Java tutorial
/* * 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; import java.util.function.Supplier; /** * 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 pre-allocates an array of length {@code maxSize+1} * and pre-fills it with elements if instantiated via the * {@link #PriorityQueue(int,Supplier)} constructor. * * <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; /** * Create an empty priority queue of the configured size. */ public PriorityQueue(int maxSize) { this(maxSize, () -> null); } /** * Create a priority queue that is pre-filled with sentinel objects, 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, the supplier 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.<br> * * If this method is extended to return a non-null value, then the following * usage pattern is recommended: * * <pre class="prettyprint"> * 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> the given supplier will be called {@code maxSize} 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 and all returned instances must {@link #lessThan compare equal}. */ public PriorityQueue(int maxSize, Supplier<T> sentinelObjectSupplier) { final int heapSize; if (0 == maxSize) { // We allocate 1 extra to avoid if statement in top() heapSize = 2; } else { if ((maxSize < 0) || (maxSize >= ArrayUtil.MAX_ARRAY_LENGTH)) { // Throw exception to prevent confusing OOME: throw new IllegalArgumentException( "maxSize must be >= 0 and < " + (ArrayUtil.MAX_ARRAY_LENGTH) + "; got: " + maxSize); } // NOTE: we add +1 because all access to heap is // 1-based not 0-based. heap[0] is unused. heapSize = maxSize + 1; } // 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 sentinel objects are supported, populate the queue with them T sentinel = sentinelObjectSupplier.get(); if (sentinel != null) { heap[1] = sentinel; for (int i = 2; i < heap.length; i++) { heap[i] = sentinelObjectSupplier.get(); } 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); /** * 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++]; } }; } }