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The Commons Collections PriorityBuffer.java source code
/*
* 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.commons.collections.buffer;
import java.io.Serializable;
import java.util.AbstractCollection;
import java.util.Comparator;
import java.util.Iterator;
import java.util.NoSuchElementException;
import org.apache.commons.collections.Buffer;
import org.apache.commons.collections.BufferUnderflowException;
/**
* Binary heap implementation of <code>Buffer that provides for
* removal based on <code>Comparator ordering.
* <p>
* The removal order of a binary heap is based on either the natural sort
* order of its elements or a specified {@link Comparator}. The
* {@link #remove()} method always returns the first element as determined
* by the sort order. (The <code>ascendingOrder flag in the constructors
* can be used to reverse the sort order, in which case {@link #remove()}
* will always remove the last element.) The removal order is
* <i>not the same as the order of iteration; elements are
* returned by the iterator in no particular order.
* <p>
* The {@link #add(Object)} and {@link #remove()} operations perform
* in logarithmic time. The {@link #get()} operation performs in constant
* time. All other operations perform in linear time or worse.
* <p>
* Note that this implementation is not synchronized. Use
* {@link org.apache.commons.collections.BufferUtils#synchronizedBuffer(Buffer)} or
* {@link org.apache.commons.collections.buffer.SynchronizedBuffer#decorate(Buffer)}
* to provide synchronized access to a <code>PriorityBuffer:
* <pre>
* Buffer heap = SynchronizedBuffer.decorate(new PriorityBuffer());
* </pre>
* <p>
* This class is Serializable from Commons Collections 3.2.
*
* @since Commons Collections 3.0 (previously BinaryHeap v1.0)
* @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
*
* @author Peter Donald
* @author Ram Chidambaram
* @author Michael A. Smith
* @author Paul Jack
* @author Stephen Colebourne
* @author Steve Phelps
*/
public class PriorityBuffer extends AbstractCollection
implements Buffer, Serializable {
/** Serialization lock. */
private static final long serialVersionUID = 6891186490470027896L;
/**
* The default capacity for the buffer.
*/
private static final int DEFAULT_CAPACITY = 13;
/**
* The elements in this buffer.
*/
protected Object[] elements;
/**
* The number of elements currently in this buffer.
*/
protected int size;
/**
* If true, the first element as determined by the sort order will
* be returned. If false, the last element as determined by the
* sort order will be returned.
*/
protected boolean ascendingOrder;
/**
* The comparator used to order the elements
*/
protected Comparator comparator;
//-----------------------------------------------------------------------
/**
* Constructs a new empty buffer that sorts in ascending order by the
* natural order of the objects added.
*/
public PriorityBuffer() {
this(DEFAULT_CAPACITY, true, null);
}
/**
* Constructs a new empty buffer that sorts in ascending order using the
* specified comparator.
*
* @param comparator the comparator used to order the elements,
* null means use natural order
*/
public PriorityBuffer(Comparator comparator) {
this(DEFAULT_CAPACITY, true, comparator);
}
/**
* Constructs a new empty buffer specifying the sort order and using the
* natural order of the objects added.
*
* @param ascendingOrder if <code>true the heap is created as a
* minimum heap; otherwise, the heap is created as a maximum heap
*/
public PriorityBuffer(boolean ascendingOrder) {
this(DEFAULT_CAPACITY, ascendingOrder, null);
}
/**
* Constructs a new empty buffer specifying the sort order and comparator.
*
* @param ascendingOrder true to use the order imposed by the given
* comparator; false to reverse that order
* @param comparator the comparator used to order the elements,
* null means use natural order
*/
public PriorityBuffer(boolean ascendingOrder, Comparator comparator) {
this(DEFAULT_CAPACITY, ascendingOrder, comparator);
}
/**
* Constructs a new empty buffer that sorts in ascending order by the
* natural order of the objects added, specifying an initial capacity.
*
* @param capacity the initial capacity for the buffer, greater than zero
* @throws IllegalArgumentException if <code>capacity is <= 0
*/
public PriorityBuffer(int capacity) {
this(capacity, true, null);
}
/**
* Constructs a new empty buffer that sorts in ascending order using the
* specified comparator and initial capacity.
