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package java.util; |
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/** |
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* An unbounded priority {@linkplain Queue queue} based on a priority heap. |
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* This queue orders |
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* elements according to an order specified at construction time, which is |
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* specified in the same manner as {@link java.util.TreeSet} and |
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* {@link java.util.TreeMap}: elements are ordered |
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* either according to their <i>natural order</i> (see {@link Comparable}), or |
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* according to a {@link java.util.Comparator}, depending on which |
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* constructor is used. |
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* An unbounded priority {@linkplain Queue queue} based on a priority heap. |
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* This queue orders elements according to an order specified at construction |
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* time, which is specified in the same manner as {@link java.util.TreeSet} |
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* and {@link java.util.TreeMap}: elements are ordered either according to |
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* their <i>natural order</i> (see {@link Comparable}), or according to a |
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* {@link java.util.Comparator}, depending on which constructor is used. |
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* <p>The <em>head</em> of this queue is the <em>least</em> element with |
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* respect to the specified ordering. |
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* If multiple elements are tied for least value, the |
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* head is one of those elements. A priority queue does not permit |
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* respect to the specified ordering. If multiple elements are tied for least |
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* value, the head is one of those elements. A priority queue does not permit |
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* <tt>null</tt> elements. |
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* |
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* <p>The {@link #remove()} and {@link #poll()} methods remove and |
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* elements are added to a priority queue, its capacity grows |
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* automatically. The details of the growth policy are not specified. |
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* |
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* <p>The Iterator provided in method {@link #iterator()} is <em>not</em> |
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* guaranteed to traverse the elements of the PriorityQueue in any |
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* particular order. If you need ordered traversal, consider using |
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* <tt>Arrays.sort(pq.toArray())</tt>. |
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* |
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* <p> <strong>Note that this implementation is not synchronized.</strong> |
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* Multiple threads should not access a <tt>PriorityQueue</tt> |
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* instance concurrently if any of the threads modifies the list |
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* structurally. Instead, use the thread-safe {@link |
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* java.util.concurrent.PriorityBlockingQueue} class. |
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* |
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* |
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* <p>Implementation note: this implementation provides O(log(n)) time |
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* for the insertion methods (<tt>offer</tt>, <tt>poll</tt>, |
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* <tt>remove()</tt> and <tt>add</tt>) methods; linear time for the |
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public class PriorityQueue<E> extends AbstractQueue<E> |
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implements Queue<E>, java.io.Serializable { |
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private static final long serialVersionUID = -7720805057305804111L; |
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private static final int DEFAULT_INITIAL_CAPACITY = 11; |
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/** |
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/** |
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* Initially fill elements of the queue array under the |
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* knowledge that it is sorted or is another PQ, in which |
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* case we can just place the elements without fixups. |
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* case we can just place the elements in the order presented. |
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*/ |
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private void fillFromSorted(Collection<? extends E> c) { |
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for (Iterator<? extends E> i = c.iterator(); i.hasNext(); ) |
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queue[++size] = i.next(); |
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} |
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/** |
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* Initially fill elements of the queue array that is |
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* not to our knowledge sorted, so we must add them |
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* one by one. |
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* Initially fill elements of the queue array that is not to our knowledge |
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* sorted, so we must rearrange the elements to guarantee the heap |
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* invariant. |
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*/ |
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private void fillFromUnsorted(Collection<? extends E> c) { |
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for (Iterator<? extends E> i = c.iterator(); i.hasNext(); ) |
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add(i.next()); |
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queue[++size] = i.next(); |
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heapify(); |
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} |
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/** |
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*/ |
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public PriorityQueue(Collection<? extends E> c) { |
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initializeArray(c); |
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if (c instanceof SortedSet<? extends E>) { |
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SortedSet<? extends E> s = (SortedSet<? extends E>) c; |
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if (c instanceof SortedSet) { |
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// @fixme double-cast workaround for compiler |
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SortedSet<? extends E> s = (SortedSet<? extends E>) (SortedSet)c; |
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comparator = (Comparator<? super E>)s.comparator(); |
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fillFromSorted(s); |
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} else if (c instanceof PriorityQueue<? extends E>) { |
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} else if (c instanceof PriorityQueue) { |
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PriorityQueue<? extends E> s = (PriorityQueue<? extends E>) c; |
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comparator = (Comparator<? super E>)s.comparator(); |
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fillFromSorted(s); |
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queue = newQueue; |
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} |
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// Queue Methods |
