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1 : tim 1.2 package java.util;
2 : tim 1.1
3 : tim 1.2 /*
4 :     * Todo
5 :     *
6 :     * 1) Make it serializable.
7 :     */
8 : tim 1.1
9 :     /**
10 : tim 1.2 * An unbounded priority queue based on a priority heap. This queue orders
11 :     * elements according to the order specified at creation time. This order is
12 :     * specified as for {@link TreeSet} and {@link TreeMap}: Elements are ordered
13 :     * either according to their <i>natural order</i> (see {@link Comparable}), or
14 :     * according to a {@link Comparator}, depending on which constructor is used.
15 :     * The {@link #peek}, {@link #poll}, and {@link #remove} methods return the
16 :     * minimal element with respect to the specified ordering. If multiple
17 :     * these elements are tied for least value, no guarantees are made as to
18 :     * which of elements is returned.
19 :     *
20 :     * <p>Each priority queue has a <i>capacity</i>. The capacity is the size of
21 :     * the array used to store the elements on the queue. It is always at least
22 :     * as large as the queue size. As elements are added to a priority list,
23 :     * its capacity grows automatically. The details of the growth policy are not
24 :     * specified.
25 :     *
26 :     *<p>Implementation note: this implementation provides O(log(n)) time for
27 :     * the <tt>offer</tt>, <tt>poll</tt>, <tt>remove()</tt> and <tt>add</tt>
28 :     * methods; linear time for the <tt>remove(Object)</tt> and
29 :     * <tt>contains</tt> methods; and constant time for the <tt>peek</tt>,
30 :     * <tt>element</tt>, and <tt>size</tt> methods.
31 :     *
32 :     * <p>This class is a member of the
33 :     * <a href="{@docRoot}/../guide/collections/index.html">
34 :     * Java Collections Framework</a>.
35 :     */
36 :     public class PriorityQueue<E> extends AbstractQueue<E>
37 :     implements Queue<E>
38 :     {
39 :     private static final int DEFAULT_INITIAL_CAPACITY = 11;
40 : tim 1.1
41 : tim 1.2 /**
42 :     * Priority queue represented as a balanced binary heap: the two children
43 :     * of queue[n] are queue[2*n] and queue[2*n + 1]. The priority queue is
44 :     * ordered by comparator, or by the elements' natural ordering, if
45 :     * comparator is null: For each node n in the heap, and each descendant
46 :     * of n, d, n <= d.
47 :     *
48 :     * The element with the lowest value is in queue[1] (assuming the queue is
49 :     * nonempty). A one-based array is used in preference to the traditional
50 :     * zero-based array to simplify parent and child calculations.
51 :     *
52 :     * queue.length must be >= 2, even if size == 0.
53 :     */
54 :     private E[] queue;
55 : tim 1.1
56 : tim 1.2 /**
57 :     * The number of elements in the priority queue.
58 :     */
59 :     private int size = 0;
60 : tim 1.1
61 : tim 1.2 /**
62 :     * The comparator, or null if priority queue uses elements'
63 :     * natural ordering.
64 :     */
65 :     private final Comparator<E> comparator;
66 :    
67 :     /**
68 :     * The number of times this priority queue has been
69 :     * <i>structurally modified</i>. See AbstractList for gory details.
70 :     */
71 :     private int modCount = 0;
72 :    
73 :     /**
74 :     * Create a new priority queue with the default initial capacity (11)
75 :     * that orders its elements according to their natural ordering.
76 :     */
77 :     public PriorityQueue() {
78 :     this(DEFAULT_INITIAL_CAPACITY);
79 : tim 1.1 }
80 : tim 1.2
81 :     /**
82 :     * Create a new priority queue with the specified initial capacity
83 :     * that orders its elements according to their natural ordering.
84 :     *
85 :     * @param initialCapacity the initial capacity for this priority queue.