*
* @param capacity the initial capacity for the buffer, greater than zero
* @param comparator the comparator used to order the elements,
* null means use natural order
* @throws IllegalArgumentException if <code>capacity is <= 0
*/
public PriorityBuffer(int capacity, Comparator comparator) {
this(capacity, true, comparator);
}
/**
* Constructs a new empty buffer that specifying initial capacity and
* sort order, using the natural order of the objects added.
*
* @param capacity the initial capacity for the buffer, greater than zero
* @param ascendingOrder if <code>true the heap is created as a
* minimum heap; otherwise, the heap is created as a maximum heap.
* @throws IllegalArgumentException if <code>capacity is <= 0
*/
public PriorityBuffer(int capacity, boolean ascendingOrder) {
this(capacity, ascendingOrder, null);
}
/**
* Constructs a new empty buffer that specifying initial capacity,
* sort order and comparator.
*
* @param capacity the initial capacity for the buffer, greater than zero
* @param ascendingOrder true to use the order imposed by the given
* comparator; false to reverse that order
* @param comparator the comparator used to order the elements,
* null means use natural order
* @throws IllegalArgumentException if <code>capacity is <= 0
*/
public PriorityBuffer(int capacity, boolean ascendingOrder, Comparator comparator) {
super();
if (capacity <= 0) {
throw new IllegalArgumentException("invalid capacity");
}
this.ascendingOrder = ascendingOrder;
//+1 as 0 is noop
this.elements = new Object[capacity + 1];
this.comparator = comparator;
}
//-----------------------------------------------------------------------
/**
* Checks whether the heap is ascending or descending order.
*
* @return true if ascending order (a min heap)
*/
public boolean isAscendingOrder() {
return ascendingOrder;
}
/**
* Gets the comparator being used for this buffer, null is natural order.
*
* @return the comparator in use, null is natural order
*/
public Comparator comparator() {
return comparator;
}
//-----------------------------------------------------------------------
/**
* Returns the number of elements in this buffer.
*
* @return the number of elements in this buffer
*/
public int size() {
return size;
}
/**
* Clears all elements from the buffer.
*/
public void clear() {
elements = new Object[elements.length]; // for gc
size = 0;
}
/**
* Adds an element to the buffer.
* <p>
* The element added will be sorted according to the comparator in use.
*
* @param element the element to be added
* @return true always
*/
public boolean add(Object element) {
if (isAtCapacity()) {
grow();
}
// percolate element to it's place in tree
if (ascendingOrder) {
percolateUpMinHeap(element);
} else {
percolateUpMaxHeap(element);
}
return true;
}
/**
* Gets the next element to be removed without actually removing it (peek).
*
* @return the next element
* @throws BufferUnderflowException if the buffer is empty
*/
public Object get() {
if (isEmpty()) {
throw new BufferUnderflowException();
} else {
return elements[1];
}
}
/**
* Gets and removes the next element (pop).
*
* @return the next element
* @throws BufferUnderflowException if the buffer is empty
*/
public Object remove() {
final Object result = get();
elements[1] = elements[size--];
// set the unused element to 'null' so that the garbage collector
// can free the object if not used anywhere else.(remove reference)
elements[size + 1] = null;
if (size != 0) {
// percolate top element to it's place in tree
if (ascendingOrder) {
percolateDownMinHeap(1);
} else {
percolateDownMaxHeap(1);
}
}
return result;
}
//-----------------------------------------------------------------------
/**
* Tests if the buffer is at capacity.
*
* @return <code>true if buffer is full; false otherwise.
*/
protected boolean isAtCapacity() {
//+1 as element 0 is noop
return elements.length == size + 1;
}
/**
* Percolates element down heap from the position given by the index.
* <p>
* Assumes it is a minimum heap.
*
* @param index the index for the element
*/
protected void percolateDownMinHeap(final int index) {
final Object element = elements[index];
int hole = index;
while ((hole * 2) <= size) {
int child = hole * 2;
// if we have a right child and that child can not be percolated
// up then move onto other child
if (child != size && compare(elements[child + 1], elements[child]) < 0) {
child++;
}
// if we found resting place of bubble then terminate search
if (compare(elements[child], element) >= 0) {
break;
}
elements[hole] = elements[child];
hole = child;
}
elements[hole] = element;
}
/**
* Percolates element down heap from the position given by the index.
* <p>
* Assumes it is a maximum heap.
