org.apache.hadoop.hdfs.util.LightWeightGSet Maven / Gradle / Ivy
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/**
* 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.hadoop.hdfs.util;
import java.io.PrintStream;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.HadoopIllegalArgumentException;
/**
* A low memory footprint {@link GSet} implementation,
* which uses an array for storing the elements
* and linked lists for collision resolution.
*
* No rehash will be performed.
* Therefore, the internal array will never be resized.
*
* This class does not support null element.
*
* This class is not thread safe.
*
* @param Key type for looking up the elements
* @param Element type, which must be
* (1) a subclass of K, and
* (2) implementing {@link LinkedElement} interface.
*/
@InterfaceAudience.Private
public class LightWeightGSet implements GSet {
/**
* Elements of {@link LightWeightGSet}.
*/
public static interface LinkedElement {
/** Set the next element. */
public void setNext(LinkedElement next);
/** Get the next element. */
public LinkedElement getNext();
}
public static final Log LOG = LogFactory.getLog(GSet.class);
static final int MAX_ARRAY_LENGTH = 1 << 30; //prevent int overflow problem
static final int MIN_ARRAY_LENGTH = 1;
/**
* An internal array of entries, which are the rows of the hash table.
* The size must be a power of two.
*/
private final LinkedElement[] entries;
/** A mask for computing the array index from the hash value of an element. */
private final int hash_mask;
/** The size of the set (not the entry array). */
private int size = 0;
/** Modification version for fail-fast.
* @see ConcurrentModificationException
*/
private volatile int modification = 0;
/**
* @param recommended_length Recommended size of the internal array.
*/
public LightWeightGSet(final int recommended_length) {
final int actual = actualArrayLength(recommended_length);
LOG.info("recommended=" + recommended_length + ", actual=" + actual);
entries = new LinkedElement[actual];
hash_mask = entries.length - 1;
}
//compute actual length
private static int actualArrayLength(int recommended) {
if (recommended > MAX_ARRAY_LENGTH) {
return MAX_ARRAY_LENGTH;
} else if (recommended < MIN_ARRAY_LENGTH) {
return MIN_ARRAY_LENGTH;
} else {
final int a = Integer.highestOneBit(recommended);
return a == recommended? a: a << 1;
}
}
@Override
public int size() {
return size;
}
private int getIndex(final K key) {
return key.hashCode() & hash_mask;
}
private E convert(final LinkedElement e){
@SuppressWarnings("unchecked")
final E r = (E)e;
return r;
}
@Override
public E get(final K key) {
//validate key
if (key == null) {
throw new NullPointerException("key == null");
}
//find element
final int index = getIndex(key);
for(LinkedElement e = entries[index]; e != null; e = e.getNext()) {
if (e.equals(key)) {
return convert(e);
}
}
//element not found
return null;
}
@Override
public boolean contains(final K key) {
return get(key) != null;
}
@Override
public E put(final E element) {
//validate element
if (element == null) {
throw new NullPointerException("Null element is not supported.");
}
if (!(element instanceof LinkedElement)) {
throw new HadoopIllegalArgumentException(
"!(element instanceof LinkedElement), element.getClass()="
+ element.getClass());
}
final LinkedElement e = (LinkedElement)element;
//find index
final int index = getIndex(element);
//remove if it already exists
final E existing = remove(index, element);
//insert the element to the head of the linked list
modification++;
size++;
e.setNext(entries[index]);
entries[index] = e;
return existing;
}
/**
* Remove the element corresponding to the key,
* given key.hashCode() == index.
*
* @return If such element exists, return it.
* Otherwise, return null.
*/
private E remove(final int index, final K key) {
if (entries[index] == null) {
return null;
} else if (entries[index].equals(key)) {
//remove the head of the linked list
modification++;
size--;
final LinkedElement e = entries[index];
entries[index] = e.getNext();
e.setNext(null);
return convert(e);
} else {
//head != null and key is not equal to head
//search the element
LinkedElement prev = entries[index];
for(LinkedElement curr = prev.getNext(); curr != null; ) {
if (curr.equals(key)) {
//found the element, remove it
modification++;
size--;
prev.setNext(curr.getNext());
curr.setNext(null);
return convert(curr);
} else {
prev = curr;
curr = curr.getNext();
}
}
//element not found
return null;
}
}
@Override
public E remove(final K key) {
//validate key
if (key == null) {
throw new NullPointerException("key == null");
}
return remove(getIndex(key), key);
}
@Override
public Iterator iterator() {
return new SetIterator();
}
@Override
public String toString() {
final StringBuilder b = new StringBuilder(getClass().getSimpleName());
b.append("(size=").append(size)
.append(String.format(", %08x", hash_mask))
.append(", modification=").append(modification)
.append(", entries.length=").append(entries.length)
.append(")");
return b.toString();
}
/** Print detailed information of this object. */
public void printDetails(final PrintStream out) {
out.print(this + ", entries = [");
for(int i = 0; i < entries.length; i++) {
if (entries[i] != null) {
LinkedElement e = entries[i];
out.print("\n " + i + ": " + e);
for(e = e.getNext(); e != null; e = e.getNext()) {
out.print(" -> " + e);
}
}
}
out.println("\n]");
}
private class SetIterator implements Iterator {
/** The starting modification for fail-fast. */
private final int startModification = modification;
/** The current index of the entry array. */
private int index = -1;
/** The next element to return. */
private LinkedElement next = nextNonemptyEntry();
/** Find the next nonempty entry starting at (index + 1). */
private LinkedElement nextNonemptyEntry() {
for(index++; index < entries.length && entries[index] == null; index++);
return index < entries.length? entries[index]: null;
}
@Override
public boolean hasNext() {
return next != null;
}
@Override
public E next() {
if (modification != startModification) {
throw new ConcurrentModificationException("modification=" + modification
+ " != startModification = " + startModification);
}
final E e = convert(next);
//find the next element
final LinkedElement n = next.getNext();
next = n != null? n: nextNonemptyEntry();
return e;
}
@Override
public void remove() {
throw new UnsupportedOperationException("Remove is not supported.");
}
}
}