/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.lucene.util;
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* A PriorityQueue maintains a partial ordering of its elements such that the
* least element can always be found in constant time. Put()'s and pop()'s
* require log(size) time but the remove() cost implemented here is linear.
*
* <p>
* <b>NOTE</b>: This class will pre-allocate a full array of length
* <code>maxSize+1</code> if instantiated via the
* {@link #PriorityQueue(int,boolean)} constructor with <code>prepopulate</code>
* set to <code>true</code>.
*
* <b>NOTE</b>: Iteration order is not specified.
*
* @lucene.internal
*/
public abstract class PriorityQueue<T> implements Iterable<T> {
private int size = 0;
private final int maxSize;
private final T[] heap;
public PriorityQueue(int maxSize) {
this(maxSize, true);
}
public PriorityQueue(int maxSize, boolean prepopulate) {
final int heapSize;
if (0 == maxSize) {
// We allocate 1 extra to avoid if statement in top()
heapSize = 2;
} else {
// NOTE: we add +1 because all access to heap is
// 1-based not 0-based. heap[0] is unused.
heapSize = maxSize + 1;
if (heapSize > ArrayUtil.MAX_ARRAY_LENGTH) {
// Throw exception to prevent confusing OOME:
throw new IllegalArgumentException("maxSize must be <= " + (ArrayUtil.MAX_ARRAY_LENGTH-1) + "; got: " + maxSize);
}
}
// T is unbounded type, so this unchecked cast works always:
@SuppressWarnings("unchecked") final T[] h = (T[]) new Object[heapSize];
this.heap = h;
this.maxSize = maxSize;
if (prepopulate) {
// If sentinel objects are supported, populate the queue with them
T sentinel = getSentinelObject();
if (sentinel != null) {
heap[1] = sentinel;
for (int i = 2; i < heap.length; i++) {
heap[i] = getSentinelObject();
}
size = maxSize;
}
}
}
/** Determines the ordering of objects in this priority queue. Subclasses
* must define this one method.
* @return <code>true</code> iff parameter <tt>a</tt> is less than parameter <tt>b</tt>.
*/
protected abstract boolean lessThan(T a, T b);
/**
* This method can be overridden by extending classes to return a sentinel
* object which will be used by the {@link PriorityQueue#PriorityQueue(int,boolean)}
* constructor to fill the queue, so that the code which uses that queue can always
* assume it's full and only change the top without attempting to insert any new
* object.<br>
*
* Those sentinel values should always compare worse than any non-sentinel
* value (i.e., {@link #lessThan} should always favor the
* non-sentinel values).<br>
*
* By default, this method returns null, which means the queue will not be
* filled with sentinel values. Otherwise, the value returned will be used to
* pre-populate the queue. Adds sentinel values to the queue.<br>
*
* If this method is extended to return a non-null value, then the following
* usage pattern is recommended:
*
* <pre class="prettyprint">
* // extends getSentinelObject() to return a non-null value.
* PriorityQueue<MyObject> pq = new MyQueue<MyObject>(numHits);
* // save the 'top' element, which is guaranteed to not be null.
* MyObject pqTop = pq.top();
* <...>
* // now in order to add a new element, which is 'better' than top (after
* // you've verified it is better), it is as simple as:
* pqTop.change().
* pqTop = pq.updateTop();
* </pre>
*
* <b>NOTE:</b> if this method returns a non-null value, it will be called by
* the {@link PriorityQueue#PriorityQueue(int,boolean)} constructor
* {@link #size()} times, relying on a new object to be returned and will not
* check if it's null again. Therefore you should ensure any call to this
* method creates a new instance and behaves consistently, e.g., it cannot
* return null if it previously returned non-null.
*
* @return the sentinel object to use to pre-populate the queue, or null if
* sentinel objects are not supported.
*/
protected T getSentinelObject() {
return null;
}
/**
* Adds an Object to a PriorityQueue in log(size) time. If one tries to add
* more objects than maxSize from initialize an
* {@link ArrayIndexOutOfBoundsException} is thrown.
*
* @return the new 'top' element in the queue.
