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28 import org.apidesign.bck2brwsr.emul.lang.System;
31 * An unbounded priority {@linkplain Queue queue} based on a priority heap.
32 * The elements of the priority queue are ordered according to their
33 * {@linkplain Comparable natural ordering}, or by a {@link Comparator}
34 * provided at queue construction time, depending on which constructor is
35 * used. A priority queue does not permit {@code null} elements.
36 * A priority queue relying on natural ordering also does not permit
37 * insertion of non-comparable objects (doing so may result in
38 * {@code ClassCastException}).
40 * <p>The <em>head</em> of this queue is the <em>least</em> element
41 * with respect to the specified ordering. If multiple elements are
42 * tied for least value, the head is one of those elements -- ties are
43 * broken arbitrarily. The queue retrieval operations {@code poll},
44 * {@code remove}, {@code peek}, and {@code element} access the
45 * element at the head of the queue.
47 * <p>A priority queue is unbounded, but has an internal
48 * <i>capacity</i> governing the size of an array used to store the
49 * elements on the queue. It is always at least as large as the queue
50 * size. As elements are added to a priority queue, its capacity
51 * grows automatically. The details of the growth policy are not
54 * <p>This class and its iterator implement all of the
55 * <em>optional</em> methods of the {@link Collection} and {@link
56 * Iterator} interfaces. The Iterator provided in method {@link
57 * #iterator()} is <em>not</em> guaranteed to traverse the elements of
58 * the priority queue in any particular order. If you need ordered
59 * traversal, consider using {@code Arrays.sort(pq.toArray())}.
61 * <p> <strong>Note that this implementation is not synchronized.</strong>
62 * Multiple threads should not access a {@code PriorityQueue}
63 * instance concurrently if any of the threads modifies the queue.
64 * Instead, use the thread-safe {@link
65 * java.util.concurrent.PriorityBlockingQueue} class.
67 * <p>Implementation note: this implementation provides
68 * O(log(n)) time for the enqueing and dequeing methods
69 * ({@code offer}, {@code poll}, {@code remove()} and {@code add});
70 * linear time for the {@code remove(Object)} and {@code contains(Object)}
71 * methods; and constant time for the retrieval methods
72 * ({@code peek}, {@code element}, and {@code size}).
74 * <p>This class is a member of the
75 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
76 * Java Collections Framework</a>.
79 * @author Josh Bloch, Doug Lea
80 * @param <E> the type of elements held in this collection
82 public class PriorityQueue<E> extends AbstractQueue<E>
83 implements java.io.Serializable {
85 private static final long serialVersionUID = -7720805057305804111L;
87 private static final int DEFAULT_INITIAL_CAPACITY = 11;
90 * Priority queue represented as a balanced binary heap: the two
91 * children of queue[n] are queue[2*n+1] and queue[2*(n+1)]. The
92 * priority queue is ordered by comparator, or by the elements'
93 * natural ordering, if comparator is null: For each node n in the
94 * heap and each descendant d of n, n <= d. The element with the
95 * lowest value is in queue[0], assuming the queue is nonempty.
97 private transient Object[] queue;
100 * The number of elements in the priority queue.
102 private int size = 0;
105 * The comparator, or null if priority queue uses elements'
108 private final Comparator<? super E> comparator;
111 * The number of times this priority queue has been
112 * <i>structurally modified</i>. See AbstractList for gory details.
114 private transient int modCount = 0;
117 * Creates a {@code PriorityQueue} with the default initial
118 * capacity (11) that orders its elements according to their
119 * {@linkplain Comparable natural ordering}.
121 public PriorityQueue() {
122 this(DEFAULT_INITIAL_CAPACITY, null);
126 * Creates a {@code PriorityQueue} with the specified initial
127 * capacity that orders its elements according to their
128 * {@linkplain Comparable natural ordering}.
130 * @param initialCapacity the initial capacity for this priority queue
131 * @throws IllegalArgumentException if {@code initialCapacity} is less
134 public PriorityQueue(int initialCapacity) {
135 this(initialCapacity, null);
139 * Creates a {@code PriorityQueue} with the specified initial capacity
140 * that orders its elements according to the specified comparator.
142 * @param initialCapacity the initial capacity for this priority queue
143 * @param comparator the comparator that will be used to order this
144 * priority queue. If {@code null}, the {@linkplain Comparable
145 * natural ordering} of the elements will be used.
