org.simmetrics.metrics.GeneralizedJaccard.java Source code

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Here is the source code for org.simmetrics.metrics.GeneralizedJaccard.java

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/*
 * #%L
 * Simmetrics Core
 * %%
 * Copyright (C) 2014 - 2016 Simmetrics Authors
 * %%
 * 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.
 * #L%
 */

package org.simmetrics.metrics;

import static org.simmetrics.metrics.Math.intersection;

import org.simmetrics.MultisetDistance;
import org.simmetrics.MultisetMetric;

import com.google.common.collect.Multiset;

/**
 * Calculates the generalized Jaccard distance and similarity coefficient over
 * two multisets. The similarity is defined as the size of the intersection
 * divided by the size of the union of the sample sets. The distance is obtained
 * by subtracting the Jaccard coefficient from 1.
 * <p>
 * <code>
 * similarity(a,b) = a  b / a  b
 * <br>
 * distance(a,b) = 1 - similarity(a,b)
 * </code>
 * <p>
 * When <code>a  b</code> is empty the multisets have no elements in common.
 * In this case the similarity is 0 by definition.
 * <p>
 * Unlike the Jaccard index the occurrence (cardinality) of an entry is taken
 * into account. E.g. {@code [hello, world]} and
 * {@code [hello, world, hello, world]} would be identical when compared with
 * the Jaccard index but are dissimilar when the generalized version is used.
 * <p>
 * This class is immutable and thread-safe.
 * 
 * @see Jaccard
 * @see <a href="http://en.wikipedia.org/wiki/Jaccard_index">Wikipedia - Jaccard
 *      index</a>
 * 
 * @param <T>
 *            type of the token
 * 
 */
public final class GeneralizedJaccard<T> implements MultisetMetric<T>, MultisetDistance<T> {
    @Override
    public float compare(Multiset<T> a, Multiset<T> b) {

        if (a.isEmpty() && b.isEmpty()) {
            return 1.0f;
        }

        if (a.isEmpty() || b.isEmpty()) {
            return 0.0f;
        }

        final int intersection = intersection(a, b).size();

        // a  b / a  b
        // Implementation note: The size of the union of two sets is equal to
        // the size of both sets minus the duplicate elements.
        return intersection / (float) (a.size() + b.size() - intersection);
    }

    @Override
    public float distance(Multiset<T> a, Multiset<T> b) {
        return 1.0f - compare(a, b);
    }

    @Override
    public String toString() {
        return "GeneralizedJaccard";
    }

}