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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;

/**
 * The hamming distance between two strings of equal length is the number of
 * positions at which the corresponding symbols are different.
 *
 * 
 * For further explanation about the Hamming Distance, take a look at its
 * Wikipedia page at http://en.wikipedia.org/wiki/Hamming_distance.
 * 
 *
 * @since 1.0
 */
public class HammingDistance implements EditDistance {

    /**
     * Find the Hamming Distance between two strings with the same
     * length.
     *
     * The distance starts with zero, and for each occurrence of a
     * different character in either String, it increments the distance
     * by 1, and finally return its value.
     *
     * Since the Hamming Distance can only be calculated between strings of equal length, input of different lengths
     * will throw IllegalArgumentException
     *
     *      * distance.apply("", "")               = 0
     * distance.apply("pappa", "pappa")     = 0
     * distance.apply("1011101", "1011111") = 1
     * distance.apply("ATCG", "ACCC")       = 2
     * distance.apply("karolin", "kerstin"  = 3
     * 
     *
     * @param left the first CharSequence, must not be null
     * @param right the second CharSequence, must not be null
     * @return distance
     * @throws IllegalArgumentException if either input is {@code null} or
     *             if they do not have the same length
     */
    @Override
    public Integer apply(final CharSequence left, final CharSequence right) {
        if (left == null || right == null) {
            throw new IllegalArgumentException("CharSequences must not be null");
        }

        if (left.length() != right.length()) {
            throw new IllegalArgumentException("CharSequences must have the same length");
        }

        int distance = 0;

        for (int i = 0; i < left.length(); i++) {
            if (left.charAt(i) != right.charAt(i)) {
                distance++;
            }
        }

        return distance;
    }

}