146 * @throws IllegalArgumentException if {@code initialCapacity} is
149 public PriorityQueue(int initialCapacity,
150 Comparator<? super E> comparator) {
151 // Note: This restriction of at least one is not actually needed,
152 // but continues for 1.5 compatibility
153 if (initialCapacity < 1)
154 throw new IllegalArgumentException();
155 this.queue = new Object[initialCapacity];
156 this.comparator = comparator;
160 * Creates a {@code PriorityQueue} containing the elements in the
161 * specified collection. If the specified collection is an instance of
162 * a {@link SortedSet} or is another {@code PriorityQueue}, this
163 * priority queue will be ordered according to the same ordering.
164 * Otherwise, this priority queue will be ordered according to the
165 * {@linkplain Comparable natural ordering} of its elements.
167 * @param c the collection whose elements are to be placed
168 * into this priority queue
169 * @throws ClassCastException if elements of the specified collection
170 * cannot be compared to one another according to the priority
172 * @throws NullPointerException if the specified collection or any
173 * of its elements are null
175 @SuppressWarnings("unchecked")
176 public PriorityQueue(Collection<? extends E> c) {
177 if (c instanceof SortedSet<?>) {
178 SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
179 this.comparator = (Comparator<? super E>) ss.comparator();
180 initElementsFromCollection(ss);
182 else if (c instanceof PriorityQueue<?>) {
183 PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
184 this.comparator = (Comparator<? super E>) pq.comparator();
185 initFromPriorityQueue(pq);
188 this.comparator = null;
189 initFromCollection(c);
194 * Creates a {@code PriorityQueue} containing the elements in the
195 * specified priority queue. This priority queue will be
196 * ordered according to the same ordering as the given priority
199 * @param c the priority queue whose elements are to be placed
200 * into this priority queue
201 * @throws ClassCastException if elements of {@code c} cannot be
202 * compared to one another according to {@code c}'s
204 * @throws NullPointerException if the specified priority queue or any
205 * of its elements are null
207 @SuppressWarnings("unchecked")
208 public PriorityQueue(PriorityQueue<? extends E> c) {
209 this.comparator = (Comparator<? super E>) c.comparator();
210 initFromPriorityQueue(c);
214 * Creates a {@code PriorityQueue} containing the elements in the
215 * specified sorted set. This priority queue will be ordered
216 * according to the same ordering as the given sorted set.
218 * @param c the sorted set whose elements are to be placed
219 * into this priority queue
220 * @throws ClassCastException if elements of the specified sorted
221 * set cannot be compared to one another according to the
222 * sorted set's ordering
223 * @throws NullPointerException if the specified sorted set or any
224 * of its elements are null
226 @SuppressWarnings("unchecked")
227 public PriorityQueue(SortedSet<? extends E> c) {
228 this.comparator = (Comparator<? super E>) c.comparator();
229 initElementsFromCollection(c);
232 private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
233 if (c.getClass() == PriorityQueue.class) {
234 this.queue = c.toArray();
235 this.size = c.size();
237 initFromCollection(c);
241 private void initElementsFromCollection(Collection<? extends E> c) {
242 Object[] a = c.toArray();
243 // If c.toArray incorrectly doesn't return Object[], copy it.
244 if (a.getClass() != Object[].class)
245 a = Arrays.copyOf(a, a.length, Object[].class);
247 if (len == 1 || this.comparator != null)
248 for (int i = 0; i < len; i++)
250 throw new NullPointerException();
252 this.size = a.length;
256 * Initializes queue array with elements from the given Collection.
258 * @param c the collection
260 private void initFromCollection(Collection<? extends E> c) {
261 initElementsFromCollection(c);
266 * The maximum size of array to allocate.
267 * Some VMs reserve some header words in an array.
268 * Attempts to allocate larger arrays may result in
269 * OutOfMemoryError: Requested array size exceeds VM limit
271 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
274 * Increases the capacity of the array.
276 * @param minCapacity the desired minimum capacity
278 private void grow(int minCapacity) {
279 int oldCapacity = queue.length;
280 // Double size if small; else grow by 50%
281 int newCapacity = oldCapacity + ((oldCapacity < 64) ?
284 // overflow-conscious code
285 if (newCapacity - MAX_ARRAY_SIZE > 0)
286 newCapacity = hugeCapacity(minCapacity);
287 queue = Arrays.copyOf(queue, newCapacity);
290 private static int hugeCapacity(int minCapacity) {
291 if (minCapacity < 0) // overflow
292 throw new OutOfMemoryError();
293 return (minCapacity > MAX_ARRAY_SIZE) ?
299 * Inserts the specified element into this priority queue.
301 * @return {@code true} (as specified by {@link Collection#add})
302 * @throws ClassCastException if the specified element cannot be
303 * compared with elements currently in this priority queue
304 * according to the priority queue's ordering
305 * @throws NullPointerException if the specified element is null
307 public boolean add(E e) {
312 * Inserts the specified element into this priority queue.