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// Queue Methods |
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/** |
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* Add the specified element to this priority queue. |
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public E poll() { |
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if (size == 0) |
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return null; |
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return (E) remove(1); |
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return remove(); |
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} |
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public E peek() { |
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} |
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/** |
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/** |
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* Removes a single instance of the specified element from this |
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* queue, if it is present. More formally, |
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* removes an element <tt>e</tt> such that <tt>(o==null ? e==null : |
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if (comparator == null) { |
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for (int i = 1; i <= size; i++) { |
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if (((Comparable<E>)queue[i]).compareTo((E)o) == 0) { |
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remove(i); |
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removeAt(i); |
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return true; |
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} |
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} |
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} else { |
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for (int i = 1; i <= size; i++) { |
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if (comparator.compare((E)queue[i], (E)o) == 0) { |
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remove(i); |
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removeAt(i); |
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return true; |
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} |
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} |
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} |
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private class Itr implements Iterator<E> { |
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/** |
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* Index (into queue array) of element to be returned by |
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* subsequent call to next. |
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private int cursor = 1; |
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/** |
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* Index of element returned by most recent call to next or |
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* previous. Reset to 0 if this element is deleted by a call |
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* to remove. |
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* Index of element returned by most recent call to next, |
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* unless that element came from the forgetMeNot list. |
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* Reset to 0 if element is deleted by a call to remove. |
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*/ |
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private int lastRet = 0; |
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*/ |
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private int expectedModCount = modCount; |
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/** |
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* A list of elements that were moved from the unvisited portion of |
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* the heap into the visited portion as a result of "unlucky" element |
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* removals during the iteration. (Unlucky element removals are those |
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* that require a fixup instead of a fixdown.) We must visit all of |
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* the elements in this list to complete the iteration. We do this |
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* after we've completed the "normal" iteration. |
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* |
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* We expect that most iterations, even those involving removals, |
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* will not use need to store elements in this field. |
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*/ |
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private ArrayList<E> forgetMeNot = null; |
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/** |
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* Element returned by the most recent call to next iff that |
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* element was drawn from the forgetMeNot list. |
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*/ |
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private Object lastRetElt = null; |
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public boolean hasNext() { |
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return cursor <= size; |
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return cursor <= size || forgetMeNot != null; |
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} |
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public E next() { |
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checkForComodification(); |
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if (cursor > size) |
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E result; |
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if (cursor <= size) { |
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result = (E) queue[cursor]; |
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lastRet = cursor++; |
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} |
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else if (forgetMeNot == null) |
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throw new NoSuchElementException(); |
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E result = (E) queue[cursor]; |
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lastRet = cursor++; |
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else { |
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int remaining = forgetMeNot.size(); |
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result = forgetMeNot.remove(remaining - 1); |
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if (remaining == 1) |
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forgetMeNot = null; |
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lastRet = 0; |
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lastRetElt = result; |
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} |
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return result; |
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} |
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public void remove() { |
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if (lastRet == 0) |
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throw new IllegalStateException(); |
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checkForComodification(); |
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PriorityQueue.this.remove(lastRet); |
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if (lastRet < cursor) |
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cursor--; |
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lastRet = 0; |
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if (lastRet != 0) { |
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E moved = PriorityQueue.this.removeAt(lastRet); |
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lastRet = 0; |
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if (moved == null) { |
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cursor--; |
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} else { |
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if (forgetMeNot == null) |
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forgetMeNot = new ArrayList(); |
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forgetMeNot.add(moved); |
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} |
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} else if (lastRetElt != null) { |