86 :     */
87 :     public PriorityQueue(int initialCapacity) {
88 :     this(initialCapacity, null);
89 : tim 1.1 }
90 : tim 1.2
91 :     /**
92 :     * Create a new priority queue with the specified initial capacity (11)
93 :     * that orders its elements according to the specified comparator.
94 :     *
95 :     * @param initialCapacity the initial capacity for this priority queue.
96 :     * @param comparator the comparator used to order this priority queue.
97 :     */
98 :     public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
99 :     if (initialCapacity < 1)
100 :     initialCapacity = 1;
101 :     queue = new E[initialCapacity + 1];
102 :     this.comparator = comparator;
103 : tim 1.1 }
104 :    
105 : tim 1.2 /**
106 :     * Create a new priority queue containing the elements in the specified
107 :     * collection. The priority queue has an initial capacity of 110% of the
108 :     * size of the specified collection. If the specified collection
109 :     * implements the {@link Sorted} interface, the priority queue will be
110 :     * sorted according to the same comparator, or according to its elements'
111 :     * natural order if the collection is sorted according to its elements'
112 :     * natural order. If the specified collection does not implement the
113 :     * <tt>Sorted</tt> interface, the priority queue is ordered according to
114 :     * its elements' natural order.
115 :     *
116 :     * @param initialElements the collection whose elements are to be placed
117 :     * into this priority queue.
118 :     * @throws ClassCastException if elements of the specified collection
119 :     * cannot be compared to one another according to the priority
120 :     * queue's ordering.
121 :     * @throws NullPointerException if the specified collection or an
122 :     * element of the specified collection is <tt>null</tt>.
123 :     */
124 :     public PriorityQueue(Collection<E> initialElements) {
125 :     int sz = initialElements.size();
126 :     int initialCapacity = (int)Math.min((sz * 110L) / 100,
127 :     Integer.MAX_VALUE - 1);
128 :     if (initialCapacity < 1)
129 :     initialCapacity = 1;
130 :     queue = new E[initialCapacity + 1];
131 :    
132 :     if (initialElements instanceof Sorted) {
133 :     comparator = ((Sorted)initialElements).comparator();
134 :     for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
135 :     queue[++size] = i.next();
136 :     } else {
137 :     comparator = null;
138 :     for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
139 :     add(i.next());
140 :     }
141 : tim 1.1 }
142 :    
143 : tim 1.2 // Queue Methods
144 :    
145 :     /**
146 :     * Remove and return the minimal element from this priority queue if
147 :     * it contains one or more elements, otherwise <tt>null</tt>. The term
148 :     * <i>minimal</i> is defined according to this priority queue's order.
149 :     *
150 :     * @return the minimal element from this priority queue if it contains
151 :     * one or more elements, otherwise <tt>null</tt>.
152 :     */
153 : tim 1.1 public E poll() {
154 : tim 1.2 if (size == 0)
155 :     return null;
156 :     return remove(1);
157 : tim 1.1 }
158 : tim 1.2
159 :     /**
160 :     * Return, but do not remove, the minimal element from the priority queue,
161 :     * or <tt>null</tt> if the queue is empty. The term <i>minimal</i> is
162 :     * defined according to this priority queue's order. This method returns
163 :     * the same object reference that would be returned by by the
164 :     * <tt>poll</tt> method. The two methods differ in that this method
165 :     * does not remove the element from the priority queue.
166 :     *
167 :     * @return the minimal element from this priority queue if it contains
168 :     * one or more elements, otherwise <tt>null</tt>.
169 :     */
170 : tim 1.1 public E peek() {
171 : tim 1.2 return queue[1];
172 : tim 1.1 }
173 :    
174 : tim 1.2 // Collection Methods
175 :    
176 :     /**
177 :     * Removes a single instance of the specified element from this priority
178 :     * queue, if it is present. Returns true if this collection contained the
179 :     * specified element (or equivalently, if this collection changed as a
180 :     * result of the call).
