/[cvs]/jsr166/src/main/java/util/PriorityQueue.java
ViewVC logotype

Contents of /jsr166/src/main/java/util/PriorityQueue.java

Parent Directory Parent Directory | Revision Log Revision Log


Revision 1.4 - (show annotations)
Mon May 19 02:45:07 2003 UTC (16 years, 3 months ago) by tim
Branch: MAIN
Changes since 1.3: +0 -4 lines
Use temp version of Sorted to allow a portion of PQ impl to be uncommented.

1 package java.util;
2
3 /*
4 * Todo
5 *
6 * 1) Make it serializable.
7 */
8
9 /**
10 * 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
41 /**
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
56 /**
57 * The number of elements in the priority queue.
58 */
59 private int size = 0;
60
61 /**
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 }
80
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 }
90
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 }
104
105 /**
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 }
142
143 // 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 public E poll() {
154 if (size == 0)
155 return null;
156 return remove(1);
157 }
158
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 public E peek() {
171 return queue[1];
172 }
173
174 // 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 return false;
210 }
211
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 public int size() {
281 return size;
282 }
283
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 }
310
311 /**
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 }
342
343 /**
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 Comparator<E> comparator() {
412 return comparator;
413 }
414 }

dl@cs.oswego.edu
ViewVC Help
Powered by ViewVC 1.1.27