org.simmetrics.metrics.SimonWhite.java Source code

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

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/*
 * #%L
 * Simmetrics Core
 * %%
 * Copyright (C) 2014 - 2015 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 java.util.List;

import org.simmetrics.ListMetric;

import com.google.common.collect.HashMultiset;
import com.google.common.collect.Multiset;

/**
 * Simon White algorithm providing a similarity measure between two lists.
 * Implementation based on the ideas as outlined in <a
 * href="http://www.catalysoft.com/articles/StrikeAMatch.html">How to Strike a
 * Match</a> by Simon White.
 * 
 * <p>
 * <code>
 * similarity(a,b) = 2 * |(a A b)|  / (|a| + |b|)
 * </code>
 * 
 * <p>
 * The A operation takes the list intersection of <code>a</code> and
 * <code>b</code>. This is a list <code>c</code> such that each element in has a
 * 1-to-1 relation to an element in both <code>a</code> and <code>b</code>. E.g.
 * the list intersection of <code>[ab,ab,ab,ac]</code> and
 * <code>[ab,ab,ad]</code> is <code>[ab,ab]</code>.
 * 
 * <p>
 * This metric is very similar to Dice's coefficient however Simon White used
 * the list intersection rather then the set intersection to prevent list of
 * duplicates from scoring a perfect match against a list with single elements.
 * E.g. 'GGGGG' should not be identical to 'GG'.
 * 
 * <p>
 * This class is immutable and thread-safe.
 * 
 * @see DiceSimilarity
 * 
 * 
 * @param <T>
 *            type of the token
 * 
 */
public class SimonWhite<T> implements ListMetric<T> {

    @Override
    public float compare(List<T> a, List<T> b) {

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

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

        // Copy for destructive list difference
        Multiset<T> bCopy = HashMultiset.create(b);

        // Count elements in the list intersection.
        // Elements are counted only once in both lists.
        // E.g. the intersection of [ab,ab,ab] and [ab,ab,ac,ad] is [ab,ab].
        // Note: this is not the same as b.retainAll(a).size()
        int intersection = 0;
        for (T token : a) {
            if (bCopy.remove(token)) {
                intersection++;
            }
        }

        return 2.0f * intersection / (a.size() + b.size());

    }

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

}