181 :     *
182 :     * @param o element to be removed from this collection, if present.
183 :     * @return <tt>true</tt> if this collection changed as a result of the
184 :     * call
185 :     * @throws ClassCastException if the specified element cannot be compared
186 :     * with elements currently in the priority queue according
187 :     * to the priority queue's ordering.
188 :     * @throws NullPointerException if the specified element is null.
189 :     */
190 :     public boolean remove(Object element) {
191 :     if (element == null)
192 :     throw new NullPointerException();
193 :    
194 :     if (comparator == null) {
195 :     for (int i = 1; i <= size; i++) {
196 :     if (((Comparable)queue[i]).compareTo(element) == 0) {
197 :     remove(i);
198 :     return true;
199 :     }
200 :     }
201 :     } else {
202 :     for (int i = 1; i <= size; i++) {
203 :     if (comparator.compare(queue[i], (E) element) == 0) {
204 :     remove(i);
205 :     return true;
206 :     }
207 :     }
208 :     }
209 : tim 1.1 return false;
210 :     }
211 : tim 1.2
212 :     /**
213 :     * Returns an iterator over the elements in this priority queue. The
214 :     * first element returned by this iterator is the same element that
215 :     * would be returned by a call to <tt>peek</tt>.
216 :     *
217 :     * @return an <tt>Iterator</tt> over the elements in this priority queue.
218 :     */
219 :     public Iterator<E> iterator() {
220 :     return new Itr();
221 :     }
222 :    
223 :     private class Itr implements Iterator<E> {
224 :     /**
225 :     * Index (into queue array) of element to be returned by
226 :     * subsequent call to next.
227 :     */
228 :     int cursor = 1;
229 :    
230 :     /**
231 :     * Index of element returned by most recent call to next or
232 :     * previous. Reset to 0 if this element is deleted by a call
233 :     * to remove.
234 :     */
235 :     int lastRet = 0;
236 :    
237 :     /**
238 :     * The modCount value that the iterator believes that the backing
239 :     * List should have. If this expectation is violated, the iterator
240 :     * has detected concurrent modification.
241 :     */
242 :     int expectedModCount = modCount;
243 :    
244 :     public boolean hasNext() {
245 :     return cursor <= size;
246 :     }
247 :    
248 :     public E next() {
249 :     checkForComodification();
250 :     if (cursor > size)
251 :     throw new NoSuchElementException();
252 :     E result = queue[cursor];
253 :     lastRet = cursor++;
254 :     return result;
255 :     }
256 :    
257 :     public void remove() {
258 :     if (lastRet == 0)
259 :     throw new IllegalStateException();
260 :     checkForComodification();
261 :    
262 :     PriorityQueue.this.remove(lastRet);
263 :     if (lastRet < cursor)
264 :     cursor--;
265 :     lastRet = 0;
266 :     expectedModCount = modCount;
267 :     }
268 :    
269 :     final void checkForComodification() {
270 :     if (modCount != expectedModCount)
271 :     throw new ConcurrentModificationException();
272 :     }
273 :     }
274 :    
275 :     /**
276 :     * Returns the number of elements in this priority queue.
277 :     *
278 :     * @return the number of elements in this priority queue.
279 :     */
280 : tim 1.1 public int size() {
281 : tim 1.2 return size;
282 : tim 1.1 }
283 : tim 1.2
284 :     /**
285 :     * Add the specified element to this priority queue.
286 :     *
287 :     * @param element the element to add.
288 :     * @return true
289 :     * @throws ClassCastException if the specified element cannot be compared
290 :     * with elements currently in the priority queue according
291 :     * to the priority queue's ordering.
292 :     * @throws NullPointerException if the specified element is null.