*/
public final T add(T element) {
size++;
heap[size] = element;
upHeap(size);
return heap[1];
}
/**
* Adds an Object to a PriorityQueue in log(size) time.
* It returns the object (if any) that was
* dropped off the heap because it was full. This can be
* the given parameter (in case it is smaller than the
* full heap's minimum, and couldn't be added), or another
* object that was previously the smallest value in the
* heap and now has been replaced by a larger one, or null
* if the queue wasn't yet full with maxSize elements.
*/
public T insertWithOverflow(T element) {
if (size < maxSize) {
add(element);
return null;
} else if (size > 0 && !lessThan(element, heap[1])) {
T ret = heap[1];
heap[1] = element;
updateTop();
return ret;
} else {
return element;
}
}
/** Returns the least element of the PriorityQueue in constant time. */
public final T top() {
// We don't need to check size here: if maxSize is 0,
// then heap is length 2 array with both entries null.
// If size is 0 then heap[1] is already null.
return heap[1];
}
/** Removes and returns the least element of the PriorityQueue in log(size)
time. */
public final T pop() {
if (size > 0) {
T result = heap[1]; // save first value
heap[1] = heap[size]; // move last to first
heap[size] = null; // permit GC of objects
size--;
downHeap(1); // adjust heap
return result;
} else {
return null;
}
}
/**
* Should be called when the Object at top changes values. Still log(n) worst
* case, but it's at least twice as fast to
*
* <pre class="prettyprint">
* pq.top().change();
* pq.updateTop();
* </pre>
*
* instead of
*
* <pre class="prettyprint">
* o = pq.pop();
* o.change();
* pq.push(o);
* </pre>
*
* @return the new 'top' element.
*/
public final T updateTop() {
downHeap(1);
return heap[1];
}
/**
* Replace the top of the pq with {@code newTop} and run {@link #updateTop()}.
*/
public final T updateTop(T newTop) {
heap[1] = newTop;
return updateTop();
}
/** Returns the number of elements currently stored in the PriorityQueue. */
public final int size() {
return size;
}
/** Removes all entries from the PriorityQueue. */
public final void clear() {
for (int i = 0; i <= size; i++) {
heap[i] = null;
}
size = 0;
}
/**
* Removes an existing element currently stored in the PriorityQueue. Cost is
* linear with the size of the queue. (A specialization of PriorityQueue which
* tracks element positions would provide a constant remove time but the
* trade-off would be extra cost to all additions/insertions)
*/
public final boolean remove(T element) {
for (int i = 1; i <= size; i++) {
if (heap[i] == element) {
heap[i] = heap[size];
heap[size] = null; // permit GC of objects
size--;
if (i <= size) {
if (!upHeap(i)) {
downHeap(i);
}
}
return true;
}
}
return false;
}
private final boolean upHeap(int origPos) {
int i = origPos;
T node = heap[i]; // save bottom node
int j = i >>> 1;
while (j > 0 && lessThan(node, heap[j])) {
heap[i] = heap[j]; // shift parents down
i = j;
j = j >>> 1;
}
heap[i] = node; // install saved node
return i != origPos;
}
private final void downHeap(int i) {
T node = heap[i]; // save top node
int j = i << 1; // find smaller child
int k = j + 1;
if (k <= size && lessThan(heap[k], heap[j])) {
j = k;
}
while (j <= size && lessThan(heap[j], node)) {
heap[i] = heap[j]; // shift up child
i = j;
j = i << 1;
k = j + 1;
if (k <= size && lessThan(heap[k], heap[j])) {
j = k;
}
}
heap[i] = node; // install saved node
}
/** This method returns the internal heap array as Object[].
* @lucene.internal
*/
protected final Object[] getHeapArray() {
return (Object[]) heap;
}
@Override
public Iterator<T> iterator() {
return new Iterator<T>() {
int i = 1;
@Override
public boolean hasNext() {
return i <= size;
}
@Override
public T next() {
if (hasNext() == false) {
throw new NoSuchElementException();
}
return heap[i++];
}
};
}
}