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/*
* Copyright (c) 2020-2023, NVIDIA CORPORATION.
*
* Licensed 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 com.nvidia.spark.rapids;
import org.apache.commons.lang3.mutable.MutableInt;
import java.util.AbstractQueue;
import java.util.Arrays;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Iterator;
/**
* Implements a priority queue based on a heap. Like many priority queue
* implementations, this provides logarithmic time for inserting elements
* and removing the top element. However unlike many implementations,
* this provides logarithmic rather than linear time for the random-access
* `contains` and `remove` methods. The queue also provides a
* mechanism for updating the heap after an element's priority has changed
* via the `priorityUpdated` method instead of requiring the element
* to be removed and re-inserted.
*
* The queue is NOT thread-safe.
*
*
The iterator does NOT return elements in priority
* order.
*/
public final class HashedPriorityQueue extends AbstractQueue {
private static final int DEFAULT_INITIAL_HEAP_SIZE = 16;
private final Comparator comparator;
/**
* An array-based heap. Given a node at index X, the indices of neighboring
* nodes can be computed via the following:
* - parent node is at (X-1)/2
* - left child is at 2*X+1
* - right child is at 2*X+2
*/
private T[] heap;
private int size;
/**
* The map of objects to their location in the heap array.
* MutableInt is used to reduce garbage creation during sifting.
*/
private final HashMap locationMap = new HashMap<>();
public HashedPriorityQueue(Comparator comparator) {
this(DEFAULT_INITIAL_HEAP_SIZE, comparator);
}
@SuppressWarnings("unchecked")
public HashedPriorityQueue(int initialHeapSize, Comparator comparator) {
this.comparator = comparator;
heap = (T[]) new Object[initialHeapSize];
size = 0;
}
@Override
public int size() {
return size;
}
@Override
public boolean offer(T obj) {
ensureCapacityToInsert();
MutableInt location = new MutableInt(size);
MutableInt oldLocation = locationMap.putIfAbsent(obj, location);
if (oldLocation == null) {
// Inserted a new object since we didn't have a prior location.
// Start with the new object at the bottom of the heap and sift it up
// until heap properties are restored.
size += 1;
siftUp(obj, location);
return true;
} else {
// we return false in the case where the object is already
// in the locationMap
return false;
}
}
@Override
public T poll() {
if (size == 0) {
return null;
}
T result = heap[0];
if (locationMap.remove(result) == null) {
throw new IllegalStateException("Object in heap without an index: " + result);
}
fillHoleWithLast(0);
return result;
}
@Override
public T peek() {
return heap[0];
}
@Override
public boolean contains(Object obj) {
return locationMap.containsKey(obj);
}
@SuppressWarnings("unchecked")
@Override
public boolean remove(Object o) {
T obj = (T) o;
MutableInt location = locationMap.remove(obj);
if (location == null) {
return false;
}
int heapIndex = location.getValue();
fillHoleWithLast(heapIndex);
return true;
}
@Override
public void clear() {
Arrays.fill(heap, 0, size, null);
locationMap.clear();
size = 0;
}
@Override
public Object[] toArray() {
return Arrays.copyOf(heap, size);
}
@SuppressWarnings("unchecked")
@Override
public T1[] toArray(T1[] a) {
if (a.length > size) {
System.arraycopy(heap, 0, a, 0, size);
return a;
}
return (T1[]) Arrays.copyOf(heap, size, a.getClass());
}
/**
* NOTE: This iterator DOES NOT iterate elements
* in priority order.
*/
@Override
public Iterator iterator() {
return Arrays.asList(Arrays.copyOf(heap, size)).iterator();
}
/**
* When an object in the heap changes priority, this must be called to
* restore proper ordering of the heap. After an object's priority
* changes, this method must be called before any elements are added
* or removed from the priority queue.
*/
public void priorityUpdated(T obj) {
MutableInt location = locationMap.get(obj);
if (location != null) {
// Try sifting up first since that operation is cheaper.
if (!siftUp(obj, location)) {
siftDown(obj, location);
}
}
}
/** Given an index of a node, return the index of the node's parent. */
private int getParentIndex(int heapIndex) {
return (heapIndex - 1) >>> 1;
}
private MutableInt getLocation(T obj) {
MutableInt location = locationMap.get(obj);
if (location == null) {
throw new IllegalStateException("Object in heap without a corresponding index: " + obj);
}
return location;
}
private void updateHeapIndex(T obj, int heapIndex) {
updateHeapIndex(obj, getLocation(obj), heapIndex);
}
private void updateHeapIndex(T obj, MutableInt location, int heapIndex) {
heap[heapIndex] = obj;
location.setValue(heapIndex);
}
/**
* Pop the last element off of the heap for placement elsewhere.
* The heap must not be empty when this is called, and it is the
* responsibility of the caller to update the index map for where
* the resulting object is ultimately placed in the heap.
*/
private T popLastElement() {
size -= 1;
T obj = heap[size];
heap[size] = null;
return obj;
}
private void ensureCapacityToInsert() {
if (heap.length == Integer.MAX_VALUE) {
throw new OutOfMemoryError("heap exceeded maximum array size");
}
int needed = size + 1;
if (heap.length <= needed) {
long newSize = Math.min((long) heap.length * 2, Integer.MAX_VALUE);
heap = Arrays.copyOf(heap, (int) newSize);
}
}
/**
* Perform a heap sift-up operation. The specified object is stored into
* the heap at its location after the sift-up operation completes.
*
* @param obj object being sifted
* @param location mutable current location that will be updated
* @return true if the object location was updated
*/
private boolean siftUp(T obj, MutableInt location) {
boolean sifted = false;
int heapIndex = location.getValue();
while (heapIndex > 0) {
int parentIndex = getParentIndex(heapIndex);
T parent = heap[parentIndex];
if (comparator.compare(obj, parent) >= 0) {
break;
}
sifted = true;
updateHeapIndex(parent, heapIndex);
heapIndex = parentIndex;
}
updateHeapIndex(obj, location, heapIndex);
return sifted;
}
/**
* Perform a heap sift-down operation. The specified object is stored into
* the heap at its location after the sift-down operation completes.
*
* @param obj object being sifted
* @param location mutable current location that will be updated
* @return true if the object location was updated
*/
private boolean siftDown(T obj, MutableInt location) {
boolean sifted = false;
int heapIndex = location.getValue();
final int parentIndexEnd = getParentIndex(size + 1);
while (heapIndex < parentIndexEnd) {
final int leftChildIndex = 2 * heapIndex + 1;
final int rightChildIndex = leftChildIndex + 1;
T leastChild = heap[leftChildIndex];
int leastChildIndex = leftChildIndex;
if (rightChildIndex < size) {
T rightChild = heap[rightChildIndex];
if (comparator.compare(leastChild, rightChild) > 0) {
leastChild = rightChild;
leastChildIndex = rightChildIndex;
}
}
if (comparator.compare(obj, leastChild) <= 0) {
break;
}
sifted = true;
updateHeapIndex(leastChild, heapIndex);
heapIndex = leastChildIndex;
}
updateHeapIndex(obj, location, heapIndex);
return sifted;
}
/**
* Move the last element into the specified index to fill a hole
* created by removing another object, then sift to restore the heap.
*/
private void fillHoleWithLast(int holeIndex) {
// Pop off the last element in the array to fill the hole.
// The element likely has a large value, so try sifting down first.
T obj = popLastElement();
if (holeIndex < size) {
MutableInt location = getLocation(obj);
location.setValue(holeIndex);
if (!siftDown(obj, location)) {
siftUp(obj, location);
}
}
}
}