293 :     */
294 :     public boolean offer(E element) {
295 :     if (element == null)
296 :     throw new NullPointerException();
297 :     modCount++;
298 :    
299 :     // Grow backing store if necessary
300 :     if (++size == queue.length) {
301 :     E[] newQueue = new E[2 * queue.length];
302 :     System.arraycopy(queue, 0, newQueue, 0, size);
303 :     queue = newQueue;
304 :     }
305 :    
306 :     queue[size] = element;
307 :     fixUp(size);
308 :     return true;
309 : tim 1.1 }
310 :    
311 : tim 1.2 /**
312 :     * Remove all elements from the priority queue.
313 :     */
314 :     public void clear() {
315 :     modCount++;
316 :    
317 :     // Null out element references to prevent memory leak
318 :     for (int i=1; i<=size; i++)
319 :     queue[i] = null;
320 :    
321 :     size = 0;
322 :     }
323 :    
324 :     /**
325 :     * Removes and returns the ith element from queue. Recall
326 :     * that queue is one-based, so 1 <= i <= size.
327 :     *
328 :     * XXX: Could further special-case i==size, but is it worth it?
329 :     * XXX: Could special-case i==0, but is it worth it?
330 :     */
331 :     private E remove(int i) {
332 :     assert i <= size;
333 :     modCount++;
334 :    
335 :     E result = queue[i];
336 :     queue[i] = queue[size];
337 :     queue[size--] = null; // Drop extra ref to prevent memory leak
338 :     if (i <= size)
339 :     fixDown(i);
340 :     return result;
341 : tim 1.1 }
342 :    
343 : tim 1.2 /**
344 :     * Establishes the heap invariant (described above) assuming the heap
345 :     * satisfies the invariant except possibly for the leaf-node indexed by k
346 :     * (which may have a nextExecutionTime less than its parent's).
347 :     *
348 :     * This method functions by "promoting" queue[k] up the hierarchy
349 :     * (by swapping it with its parent) repeatedly until queue[k]
350 :     * is greater than or equal to its parent.
351 :     */
352 :     private void fixUp(int k) {
353 :     if (comparator == null) {
354 :     while (k > 1) {
355 :     int j = k >> 1;
356 :     if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
357 :     break;
358 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
359 :     k = j;
360 :     }
361 :     } else {
362 :     while (k > 1) {
363 :     int j = k >> 1;
364 :     if (comparator.compare(queue[j], queue[k]) <= 0)
365 :     break;
366 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
367 :     k = j;
368 :     }
369 :     }
370 :     }
371 :    
372 :     /**
373 :     * Establishes the heap invariant (described above) in the subtree
374 :     * rooted at k, which is assumed to satisfy the heap invariant except
375 :     * possibly for node k itself (which may be greater than its children).
376 :     *
377 :     * This method functions by "demoting" queue[k] down the hierarchy
378 :     * (by swapping it with its smaller child) repeatedly until queue[k]
379 :     * is less than or equal to its children.
380 :     */
381 :     private void fixDown(int k) {
382 :     int j;
383 :     if (comparator == null) {
384 :     while ((j = k << 1) <= size) {
385 :     if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
386 :     j++; // j indexes smallest kid
387 :     if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
388 :     break;
389 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
390 :     k = j;
391 :     }
392 :     } else {
393 :     while ((j = k << 1) <= size) {
394 :     if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
395 :     j++; // j indexes smallest kid
396 :     if (comparator.compare(queue[k], queue[j]) <= 0)
397 :     break;
398 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
399 :     k = j;
400 :     }
401 :     }
402 :     }
403 :    
404 :     /**
405 :     * Returns the comparator associated with this priority queue, or
406 :     * <tt>null</tt> if it uses its elements' natural ordering.
407 :     *
408 :     * @return the comparator associated with this priority queue, or
409 :     * <tt>null</tt> if it uses its elements' natural ordering.
410 :     */
411 : tim 1.3 Comparator<E> comparator() {
412 : tim 1.2 return comparator;
413 :     }
414 : tim 1.1 }

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