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Apache Commons Text is a library focused on algorithms working on strings.
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.
 */
package org.apache.commons.text.similarity;

import java.util.Locale;

/**
 * A matching algorithm that is similar to the searching algorithms implemented in editors such
 * as Sublime Text, TextMate, Atom and others.
 *
 * 
 * One point is given for every matched character. Subsequent matches yield two bonus points. A higher score
 * indicates a higher similarity.
 * 
 *
 * 
 * This code has been adapted from Apache Commons Lang 3.3.
 * 
 *
 * @since 1.0
 */
public class FuzzyScore {

    /**
     * Locale used to change the case of text.
     */
    private final Locale locale;


    /**
     * This returns a {@link Locale}-specific {@link FuzzyScore}.
     *
     * @param locale The string matching logic is case insensitive.
                     A {@link Locale} is necessary to normalize both Strings to lower case.
     * @throws IllegalArgumentException
     *         This is thrown if the {@link Locale} parameter is {@code null}.
     */
    public FuzzyScore(final Locale locale) {
        if (locale == null) {
            throw new IllegalArgumentException("Locale must not be null");
        }
        this.locale = locale;
    }

    /**
     * 
     * Find the Fuzzy Score which indicates the similarity score between two
     * Strings.
     * 
     *
     *      * score.fuzzyScore(null, null, null)                                    = IllegalArgumentException
     * score.fuzzyScore("", "", Locale.ENGLISH)                              = 0
     * score.fuzzyScore("Workshop", "b", Locale.ENGLISH)                     = 0
     * score.fuzzyScore("Room", "o", Locale.ENGLISH)                         = 1
     * score.fuzzyScore("Workshop", "w", Locale.ENGLISH)                     = 1
     * score.fuzzyScore("Workshop", "ws", Locale.ENGLISH)                    = 2
     * score.fuzzyScore("Workshop", "wo", Locale.ENGLISH)                    = 4
     * score.fuzzyScore("Apache Software Foundation", "asf", Locale.ENGLISH) = 3
     * 
     *
     * @param term a full term that should be matched against, must not be null
     * @param query the query that will be matched against a term, must not be
     *            null
     * @return result score
     * @throws IllegalArgumentException if either CharSequence input is {@code null}
     */
    public Integer fuzzyScore(final CharSequence term, final CharSequence query) {
        if (term == null || query == null) {
            throw new IllegalArgumentException("CharSequences must not be null");
        }

        // fuzzy logic is case insensitive. We normalize the Strings to lower
        // case right from the start. Turning characters to lower case
        // via Character.toLowerCase(char) is unfortunately insufficient
        // as it does not accept a locale.
        final String termLowerCase = term.toString().toLowerCase(locale);
        final String queryLowerCase = query.toString().toLowerCase(locale);

        // the resulting score
        int score = 0;

        // the position in the term which will be scanned next for potential
        // query character matches
        int termIndex = 0;

        // index of the previously matched character in the term
        int previousMatchingCharacterIndex = Integer.MIN_VALUE;

        for (int queryIndex = 0; queryIndex < queryLowerCase.length(); queryIndex++) {
            final char queryChar = queryLowerCase.charAt(queryIndex);

            boolean termCharacterMatchFound = false;
            for (; termIndex < termLowerCase.length()
                    && !termCharacterMatchFound; termIndex++) {
                final char termChar = termLowerCase.charAt(termIndex);

                if (queryChar == termChar) {
                    // simple character matches result in one point
                    score++;

                    // subsequent character matches further improve
                    // the score.
                    if (previousMatchingCharacterIndex + 1 == termIndex) {
                        score += 2;
                    }

                    previousMatchingCharacterIndex = termIndex;

                    // we can leave the nested loop. Every character in the
                    // query can match at most one character in the term.
                    termCharacterMatchFound = true;
                }
            }
        }

        return score;
    }

    /**
     * Gets the locale.
     *
     * @return The locale
     */
    public Locale getLocale() {
        return locale;
    }

}