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PriorityQueue.this.remove(lastRetElt); |
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lastRetElt = null; |
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} else { |
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throw new IllegalStateException(); |
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} |
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expectedModCount = modCount; |
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} |
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} |
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/** |
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* Removes and returns the ith element from queue. Recall |
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* that queue is one-based, so 1 <= i <= size. |
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* Removes and returns the first element from queue. |
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*/ |
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public E remove() { |
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if (size == 0) |
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throw new NoSuchElementException(); |
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modCount++; |
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|
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E result = (E) queue[1]; |
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queue[1] = queue[size]; |
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queue[size--] = null; // Drop extra ref to prevent memory leak |
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if (size > 1) |
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fixDown(1); |
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return result; |
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} |
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|
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/** |
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* Removes and returns the ith element from queue. (Recall that queue |
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* is one-based, so 1 <= i <= size.) |
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* |
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* XXX: Could further special-case i==size, but is it worth it? |
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* XXX: Could special-case i==0, but is it worth it? |
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* Normally this method leaves the elements at positions from 1 up to i-1, |
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* inclusive, untouched. Under these circumstances, it returns null. |
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* Occasionally, in order to maintain the heap invariant, it must move |
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* the last element of the list to some index in the range [2, i-1], |
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* and move the element previously at position (i/2) to position i. |
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* Under these circumstances, this method returns the element that was |
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* previously at the end of the list and is now at some position between |
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* 2 and i-1 inclusive. |
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*/ |
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private E remove(int i) { |
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assert i <= size; |
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private E removeAt(int i) { |
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assert i > 0 && i <= size; |
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modCount++; |
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E result = (E) queue[i]; |
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queue[i] = queue[size]; |
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E moved = (E) queue[size]; |
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queue[i] = moved; |
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queue[size--] = null; // Drop extra ref to prevent memory leak |
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if (i <= size) |
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if (i <= size) { |
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fixDown(i); |
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return result; |
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if (queue[i] == moved) { |
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fixUp(i); |
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if (queue[i] != moved) |
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return moved; |
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} |
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} |
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return null; |
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} |
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/** |
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} |
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} else { |
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while (k > 1) { |
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int j = k >> 1; |
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int j = k >>> 1; |
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if (comparator.compare((E)queue[j], (E)queue[k]) <= 0) |
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break; |
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Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
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private void fixDown(int k) { |
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int j; |
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if (comparator == null) { |
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while ((j = k << 1) <= size) { |
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if (j<size && ((Comparable<E>)queue[j]).compareTo((E)queue[j+1]) > 0) |
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while ((j = k << 1) <= size && (j > 0)) { |
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if (j<size && |
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((Comparable<E>)queue[j]).compareTo((E)queue[j+1]) > 0) |
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j++; // j indexes smallest kid |
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|
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if (((Comparable<E>)queue[k]).compareTo((E)queue[j]) <= 0) |
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break; |
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Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
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k = j; |
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} |
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} else { |
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while ((j = k << 1) <= size) { |
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if (j < size && comparator.compare((E)queue[j], (E)queue[j+1]) > 0) |
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while ((j = k << 1) <= size && (j > 0)) { |
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if (j<size && |
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comparator.compare((E)queue[j], (E)queue[j+1]) > 0) |
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j++; // j indexes smallest kid |
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if (comparator.compare((E)queue[k], (E)queue[j]) <= 0) |
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break; |
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} |
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} |
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/** |
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* Establishes the heap invariant (described above) in the entire tree, |
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* assuming nothing about the order of the elements prior to the call. |
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*/ |
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private void heapify() { |
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for (int i = size/2; i >= 1; i--) |
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fixDown(i); |
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} |
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/** |
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* Returns the comparator used to order this collection, or <tt>null</tt> |
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} |
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} |
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