]).compareTo((E)queue[j+1]) > 0) j++; // j indexes smallest kid if (((Comparable<? super 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 the elements in this * queue, or <tt>null</tt> if this queue is sorted according to * the {@linkplain Comparable natural ordering} of its elements. * * @return the comparator used to order this queue, or * <tt>null</tt> if this queue is sorted according to the * natural ordering of its elements. */ 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>PriorityQueue</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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