314 * @return {@code true} (as specified by {@link Queue#offer})
315 * @throws ClassCastException if the specified element cannot be
316 * compared with elements currently in this priority queue
317 * according to the priority queue's ordering
318 * @throws NullPointerException if the specified element is null
320 public boolean offer(E e) {
322 throw new NullPointerException();
325 if (i >= queue.length)
341 private int indexOf(Object o) {
343 for (int i = 0; i < size; i++)
344 if (o.equals(queue[i]))
351 * Removes a single instance of the specified element from this queue,
352 * if it is present. More formally, removes an element {@code e} such
353 * that {@code o.equals(e)}, if this queue contains one or more such
354 * elements. Returns {@code true} if and only if this queue contained
355 * the specified element (or equivalently, if this queue changed as a
356 * result of the call).
358 * @param o element to be removed from this queue, if present
359 * @return {@code true} if this queue changed as a result of the call
361 public boolean remove(Object o) {
372 * Version of remove using reference equality, not equals.
373 * Needed by iterator.remove.
375 * @param o element to be removed from this queue, if present
376 * @return {@code true} if removed
378 boolean removeEq(Object o) {
379 for (int i = 0; i < size; i++) {
389 * Returns {@code true} if this queue contains the specified element.
390 * More formally, returns {@code true} if and only if this queue contains
391 * at least one element {@code e} such that {@code o.equals(e)}.
393 * @param o object to be checked for containment in this queue
394 * @return {@code true} if this queue contains the specified element
396 public boolean contains(Object o) {
397 return indexOf(o) != -1;
401 * Returns an array containing all of the elements in this queue.
402 * The elements are in no particular order.
404 * <p>The returned array will be "safe" in that no references to it are
405 * maintained by this queue. (In other words, this method must allocate
406 * a new array). The caller is thus free to modify the returned array.
408 * <p>This method acts as bridge between array-based and collection-based
411 * @return an array containing all of the elements in this queue
413 public Object[] toArray() {
414 return Arrays.copyOf(queue, size);
418 * Returns an array containing all of the elements in this queue; the
419 * runtime type of the returned array is that of the specified array.
420 * The returned array elements are in no particular order.
421 * If the queue fits in the specified array, it is returned therein.
422 * Otherwise, a new array is allocated with the runtime type of the
423 * specified array and the size of this queue.
425 * <p>If the queue fits in the specified array with room to spare
426 * (i.e., the array has more elements than the queue), the element in
427 * the array immediately following the end of the collection is set to
430 * <p>Like the {@link #toArray()} method, this method acts as bridge between
431 * array-based and collection-based APIs. Further, this method allows
432 * precise control over the runtime type of the output array, and may,
433 * under certain circumstances, be used to save allocation costs.
435 * <p>Suppose <tt>x</tt> is a queue known to contain only strings.
436 * The following code can be used to dump the queue into a newly
437 * allocated array of <tt>String</tt>:
440 * String[] y = x.toArray(new String[0]);</pre>
442 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
443 * <tt>toArray()</tt>.
445 * @param a the array into which the elements of the queue are to
446 * be stored, if it is big enough; otherwise, a new array of the
447 * same runtime type is allocated for this purpose.
448 * @return an array containing all of the elements in this queue
449 * @throws ArrayStoreException if the runtime type of the specified array
450 * is not a supertype of the runtime type of every element in
452 * @throws NullPointerException if the specified array is null
454 public <T> T[] toArray(T[] a) {
456 // Make a new array of a's runtime type, but my contents:
457 return (T[]) Arrays.copyOf(queue, size, a.getClass());
458 System.arraycopy(queue, 0, a, 0, size);
465 * Returns an iterator over the elements in this queue. The iterator
466 * does not return the elements in any particular order.
468 * @return an iterator over the elements in this queue
470 public Iterator<E> iterator() {
474 private final class Itr implements Iterator<E> {
476 * Index (into queue array) of element to be returned by
477 * subsequent call to next.
479 private int cursor = 0;
482 * Index of element returned by most recent call to next,
483 * unless that element came from the forgetMeNot list.
484 * Set to -1 if element is deleted by a call to remove.
486 private int lastRet = -1;
489 * A queue of elements that were moved from the unvisited portion of
490 * the heap into the visited portion as a result of "unlucky" element
491 * removals during the iteration. (Unlucky element removals are those
492 * that require a siftup instead of a siftdown.) We must visit all of
493 * the elements in this list to complete the iteration. We do this
494 * after we've completed the "normal" iteration.
496 * We expect that most iterations, even those involving removals,
497 * will not need to store elements in this field.
499 private ArrayDeque<E> forgetMeNot = null;
502 * Element returned by the most recent call to next iff that
503 * element was drawn from the forgetMeNot list.
