package java.util;
/*
* Todo
*
* 1) Make it serializable.
*/
/**
* An unbounded priority queue based on a priority heap. This queue orders
* elements according to the order specified at creation time. This order is
* specified as for {@link TreeSet} and {@link TreeMap}: Elements are ordered
* either according to their natural order (see {@link Comparable}), or
* according to a {@link Comparator}, depending on which constructor is used.
* The {@link #peek}, {@link #poll}, and {@link #remove} methods return the
* minimal element with respect to the specified ordering. If multiple
* these elements are tied for least value, no guarantees are made as to
* which of elements is returned.
*
*
Each priority queue has a capacity. The capacity is the size of
* the 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 list,
* its capacity grows automatically. The details of the growth policy are not
* specified.
*
*
Implementation note: this implementation provides O(log(n)) time for
* the offer, poll, remove() and add
* methods; linear time for the remove(Object) and
* contains methods; and constant time for the peek,
* element, and size methods.
*
*
This class is a member of the
*
* Java Collections Framework.
*/
public class PriorityQueue extends AbstractQueue
implements Queue
{
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
* of n, d, 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 E[] 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 comparator;
/**
* The number of times this priority queue has been
* structurally modified. See AbstractList for gory details.
*/
private int modCount = 0;
/**
* Create a new priority queue with the default initial capacity (11)
* that orders its elements according to their natural ordering.
*/
public PriorityQueue() {
this(DEFAULT_INITIAL_CAPACITY);
}
/**
* Create a new priority queue with the specified initial capacity
* that orders its elements according to their natural ordering.
*
* @param initialCapacity the initial capacity for this priority queue.
*/
public PriorityQueue(int initialCapacity) {
this(initialCapacity, null);
}
/**
* Create a new priority queue with the specified initial capacity (11)
* that orders its elements according to the specified comparator.
*
* @param initialCapacity the initial capacity for this priority queue.
* @param comparator the comparator used to order this priority queue.
*/
public PriorityQueue(int initialCapacity, Comparator comparator) {
if (initialCapacity < 1)
initialCapacity = 1;
queue = new E[initialCapacity + 1];
this.comparator = comparator;
}
/**
* Create a new priority queue containing the elements in the specified
* collection. The priority queue has an initial capacity of 110% of the
* size of the specified collection. If the specified collection
* implements the {@link Sorted} interface, the priority queue will be
* sorted according to the same comparator, or according to its elements'
* natural order if the collection is sorted according to its elements'
* natural order. If the specified collection does not implement the
* Sorted interface, the priority queue is ordered according to
* its elements' natural order.
*
* @param initialElements 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 an
* element of the specified collection is null.
*/
public PriorityQueue(Collection initialElements) {
int sz = initialElements.size();
int initialCapacity = (int)Math.min((sz * 110L) / 100,
Integer.MAX_VALUE - 1);
if (initialCapacity < 1)
initialCapacity = 1;
queue = new E[initialCapacity + 1];
/* Commented out to compile with generics compiler
if (initialElements instanceof Sorted) {
comparator = ((Sorted)initialElements).comparator();
for (Iterator i = initialElements.iterator(); i.hasNext(); )
queue[++size] = i.next();
} else {
*/
{
comparator = null;
for (Iterator i = initialElements.iterator(); i.hasNext(); )
add(i.next());
}
}
// Queue Methods
/**
* Remove and return the minimal element from this priority queue if
* it contains one or more elements, otherwise null. The term
* minimal is defined according to this priority queue's order.
*
* @return the minimal element from this priority queue if it contains
* one or more elements, otherwise null.
*/
public E poll() {
if (size == 0)
return null;
return remove(1);
}
/**
* Return, but do not remove, the minimal element from the priority queue,
* or null if the queue is empty. The term minimal is
* defined according to this priority queue's order. This method returns
* the same object reference that would be returned by by the
* poll method. The two methods differ in that this method
* does not remove the element from the priority queue.
*
* @return the minimal element from this priority queue if it contains
* one or more elements, otherwise null.
