edu.harvard.med.screensaver.util.PowerSet.java Source code

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Here is the source code for edu.harvard.med.screensaver.util.PowerSet.java

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// $HeadURL$
// $Id$
//
// Copyright  2006, 2010, 2011, 2012 by the President and Fellows of Harvard College.
// 
// Screensaver is an open-source project developed by the ICCB-L and NSRB labs
// at Harvard Medical School. This software is distributed under the terms of
// the GNU General Public License.

package edu.harvard.med.screensaver.util;

import java.util.Comparator;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import java.util.SortedSet;
import java.util.TreeSet;

import org.apache.log4j.Logger;

import com.google.common.collect.Iterators;
import com.google.common.collect.Sets;

/**
 * Utility class for generating powersets. 
 * @author <a mailto="andrew_tolopko@hms.harvard.edu">Andrew Tolopko</a>
 */
public class PowerSet {
    private static final Logger log = Logger.getLogger(PowerSet.class);

    /**
     * Return an iterator that generates the powerset of the specified set.  
     */
    public static <T> Iterator<Set<T>> powerSetIterator(final Set<T> values) {
        return new Iterator<Set<T>>() {
            private int i;
            private long n = 1 << Math.max(0, values.size());

            public boolean hasNext() {
                return i < n;
            }

            public Set<T> next() {
                Set<T> set = new HashSet<T>();
                int j = 0;
                for (T e : values) {
                    if (((1 << j++) & i) != 0) {
                        set.add(e);
                    }
                }
                ++i;
                return set;
            }

            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
    }

    public static <T> Set<Set<T>> powerSet(Set<T> values) {
        Set<Set<T>> powerSet = new HashSet<Set<T>>();
        Iterators.addAll(powerSet, powerSetIterator(values));
        return powerSet;
    }

    /**
     * Returns the power set of the specified set, ordering the power set elements
     * (themselves sets) by size, then minimum value. Each the power set elements
     * (sets) are also ordered by the natural ordering of their values.
     */
    public static <T extends Comparable<T>> SortedSet<SortedSet<T>> orderedPowerset(Set<T> values) {
        SortedSet<SortedSet<T>> orderedPowerset = new TreeSet<SortedSet<T>>(new Comparator<SortedSet<T>>() {
            public int compare(SortedSet<T> s1, SortedSet<T> s2) {
                if (s1.size() < s2.size()) {
                    return -1;
                } else if (s1.size() > s2.size()) {
                    return 1;
                } else {
                    Iterator<T> i1 = s1.iterator();
                    Iterator<T> i2 = s2.iterator();
                    int c = 0;
                    while (i1.hasNext() && i2.hasNext() && c == 0) {
                        c = i1.next().compareTo(i2.next());
                    }
                    return c;
                }
            }
        });
        Iterator<Set<T>> powerSetIterator = powerSetIterator(values);
        while (powerSetIterator.hasNext()) {
            TreeSet<T> orderedSetElement = Sets.newTreeSet(powerSetIterator.next());
            orderedPowerset.add(orderedSetElement);
            if (log.isDebugEnabled()) {
                log.debug("added " + orderedSetElement);
                log.debug("powerset = " + orderedPowerset);
            }
        }
        return orderedPowerset;
    }

}