    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.Map;
    021import java.util.Set;
    022
    023/**
    024 * Measures the Cosine similarity of two vectors of an inner product space and compares the angle between them.
    025 * <p>
    026 * For further explanation about the Cosine Similarity, refer to http://en.wikipedia.org/wiki/Cosine_similarity.
    027 * </p>
    028 * <p>
    029 * Instances of this class are immutable and are safe for use by multiple concurrent threads.
    030 * </p>
    031 *
    032 * @since 1.0
    033 */
    034public class CosineSimilarity {
    035
    036    /**
    037     * Singleton instance.
    038     */
    039    static final CosineSimilarity INSTANCE = new CosineSimilarity();
    040
    041    /**
    042     * Calculates the cosine similarity for two given vectors.
    043     *
    044     * @param leftVector left vector
    045     * @param rightVector right vector
    046     * @return cosine similarity between the two vectors
    047     */
    048    public Double cosineSimilarity(final Map<CharSequence, Integer> leftVector,
    049                                   final Map<CharSequence, Integer> rightVector) {
    050        if (leftVector == null || rightVector == null) {
    051            throw new IllegalArgumentException("Vectors must not be null");
    052        }
    053
    054        final Set<CharSequence> intersection = getIntersection(leftVector, rightVector);
    055
    056        final double dotProduct = dot(leftVector, rightVector, intersection);
    057        double d1 = 0.0d;
    058        for (final Integer value : leftVector.values()) {
    059            d1 += Math.pow(value, 2);
    060        }
    061        double d2 = 0.0d;
    062        for (final Integer value : rightVector.values()) {
    063            d2 += Math.pow(value, 2);
    064        }
    065        final double cosineSimilarity;
    066        if (d1 <= 0.0 || d2 <= 0.0) {
    067            cosineSimilarity = 0.0;
    068        } else {
    069            cosineSimilarity = dotProduct / (Math.sqrt(d1) * Math.sqrt(d2));
    070        }
    071        return cosineSimilarity;
    072    }
    073
    074    /**
    075     * Computes the dot product of two vectors. It ignores remaining elements. It means
    076     * that if a vector is longer than other, then a smaller part of it will be used to compute
    077     * the dot product.
    078     *
    079     * @param leftVector left vector
    080     * @param rightVector right vector
    081     * @param intersection common elements
    082     * @return The dot product
    083     */
    084    private double dot(final Map<CharSequence, Integer> leftVector, final Map<CharSequence, Integer> rightVector,
    085            final Set<CharSequence> intersection) {
    086        long dotProduct = 0;
    087        for (final CharSequence key : intersection) {
    088            dotProduct += leftVector.get(key) * (long) rightVector.get(key);
    089        }
    090        return dotProduct;
    091    }
    092
    093    /**
    094     * Returns a set with strings common to the two given maps.
    095     *
    096     * @param leftVector left vector map
    097     * @param rightVector right vector map
    098     * @return common strings
    099     */
    100    private Set<CharSequence> getIntersection(final Map<CharSequence, Integer> leftVector,
    101            final Map<CharSequence, Integer> rightVector) {
    102        final Set<CharSequence> intersection = new HashSet<>(leftVector.keySet());
    103        intersection.retainAll(rightVector.keySet());
    104        return intersection;
    105    }
    106
    107}