Mega Code Archive
A memory-efficient hash set
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/*
* Copyright 2009 Google Inc.
*
* 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.
*/
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.reflect.Array;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* A memory-efficient hash set.
*
* @param <E> the element type
*/
public class HashSet<E> extends AbstractSet<E> implements Serializable {
private class SetIterator implements Iterator<E> {
private int index = 0;
private int last = -1;
public SetIterator() {
advanceToItem();
}
public boolean hasNext() {
return index < table.length;
}
@SuppressWarnings("unchecked")
public E next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
last = index;
E toReturn = (E) unmaskNull(table[index++]);
advanceToItem();
return toReturn;
}
public void remove() {
if (last < 0) {
throw new IllegalStateException();
}
internalRemove(last);
if (table[last] != null) {
index = last;
}
last = -1;
}
private void advanceToItem() {
for (; index < table.length; ++index) {
if (table[index] != null) {
return;
}
}
}
}
/**
* In the interest of memory-savings, we start with the smallest feasible
* power-of-two table size that can hold three items without rehashing. If we
* started with a size of 2, we'd have to expand as soon as the second item
* was added.
*/
private static final int INITIAL_TABLE_SIZE = 4;
private static final Object NULL_ITEM = new Serializable() {
Object readResolve() {
return NULL_ITEM;
}
};
static Object maskNull(Object o) {
return (o == null) ? NULL_ITEM : o;
}
static Object unmaskNull(Object o) {
return (o == NULL_ITEM) ? null : o;
}
/**
* Number of objects in this set; transient due to custom serialization.
* Default access to avoid synthetic accessors from inner classes.
*/
transient int size = 0;
/**
* Backing store for all the objects; transient due to custom serialization.
* Default access to avoid synthetic accessors from inner classes.
*/
transient Object[] table;
public HashSet() {
table = new Object[INITIAL_TABLE_SIZE];
}
public HashSet(Collection<? extends E> c) {
int newCapacity = INITIAL_TABLE_SIZE;
int expectedSize = c.size();
while (newCapacity * 3 < expectedSize * 4) {
newCapacity <<= 1;
}
table = new Object[newCapacity];
super.addAll(c);
}
@Override
public boolean add(E e) {
ensureSizeFor(size + 1);
int index = findOrEmpty(e);
if (table[index] == null) {
++size;
table[index] = maskNull(e);
return true;
}
return false;
}
@Override
public boolean addAll(Collection<? extends E> c) {
ensureSizeFor(size + c.size());
return super.addAll(c);
}
@Override
public void clear() {
table = new Object[INITIAL_TABLE_SIZE];
size = 0;
}
@Override
public boolean contains(Object o) {
return find(o) >= 0;
}
@Override
public Iterator<E> iterator() {
return new SetIterator();
}
@Override
public boolean remove(Object o) {
int index = find(o);
if (index < 0) {
return false;
}
internalRemove(index);
return true;
}
@Override
public int size() {
return size;
}
@Override
public Object[] toArray() {
return toArray(new Object[size]);
}
@SuppressWarnings("unchecked")
@Override
public <T> T[] toArray(T[] a) {
if (a.length < size) {
a = (T[]) Array.newInstance(a.getClass().getComponentType(), size);
}
int index = 0;
for (int i = 0; i < table.length; ++i) {
Object e = table[i];
if (e != null) {
a[index++] = (T) unmaskNull(e);
}
}
while (index < a.length) {
a[index++] = null;
}
return a;
}
/**
* Adapted from {@link org.apache.commons.collections.map.AbstractHashedMap}.
*/
@SuppressWarnings("unchecked")
protected void doReadObject(ObjectInputStream in) throws IOException,
ClassNotFoundException {
table = new Object[in.readInt()];
int items = in.readInt();
for (int i = 0; i < items; i++) {
add((E) in.readObject());
}
}
/**
* Adapted from {@link org.apache.commons.collections.map.AbstractHashedMap}.
