TinyHashSet.java

/*-
 * See the file LICENSE for redistribution information.
 *
 * Copyright (c) 2002-2006
 *      Sleepycat Software.  All rights reserved.
 *
 * $Id: TinyHashSet.java,v 1.1 2006/05/06 09:00:28 ckaestne Exp $
 */

package com.sleepycat.je.utilint;

import java.util.HashSet;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Set;

/**
 * TinyHashSet is used to optimize (for speed, not space) the case where a
 * HashSet generally holds a single element.  This saves us the cost of
 * creating the HashSet and related elements as well as call Object.hashCode().
 *
 * If single != null, it's the only element in the TinyHashSet.  If set != null
 * then there are multiple elements in the TinyHashSet.  It should never be
 * true that (single != null) && (set != null).
 */
public class TinyHashSet {

    private Set set;
    private Object single;

    /*
     * Will return a fuzzy value if the not under synchronized control.
     */
    public int size() {
	if (single != null) {
	    return 1;
	} else if (set != null) {
	    return set.size();
	} else {
	    return 0;
	}
    }

    public boolean remove(Object o) {
	assert (single == null) || (set == null);
	if (single != null) {
	    if (single == o ||
		single.equals(o)) {
		single = null;
		return true;
	    } else {
		return false;
	    }
	} else if (set != null) {
	    return set.remove(o);
	} else {
	    return false;
	}
    }

    public boolean add(Object o) {
	assert (single == null) || (set == null);
	if (set != null) {
	    return set.add(o);
	} else if (single == null) {
	    single = o;
	    return true;
	} else {
	    set = new HashSet();
	    set.add(single);
	    single = null;
	    return set.add(o);
	}
    }

    public Set copy() {
	assert (single == null) || (set == null);
	if (set != null) {
	    return new HashSet(set);
	} else {
	    Set ret = new HashSet();
	    if (single != null) {
		ret.add(single);
	    }
	    return ret;
	}
    }

    public Iterator iterator() {
	assert (single == null) || (set == null);
	if (set != null) {
	    return set.iterator();
	} else {
	    return new SingleElementIterator(single, this);
	}
    }

    /*
     * Iterator that is used to just return one element.
     */
    public static class SingleElementIterator implements Iterator {
	Object theObject;
	TinyHashSet theSet;
	boolean returnedTheObject = false;

	SingleElementIterator(Object o, TinyHashSet theSet) {
	    theObject = o;
	    this.theSet = theSet;
	    returnedTheObject = (o == null);
	}

	public boolean hasNext() {
	    return !returnedTheObject;
	}

	public Object next() {
	    if (returnedTheObject) {
		throw new NoSuchElementException();
	    }

	    returnedTheObject = true;
	    return theObject;
	}

	public void remove() {
	    if (theObject == null ||
		!returnedTheObject) {
		throw new IllegalStateException();
	    }
	    theSet.remove(theObject);
	}
    }
}