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     * Singleton instance.
038     */
039    static final JaccardSimilarity INSTANCE = new JaccardSimilarity();
040
041    /**
042     * Calculates Jaccard Similarity of two set character sequence passed as
043     * input.
044     *
045     * @param left first character sequence
046     * @param right second character sequence
047     * @return index
048     * @throws IllegalArgumentException
049     *             if either String input {@code null}
050     */
051    @Override
052    public Double apply(final CharSequence left, final CharSequence right) {
053        if (left == null || right == null) {
054            throw new IllegalArgumentException("Input cannot be null");
055        }
056        return calculateJaccardSimilarity(left, right);
057    }
058
059    /**
060     * Calculates Jaccard Similarity of two character sequences passed as
061     * input. Does the calculation by identifying the union (characters in at
062     * least one of the two sets) of the two sets and intersection (characters
063     * which are present in set one which are present in set two)
064     *
065     * @param left first character sequence
066     * @param right second character sequence
067     * @return index
068     */
069    private Double calculateJaccardSimilarity(final CharSequence left, final CharSequence right) {
070        final int leftLength = left.length();
071        final int rightLength = right.length();
072        if (leftLength == 0 && rightLength == 0) {
073            return 1d;
074        }
075        if (leftLength == 0 || rightLength == 0) {
076            return 0d;
077        }
078        final Set<Character> leftSet = new HashSet<>();
079        for (int i = 0; i < leftLength; i++) {
080            leftSet.add(left.charAt(i));
081        }
082        final Set<Character> rightSet = new HashSet<>();
083        for (int i = 0; i < rightLength; i++) {
084            rightSet.add(right.charAt(i));
085        }
086        final Set<Character> unionSet = new HashSet<>(leftSet);
087        unionSet.addAll(rightSet);
088        final int intersectionSize = leftSet.size() + rightSet.size() - unionSet.size();
089        return 1.0d * intersectionSize / unionSet.size();
090    }
091}