gnu.trove.THash Maven / Gradle / Ivy
///////////////////////////////////////////////////////////////////////////////
// Copyright (c) 2001, Eric D. Friedman All Rights Reserved.
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this program; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
///////////////////////////////////////////////////////////////////////////////
package gnu.trove;
import java.io.Externalizable;
import java.io.ObjectOutput;
import java.io.IOException;
import java.io.ObjectInput;
/**
* Base class for hashtables that use open addressing to resolve
* collisions.
*
* Created: Wed Nov 28 21:11:16 2001
*
* @author Eric D. Friedman
* @author Rob Eden (auto-compaction)
*
* @version $Id: THash.java,v 1.14 2008/10/08 16:39:10 robeden Exp $
*/
abstract public class THash implements Cloneable, Externalizable {
static final long serialVersionUID = -1792948471915530295L;
/** the load above which rehashing occurs. */
protected static final float DEFAULT_LOAD_FACTOR = 0.5f;
/** the default initial capacity for the hash table. This is one
* less than a prime value because one is added to it when
* searching for a prime capacity to account for the free slot
* required by open addressing. Thus, the real default capacity is
* 11. */
protected static final int DEFAULT_INITIAL_CAPACITY = 10;
/** the current number of occupied slots in the hash. */
protected transient int _size;
/** the current number of free slots in the hash. */
protected transient int _free;
/** Determines how full the internal table can become before
* rehashing is required. This must be a value in the range: 0.0 <
* loadFactor < 1.0. The default value is 0.5, which is about as
* large as you can get in open addressing without hurting
* performance. Cf. Knuth, Volume 3., Chapter 6.
*/
protected float _loadFactor;
/**
* The maximum number of elements allowed without allocating more
* space.
*/
protected int _maxSize;
/**
* The number of removes that should be performed before an auto-compaction occurs.
*/
protected int _autoCompactRemovesRemaining;
/**
* The auto-compaction factor for the table.
*
* @see #setAutoCompactionFactor
*/
protected float _autoCompactionFactor;
/**
* @see #tempDisableAutoCompaction
*/
private transient boolean _autoCompactTemporaryDisable = false;
/**
* Creates a new THash instance with the default
* capacity and load factor.
*/
public THash() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
/**
* Creates a new THash instance with a prime capacity
* at or near the specified capacity and with the default load
* factor.
*
* @param initialCapacity an int value
*/
public THash(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Creates a new THash instance with a prime capacity
* at or near the minimum needed to hold initialCapacity
* elements with load factor loadFactor without triggering
* a rehash.
*
* @param initialCapacity an int value
* @param loadFactor a float value
*/
public THash(int initialCapacity, float loadFactor) {
super();
_loadFactor = loadFactor;
// Through testing, the load factor (especially the default load factor) has been
// found to be a pretty good starting auto-compaction factor.
_autoCompactionFactor = loadFactor;
setUp(HashFunctions.fastCeil(initialCapacity / loadFactor));
}
public Object clone() {
try {
return super.clone();
} catch (CloneNotSupportedException cnse) {
return null; // it's supported
}
}
/**
* Tells whether this set is currently holding any elements.
*
* @return a boolean value
*/
public boolean isEmpty() {
return 0 == _size;
}
/**
* Returns the number of distinct elements in this collection.
*
* @return an int value
*/
public int size() {
return _size;
}
/**
* @return the current physical capacity of the hash table.
*/
abstract protected int capacity();
/**
* Ensure that this hashtable has sufficient capacity to hold
* desiredCapacity additional elements without
* requiring a rehash. This is a tuning method you can call
* before doing a large insert.
*
* @param desiredCapacity an int value
*/
public void ensureCapacity(int desiredCapacity) {
if (desiredCapacity > (_maxSize - size())) {
rehash(PrimeFinder.nextPrime(HashFunctions.fastCeil(
(desiredCapacity + size()) / _loadFactor) + 1));
computeMaxSize(capacity());
}
}
/**
* Compresses the hashtable to the minimum prime size (as defined
* by PrimeFinder) that will hold all of the elements currently in
* the table. If you have done a lot of remove
* operations and plan to do a lot of queries or insertions or
* iteration, it is a good idea to invoke this method. Doing so
* will accomplish two things:
*
*
* - You'll free memory allocated to the table but no
* longer needed because of the remove()s.
*
* - You'll get better query/insert/iterator performance
* because there won't be any REMOVED slots to skip
* over when probing for indices in the table.
*
*/
public void compact() {
// need at least one free spot for open addressing
rehash(PrimeFinder.nextPrime(HashFunctions.fastCeil(size()/_loadFactor) + 1));
computeMaxSize(capacity());
// If auto-compaction is enabled, re-determine the compaction interval
if ( _autoCompactionFactor != 0 ) {
computeNextAutoCompactionAmount(size());
}
}
/**
* The auto-compaction factor controls whether and when a table performs a
* {@link #compact} automatically after a certain number of remove operations.
