    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.text.similarity;
    018
    019import java.util.HashSet;
    020import java.util.Set;
    021
    022/**
    023 * Measures the Jaccard similarity (aka Jaccard index) of two sets of character
    024 * sequence. Jaccard similarity is the size of the intersection divided by the
    025 * size of the union of the two sets.
    026 *
    027 * <p>
    028 * For further explanation about Jaccard Similarity, refer
    029 * https://en.wikipedia.org/wiki/Jaccard_index
    030 * </p>
    031 *
    032 * @since 1.0
    033 */
    034public class JaccardSimilarity implements SimilarityScore<Double> {
    035
    036    /**
    037     * Calculates Jaccard Similarity of two set character sequence passed as
    038     * input.
    039     *
    040     * @param left first character sequence
    041     * @param right second character sequence
    042     * @return index
    043     * @throws IllegalArgumentException
    044     *             if either String input {@code null}
    045     */
    046    @Override
    047    public Double apply(final CharSequence left, final CharSequence right) {
    048        if (left == null || right == null) {
    049            throw new IllegalArgumentException("Input cannot be null");
    050        }
    051        return Math.round(calculateJaccardSimilarity(left, right) * 100d) / 100d;
    052    }
    053
    054    /**
    055     * Calculates Jaccard Similarity of two character sequences passed as
    056     * input. Does the calculation by identifying the union (characters in at
    057     * least one of the two sets) of the two sets and intersection (characters
    058     * which are present in set one which are present in set two)
    059     *
    060     * @param left first character sequence
    061     * @param right second character sequence
    062     * @return index
    063     */
    064    private Double calculateJaccardSimilarity(final CharSequence left, final CharSequence right) {
    065        final int leftLength = left.length();
    066        final int rightLength = right.length();
    067        if (leftLength == 0 || rightLength == 0) {
    068            return 0d;
    069        }
    070        final Set<Character> leftSet = new HashSet<>();
    071        for (int i = 0; i < leftLength; i++) {
    072            leftSet.add(left.charAt(i));
    073        }
    074        final Set<Character> rightSet = new HashSet<>();
    075        for (int i = 0; i < rightLength; i++) {
    076            rightSet.add(right.charAt(i));
    077        }
    078        final Set<Character> unionSet = new HashSet<>(leftSet);
    079        unionSet.addAll(rightSet);
    080        final int intersectionSize = leftSet.size() + rightSet.size() - unionSet.size();
    081        return 1.0d * intersectionSize / unionSet.size();
    082    }
    083}