*/
public E peek() {
return queue[1];
}
// Collection Methods
/**
* Removes a single instance of the specified element from this priority
* queue, if it is present. Returns true if this collection contained the
* specified element (or equivalently, if this collection changed as a
* result of the call).
*
* @param o element to be removed from this collection, if present.
* @return true if this collection changed as a result of the
* call
* @throws ClassCastException if the specified element cannot be compared
* with elements currently in the priority queue according
* to the priority queue's ordering.
* @throws NullPointerException if the specified element is null.
*/
public boolean remove(Object element) {
if (element == null)
throw new NullPointerException();
if (comparator == null) {
for (int i = 1; i <= size; i++) {
if (((Comparable)queue[i]).compareTo(element) == 0) {
remove(i);
return true;
}
}
} else {
for (int i = 1; i <= size; i++) {
if (comparator.compare(queue[i], (E) element) == 0) {
remove(i);
return true;
}
}
}
return false;
}
/**
* Returns an iterator over the elements in this priority queue. The
* first element returned by this iterator is the same element that
* would be returned by a call to peek.
*
* @return an Iterator over the elements in this priority queue.
*/
public Iterator iterator() {
return new Itr();
}
private class Itr implements Iterator {
/**
* Index (into queue array) of element to be returned by
* subsequent call to next.
*/
int cursor = 1;
/**
* Index of element returned by most recent call to next or
* previous. Reset to 0 if this element is deleted by a call
* to remove.
*/
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.
*/
int expectedModCount = modCount;
public boolean hasNext() {
return cursor <= size;
}
public E next() {
checkForComodification();
if (cursor > size)
throw new NoSuchElementException();
E result = queue[cursor];
lastRet = cursor++;
return result;
}
public void remove() {
if (lastRet == 0)
throw new IllegalStateException();
checkForComodification();
PriorityQueue.this.remove(lastRet);
if (lastRet < cursor)
cursor--;
lastRet = 0;
expectedModCount = modCount;
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
/**
* Returns the number of elements in this priority queue.
*
* @return the number of elements in this priority queue.
*/
public int size() {
return size;
}
/**
* Add the specified element to this priority queue.
*
* @param element the element to add.
* @return true
* @throws ClassCastException if the specified element cannot be compared
* with elements currently in the priority queue according
* to the priority queue's ordering.
* @throws NullPointerException if the specified element is null.
*/
public boolean offer(E element) {
if (element == null)
throw new NullPointerException();
modCount++;
// Grow backing store if necessary
if (++size == queue.length) {
E[] newQueue = new E[2 * queue.length];
System.arraycopy(queue, 0, newQueue, 0, size);
queue = newQueue;
}
queue[size] = element;
fixUp(size);
return true;
}
/**
* Remove all elements from the priority queue.
*/
public void clear() {
modCount++;
// Null out element references to prevent memory leak
for (int i=1; i<=size; i++)
queue[i] = null;
size = 0;
}
/**
* Removes and returns the ith element from queue. Recall
* that queue is one-based, so 1 <= i <= size.
*
* XXX: Could further special-case i==size, but is it worth it?
* XXX: Could special-case i==0, but is it worth it?
*/
private E remove(int i) {
assert i <= size;
modCount++;
E result = queue[i];
queue[i] = queue[size];
queue[size--] = null; // Drop extra ref to prevent memory leak
if (i <= size)
fixDown(i);
return result;
}
/**
* 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)queue[j]).compareTo(queue[k]) <= 0)
break;
E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
k = j;
}
} else {
while (k > 1) {
int j = k >> 1;
if (comparator.compare(queue[j], queue[k]) <= 0)
break;
E 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) {
if (j 0)
j++; // j indexes smallest kid
if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
break;
E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
k = j;
}
} else {
while ((j = k << 1) <= size) {
if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
j++; // j indexes smallest kid
if (comparator.compare(queue[k], queue[j]) <= 0)
break;
E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
k = j;
}
}
}
/**
* Returns the comparator associated with this priority queue, or
* null if it uses its elements' natural ordering.
*
* @return the comparator associated with this priority queue, or
* null if it uses its elements' natural ordering.
*/
Comparator comparator() {
return comparator;
}
}