505 private E lastRetElt = null;
508 * The modCount value that the iterator believes that the backing
509 * Queue should have. If this expectation is violated, the iterator
510 * has detected concurrent modification.
512 private int expectedModCount = modCount;
514 public boolean hasNext() {
515 return cursor < size ||
516 (forgetMeNot != null && !forgetMeNot.isEmpty());
520 if (expectedModCount != modCount)
521 throw new ConcurrentModificationException();
523 return (E) queue[lastRet = cursor++];
524 if (forgetMeNot != null) {
526 lastRetElt = forgetMeNot.poll();
527 if (lastRetElt != null)
530 throw new NoSuchElementException();
533 public void remove() {
534 if (expectedModCount != modCount)
535 throw new ConcurrentModificationException();
537 E moved = PriorityQueue.this.removeAt(lastRet);
542 if (forgetMeNot == null)
543 forgetMeNot = new ArrayDeque<>();
544 forgetMeNot.add(moved);
546 } else if (lastRetElt != null) {
547 PriorityQueue.this.removeEq(lastRetElt);
550 throw new IllegalStateException();
552 expectedModCount = modCount;
561 * Removes all of the elements from this priority queue.
562 * The queue will be empty after this call returns.
564 public void clear() {
566 for (int i = 0; i < size; i++)
576 E result = (E) queue[0];
585 * Removes the ith element from queue.
587 * Normally this method leaves the elements at up to i-1,
588 * inclusive, untouched. Under these circumstances, it returns
589 * null. Occasionally, in order to maintain the heap invariant,
590 * it must swap a later element of the list with one earlier than
591 * i. Under these circumstances, this method returns the element
592 * that was previously at the end of the list and is now at some
593 * position before i. This fact is used by iterator.remove so as to
594 * avoid missing traversing elements.
596 private E removeAt(int i) {
597 assert i >= 0 && i < size;
600 if (s == i) // removed last element
603 E moved = (E) queue[s];
606 if (queue[i] == moved) {
608 if (queue[i] != moved)
616 * Inserts item x at position k, maintaining heap invariant by
617 * promoting x up the tree until it is greater than or equal to
618 * its parent, or is the root.
620 * To simplify and speed up coercions and comparisons. the
621 * Comparable and Comparator versions are separated into different
622 * methods that are otherwise identical. (Similarly for siftDown.)
624 * @param k the position to fill
625 * @param x the item to insert
627 private void siftUp(int k, E x) {
628 if (comparator != null)
629 siftUpUsingComparator(k, x);
631 siftUpComparable(k, x);
634 private void siftUpComparable(int k, E x) {
635 Comparable<? super E> key = (Comparable<? super E>) x;
637 int parent = (k - 1) >>> 1;
638 Object e = queue[parent];
639 if (key.compareTo((E) e) >= 0)
647 private void siftUpUsingComparator(int k, E x) {
649 int parent = (k - 1) >>> 1;
650 Object e = queue[parent];
651 if (comparator.compare(x, (E) e) >= 0)
660 * Inserts item x at position k, maintaining heap invariant by
661 * demoting x down the tree repeatedly until it is less than or
662 * equal to its children or is a leaf.
664 * @param k the position to fill
665 * @param x the item to insert
667 private void siftDown(int k, E x) {
668 if (comparator != null)
669 siftDownUsingComparator(k, x);
671 siftDownComparable(k, x);
674 private void siftDownComparable(int k, E x) {
675 Comparable<? super E> key = (Comparable<? super E>)x;
676 int half = size >>> 1; // loop while a non-leaf
678 int child = (k << 1) + 1; // assume left child is least
679 Object c = queue[child];
680 int right = child + 1;
682 ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
683 c = queue[child = right];
684 if (key.compareTo((E) c) <= 0)
692 private void siftDownUsingComparator(int k, E x) {
693 int half = size >>> 1;
695 int child = (k << 1) + 1;
696 Object c = queue[child];
697 int right = child + 1;
699 comparator.compare((E) c, (E) queue[right]) > 0)
700 c = queue[child = right];
701 if (comparator.compare(x, (E) c) <= 0)
710 * Establishes the heap invariant (described above) in the entire tree,
711 * assuming nothing about the order of the elements prior to the call.
713 private void heapify() {
714 for (int i = (size >>> 1) - 1; i >= 0; i--)
715 siftDown(i, (E) queue[i]);
719 * Returns the comparator used to order the elements in this
720 * queue, or {@code null} if this queue is sorted according to
721 * the {@linkplain Comparable natural ordering} of its elements.
723 * @return the comparator used to order this queue, or
724 * {@code null} if this queue is sorted according to the
725 * natural ordering of its elements
727 public Comparator<? super E> comparator() {