*/
protected void doWriteObject(ObjectOutputStream out) throws IOException {
out.writeInt(table.length);
out.writeInt(size);
for (int i = 0; i < table.length; ++i) {
Object e = table[i];
if (e != null) {
out.writeObject(unmaskNull(e));
}
}
}
/**
* Returns whether two items are equal for the purposes of this set.
*/
protected boolean itemEquals(Object a, Object b) {
return (a == null) ? (b == null) : a.equals(b);
}
/**
* Return the hashCode for an item.
*/
protected int itemHashCode(Object o) {
return (o == null) ? 0 : o.hashCode();
}
/**
* Works just like {@link #addAll(Collection)}, but for arrays. Used to avoid
* having to synthesize a collection in {@link Sets}.
*/
void addAll(E[] elements) {
ensureSizeFor(size + elements.length);
for (E e : elements) {
int index = findOrEmpty(e);
if (table[index] == null) {
++size;
table[index] = maskNull(e);
}
}
}
/**
* Removes the item at the specified index, and performs internal management
* to make sure we don't wind up with a hole in the table. Default access to
* avoid synthetic accessors from inner classes.
*/
void internalRemove(int index) {
table[index] = null;
--size;
plugHole(index);
}
/**
* Ensures the set is large enough to contain the specified number of entries.
*/
private void ensureSizeFor(int expectedSize) {
if (table.length * 3 >= expectedSize * 4) {
return;
}
int newCapacity = table.length << 1;
while (newCapacity * 3 < expectedSize * 4) {
newCapacity <<= 1;
}
Object[] oldTable = table;
table = new Object[newCapacity];
for (Object o : oldTable) {
if (o != null) {
int newIndex = getIndex(unmaskNull(o));
while (table[newIndex] != null) {
if (++newIndex == table.length) {
newIndex = 0;
}
}
table[newIndex] = o;
}
}
}
/**
* Returns the index in the table at which a particular item resides, or -1 if
* the item is not in the table.
*/
private int find(Object o) {
int index = getIndex(o);
while (true) {
Object existing = table[index];
if (existing == null) {
return -1;
}
if (itemEquals(o, unmaskNull(existing))) {
return index;
}
if (++index == table.length) {
index = 0;
}
}
}
/**
* Returns the index in the table at which a particular item resides, or the
* index of an empty slot in the table where this item should be inserted if
* it is not already in the table.
*/
private int findOrEmpty(Object o) {
int index = getIndex(o);
while (true) {
Object existing = table[index];
if (existing == null) {
return index;
}
if (itemEquals(o, unmaskNull(existing))) {
return index;
}
if (++index == table.length) {
index = 0;
}
}
}
private int getIndex(Object o) {
int h = itemHashCode(o);
// Copied from Apache's AbstractHashedMap; prevents power-of-two collisions.
h += ~(h << 9);
h ^= (h >>> 14);
h += (h << 4);
h ^= (h >>> 10);
// Power of two trick.
return h & (table.length - 1);
}
/**
* Tricky, we left a hole in the map, which we have to fill. The only way to
* do this is to search forwards through the map shuffling back values that
* match this index until we hit a null.
*/
private void plugHole(int hole) {
int index = hole + 1;
if (index == table.length) {
index = 0;
}
while (table[index] != null) {
int targetIndex = getIndex(unmaskNull(table[index]));
if (hole < index) {
/*
* "Normal" case, the index is past the hole and the "bad range" is from
* hole (exclusive) to index (inclusive).
*/
if (!(hole < targetIndex && targetIndex <= index)) {
// Plug it!
table[hole] = table[index];
table[index] = null;
hole = index;
}
} else {
/*
* "Wrapped" case, the index is before the hole (we've wrapped) and the
* "good range" is from index (exclusive) to hole (inclusive).
*/
if (index < targetIndex && targetIndex <= hole) {
// Plug it!
table[hole] = table[index];
table[index] = null;
hole = index;
}
}
if (++index == table.length) {
index = 0;
}
}
}
private void readObject(ObjectInputStream in) throws IOException,
ClassNotFoundException {
in.defaultReadObject();
doReadObject(in);
}
private void writeObject(ObjectOutputStream out) throws IOException {
out.defaultWriteObject();
doWriteObject(out);
}
}
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