* If the value is non-zero, the number of removes that need to occur for
* auto-compaction is the size of table at the time of the previous compaction
* (or the initial capacity) multiplied by this factor.
*
* Setting this value to zero will disable auto-compaction.
*/
public void setAutoCompactionFactor( float factor ) {
if ( factor < 0 ) {
throw new IllegalArgumentException( "Factor must be >= 0: " + factor );
}
_autoCompactionFactor = factor;
}
/**
* @see #setAutoCompactionFactor
*/
public float getAutoCompactionFactor() {
return _autoCompactionFactor;
}
/**
* This simply calls {@link #compact compact}. It is included for
* symmetry with other collection classes. Note that the name of this
* method is somewhat misleading (which is why we prefer
* compact) as the load factor may require capacity above
* and beyond the size of this collection.
*
* @see #compact
*/
public final void trimToSize() {
compact();
}
/**
* Delete the record at index. Reduces the size of the
* collection by one.
*
* @param index an int value
*/
protected void removeAt(int index) {
_size--;
// If auto-compaction is enabled, see if we need to compact
if ( _autoCompactionFactor != 0 ) {
_autoCompactRemovesRemaining--;
if ( !_autoCompactTemporaryDisable && _autoCompactRemovesRemaining <= 0 ) {
// Do the compact
// NOTE: this will cause the next compaction interval to be calculated
compact();
}
}
}
/**
* Empties the collection.
*/
public void clear() {
_size = 0;
_free = capacity();
}
/**
* initializes the hashtable to a prime capacity which is at least
* initialCapacity + 1.
*
* @param initialCapacity an int value
* @return the actual capacity chosen
*/
protected int setUp(int initialCapacity) {
int capacity;
capacity = PrimeFinder.nextPrime(initialCapacity);
computeMaxSize(capacity);
computeNextAutoCompactionAmount(initialCapacity);
return capacity;
}
/**
* Rehashes the set.
*
* @param newCapacity an int value
*/
protected abstract void rehash(int newCapacity);
/**
* Temporarily disables auto-compaction. MUST be followed by calling
* {@link #reenableAutoCompaction}.
*/
protected void tempDisableAutoCompaction() {
_autoCompactTemporaryDisable = true;
}
/**
* Re-enable auto-compaction after it was disabled via
* {@link #tempDisableAutoCompaction()}.
*
* @param check_for_compaction True if compaction should be performed if needed
* before returning. If false, no compaction will be
* performed.
*/
protected void reenableAutoCompaction( boolean check_for_compaction ) {
_autoCompactTemporaryDisable = false;
if ( check_for_compaction && _autoCompactRemovesRemaining <= 0 &&
_autoCompactionFactor != 0 ) {
// Do the compact
// NOTE: this will cause the next compaction interval to be calculated
compact();
}
}
/**
* Computes the values of maxSize. There will always be at least
* one free slot required.
*
* @param capacity an int value
*/
private void computeMaxSize(int capacity) {
// need at least one free slot for open addressing
_maxSize = Math.min(capacity - 1, (int) (capacity * _loadFactor));
_free = capacity - _size; // reset the free element count
}
/**
* Computes the number of removes that need to happen before the next auto-compaction
* will occur.
*/
private void computeNextAutoCompactionAmount( int size ) {
if ( _autoCompactionFactor != 0 ) {
// NOTE: doing the round ourselves has been found to be faster than using
// Math.round.
_autoCompactRemovesRemaining =
( int ) ( ( size * _autoCompactionFactor ) + 0.5f );
}
}
/**
* After an insert, this hook is called to adjust the size/free
* values of the set and to perform rehashing if necessary.
*/
protected final void postInsertHook(boolean usedFreeSlot) {
if (usedFreeSlot) {
_free--;
}
// rehash whenever we exhaust the available space in the table
if (++_size > _maxSize || _free == 0) {
// choose a new capacity suited to the new state of the table
// if we've grown beyond our maximum size, double capacity;
// if we've exhausted the free spots, rehash to the same capacity,
// which will free up any stale removed slots for reuse.
int newCapacity = _size > _maxSize ? PrimeFinder.nextPrime(capacity() << 1) : capacity();
rehash(newCapacity);
computeMaxSize(capacity());
}
}
protected int calculateGrownCapacity() {
return capacity() << 1;
}
public void writeExternal(ObjectOutput out) throws IOException {
// VERSION
out.writeByte( 0 );
// LOAD FACTOR
out.writeFloat( _loadFactor );
// AUTO COMPACTION LOAD FACTOR
out.writeFloat( _autoCompactionFactor );
}
public void readExternal(ObjectInput in)
throws IOException, ClassNotFoundException {
// VERSION
in.readByte();
// LOAD FACTOR
float old_factor = _loadFactor;
_loadFactor = in.readFloat();
// AUTO COMPACTION LOAD FACTOR
_autoCompactionFactor = in.readFloat();
// If we change the laod factor from the default, re-setup
if ( old_factor != _loadFactor ) {
setUp((int)Math.ceil(DEFAULT_INITIAL_CAPACITY / _loadFactor));
}
}
}// THash