Concurrent Cuckoo hashing using lock striping. Uses R/W locks for resizing. Exercise solution.
/*
* RWStripedHashSet.java
*
* Created on November 15, 2006, 3:35 PM
*
* From "The Art of Multiprocessor Programming",
* by Maurice Herlihy and Nir Shavit.
*
* This work is licensed under a Creative Commons Attribution-Share Alike 3.0 United States License.
* http://i.creativecommons.org/l/by-sa/3.0/us/88x31.png
*/
//package xbird.util.concurrent.set;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
/**
* Concurrent Cuckoo hashing using lock striping.
* Uses R/W locks for resizing. Exercise solution.
* @param <T> type
* @author Maurice Herlihy
*/
public class RWStripedHashSet<T> extends BaseHashSet<T> {
final Lock readLock;
final Lock writeLock;
volatile Lock[] locks;
/**
* Constructor
* @param capacity Initial number of buckets.
*/
public RWStripedHashSet(int capacity) {
super(capacity);
locks = new Lock[capacity];
for(int j = 0; j < locks.length; j++) {
locks[j] = new ReentrantLock();
}
ReadWriteLock rwLock = new ReentrantReadWriteLock();
readLock = rwLock.readLock();
writeLock = rwLock.writeLock();
}
/**
* double the set size
*/
@Override
public void resize() {
int oldCapacity = table.length;
writeLock.lock();
try {
if(oldCapacity != table.length) {
return; // someone beat us to it
}
int newCapacity = 2 * oldCapacity;
List<T>[] oldTable = table;
table = (List<T>[]) new List[newCapacity];
for(int i = 0; i < newCapacity; i++)
table[i] = new ArrayList<T>();
initializeFrom(oldTable);
} finally {
writeLock.unlock();
}
}
private void initializeFrom(List<T>[] oldTable) {
for(List<T> bucket : oldTable) {
for(T x : bucket) {
int myBucket = Math.abs(x.hashCode() % table.length);
table[myBucket].add(x);
}
}
}
/**
* Synchronize before adding, removing, or testing for item
* @param x item involved
*/
@Override
public final void acquire(T x) {
readLock.lock();
int myBucket = Math.abs(x.hashCode() % locks.length);
locks[myBucket].lock();
}
/**
* synchronize after adding, removing, or testing for item
* @param x item involved
*/
@Override
public void release(T x) {
readLock.unlock();
int myBucket = Math.abs(x.hashCode() % locks.length);
locks[myBucket].unlock();
}
@Override
public boolean policy() {
return size / table.length > 4;
}
}
/**
* Simple fine-grained hash map.
* @param <T> type
* @author Maurice Herlihy
*/
abstract class BaseHashSet<T> {
protected List<T>[] table;
protected int size;
public BaseHashSet(int capacity) {
size = 0;
table = (List<T>[]) new List[capacity];
for(int i = 0; i < capacity; i++) {
table[i] = new ArrayList<T>();
}
}
/**
* Is item in set?
* @param x item to test
* @return <code>true</code> iff item present
*/
public boolean contains(T x) {
acquire(x);
try {
int myBucket = Math.abs(x.hashCode() % table.length);
return table[myBucket].contains(x);
} finally {
release(x);
}
}
/**
* Add item to set
* @param x item to add
* @return <code>true</code> iff set changed
*/
public boolean add(T x) {
boolean result = false;
acquire(x);
try {
int myBucket = Math.abs(x.hashCode() % table.length);
result = table[myBucket].add(x);
size = result ? size + 1 : size;
} finally {
release(x); // always unlock
}
if(policy())
resize();
return result;
}
/**
* Remove item from set
* @param x item to remove
* @return <code>true</code> iff set changed
*/
public boolean remove(T x) {
acquire(x);
try {
int myBucket = Math.abs(x.hashCode() % table.length);
boolean result = table[myBucket].remove(x);
size = result ? size - 1 : size;
return result;
} finally {
release(x); // always unlock
}
}
/**
* Synchronize before adding, removing, or testing for item
* @param x item involved
*/
public abstract void acquire(T x);
/**
* synchronize after adding, removing, or testing for item
* @param x item involved
*/
public abstract void release(T x);
/**
* double the set size
*/
public abstract void resize();
/**
* decide whether to resize
* @return whether to resize
*/
public abstract boolean policy();
}
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