*
* @param index the index of the element
*/
protected void percolateDownMaxHeap(final int index) {
final Object element = elements[index];
int hole = index;
while ((hole * 2) <= size) {
int child = hole * 2;
// if we have a right child and that child can not be percolated
// up then move onto other child
if (child != size && compare(elements[child + 1], elements[child]) > 0) {
child++;
}
// if we found resting place of bubble then terminate search
if (compare(elements[child], element) <= 0) {
break;
}
elements[hole] = elements[child];
hole = child;
}
elements[hole] = element;
}
/**
* Percolates element up heap from the position given by the index.
* <p>
* Assumes it is a minimum heap.
*
* @param index the index of the element to be percolated up
*/
protected void percolateUpMinHeap(final int index) {
int hole = index;
Object element = elements[hole];
while (hole > 1 && compare(element, elements[hole / 2]) < 0) {
// save element that is being pushed down
// as the element "bubble" is percolated up
final int next = hole / 2;
elements[hole] = elements[next];
hole = next;
}
elements[hole] = element;
}
/**
* Percolates a new element up heap from the bottom.
* <p>
* Assumes it is a minimum heap.
*
* @param element the element
*/
protected void percolateUpMinHeap(final Object element) {
elements[++size] = element;
percolateUpMinHeap(size);
}
/**
* Percolates element up heap from from the position given by the index.
* <p>
* Assume it is a maximum heap.
*
* @param index the index of the element to be percolated up
*/
protected void percolateUpMaxHeap(final int index) {
int hole = index;
Object element = elements[hole];
while (hole > 1 && compare(element, elements[hole / 2]) > 0) {
// save element that is being pushed down
// as the element "bubble" is percolated up
final int next = hole / 2;
elements[hole] = elements[next];
hole = next;
}
elements[hole] = element;
}
/**
* Percolates a new element up heap from the bottom.
* <p>
* Assume it is a maximum heap.
*
* @param element the element
*/
protected void percolateUpMaxHeap(final Object element) {
elements[++size] = element;
percolateUpMaxHeap(size);
}
/**
* Compares two objects using the comparator if specified, or the
* natural order otherwise.
*
* @param a the first object
* @param b the second object
* @return -ve if a less than b, 0 if they are equal, +ve if a greater than b
*/
protected int compare(Object a, Object b) {
if (comparator != null) {
return comparator.compare(a, b);
} else {
return ((Comparable) a).compareTo(b);
}
}
/**
* Increases the size of the heap to support additional elements
*/
protected void grow() {
final Object[] array = new Object[elements.length * 2];
System.arraycopy(elements, 0, array, 0, elements.length);
elements = array;
}
//-----------------------------------------------------------------------
/**
* Returns an iterator over this heap's elements.
*
* @return an iterator over this heap's elements
*/
public Iterator iterator() {
return new Iterator() {
private int index = 1;
private int lastReturnedIndex = -1;
public boolean hasNext() {
return index <= size;
}
public Object next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
lastReturnedIndex = index;
index++;
return elements[lastReturnedIndex];
}
public void remove() {
if (lastReturnedIndex == -1) {
throw new IllegalStateException();
}
elements[ lastReturnedIndex ] = elements[ size ];
elements[ size ] = null;
size--;
if( size != 0 && lastReturnedIndex <= size) {
int compareToParent = 0;
if (lastReturnedIndex > 1) {
compareToParent = compare(elements[lastReturnedIndex],
elements[lastReturnedIndex / 2]);
}
if (ascendingOrder) {
if (lastReturnedIndex > 1 && compareToParent < 0) {
percolateUpMinHeap(lastReturnedIndex);
} else {
percolateDownMinHeap(lastReturnedIndex);
}
} else { // max heap
if (lastReturnedIndex > 1 && compareToParent > 0) {
percolateUpMaxHeap(lastReturnedIndex);
} else {
percolateDownMaxHeap(lastReturnedIndex);
}
}
}
index--;
lastReturnedIndex = -1;
}
};
}
/**
* Returns a string representation of this heap. The returned string
* is similar to those produced by standard JDK collections.
*
* @return a string representation of this heap
*/
public String toString() {
final StringBuffer sb = new StringBuffer();
sb.append("[ ");
for (int i = 1; i < size + 1; i++) {
if (i != 1) {
sb.append(", ");
}
sb.append(elements[i]);
}
sb.append(" ]");
return sb.toString();
}
}
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