Nysiis.java

/*
 * 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
 *
 *      https://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.codec.language;

import java.util.regex.Pattern;

import org.apache.commons.codec.EncoderException;
import org.apache.commons.codec.StringEncoder;

/**
 * Encodes a string into a NYSIIS value. NYSIIS is an encoding used to relate similar names, but can also be used as a
 * general purpose scheme to find word with similar phonemes.
 * <p>
 * NYSIIS features an accuracy increase of 2.7% over the traditional Soundex algorithm.
 * </p>
 * <p>
 * Algorithm description:
 * </p>
 * <pre>
 * 1. Transcode first characters of name
 *   1a. MAC -&gt;   MCC
 *   1b. KN  -&gt;   NN
 *   1c. K   -&gt;   C
 *   1d. PH  -&gt;   FF
 *   1e. PF  -&gt;   FF
 *   1f. SCH -&gt;   SSS
 * 2. Transcode last characters of name
 *   2a. EE, IE          -&gt;   Y
 *   2b. DT,RT,RD,NT,ND  -&gt;   D
 * 3. First character of key = first character of name
 * 4. Transcode remaining characters by following these rules, incrementing by one character each time
 *   4a. EV  -&gt;   AF  else A,E,I,O,U -&gt; A
 *   4b. Q   -&gt;   G
 *   4c. Z   -&gt;   S
 *   4d. M   -&gt;   N
 *   4e. KN  -&gt;   N   else K -&gt; C
 *   4f. SCH -&gt;   SSS
 *   4g. PH  -&gt;   FF
 *   4h. H   -&gt;   If previous or next is non-vowel, previous
 *   4i. W   -&gt;   If previous is vowel, previous
 *   4j. Add current to key if current != last key character
 * 5. If last character is S, remove it
 * 6. If last characters are AY, replace with Y
 * 7. If last character is A, remove it
 * 8. Collapse all strings of repeated characters
 * 9. Add original first character of name as first character of key
 * </pre>
 * <p>
 * This class is immutable and thread-safe.
 * </p>
 *
 * @see <a href="https://en.wikipedia.org/wiki/NYSIIS">NYSIIS on Wikipedia</a>
 * @see <a href="http://www.dropby.com/NYSIIS.html">NYSIIS on dropby.com</a>
 * @see Soundex
 * @since 1.7
 */
public class Nysiis implements StringEncoder {

    private static final char[] CHARS_A   = { 'A' };
    private static final char[] CHARS_AF  = { 'A', 'F' };
    private static final char[] CHARS_C   = { 'C' };
    private static final char[] CHARS_FF  = { 'F', 'F' };
    private static final char[] CHARS_G   = { 'G' };
    private static final char[] CHARS_N   = { 'N' };
    private static final char[] CHARS_NN  = { 'N', 'N' };
    private static final char[] CHARS_S   = { 'S' };
    private static final char[] CHARS_SSS = { 'S', 'S', 'S' };

    private static final Pattern PAT_MAC    = Pattern.compile("^MAC");
    private static final Pattern PAT_KN     = Pattern.compile("^KN");
    private static final Pattern PAT_K      = Pattern.compile("^K");
    private static final Pattern PAT_PH_PF  = Pattern.compile("^(PH|PF)");
    private static final Pattern PAT_SCH    = Pattern.compile("^SCH");
    private static final Pattern PAT_EE_IE  = Pattern.compile("(EE|IE)$");
    private static final Pattern PAT_DT_ETC = Pattern.compile("(DT|RT|RD|NT|ND)$");

    private static final char SPACE = ' ';
    private static final int TRUE_LENGTH = 6;

    /**
     * Tests if the given character is a vowel.
     *
     * @param c
     *            the character to test.
     * @return {@code true} if the character is a vowel, {@code false} otherwise.
     */
    private static boolean isVowel(final char c) {
        return c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U';
    }

    /**
     * Transcodes the remaining parts of the String. The method operates on a sliding window, looking at 4 characters at
     * a time: [i-1, i, i+1, i+2].
     *
     * @param prev
     *            the previous character.
     * @param curr
     *            the current character.
     * @param next
     *            the next character.
     * @param aNext
     *            the after next character.
     * @return a transcoded array of characters, starting from the current position.
     */
    private static char[] transcodeRemaining(final char prev, final char curr, final char next, final char aNext) {
        // 1. EV -> AF
        if (curr == 'E' && next == 'V') {
            return CHARS_AF;
        }

        // A, E, I, O, U -> A
        if (isVowel(curr)) {
            return CHARS_A;
        }

        // 2. Q -> G, Z -> S, M -> N

        // 3. KN -> NN else K -> C
        switch (curr) {
        case 'Q':
            return CHARS_G;
        case 'Z':
            return CHARS_S;
        case 'M':
            return CHARS_N;
        case 'K':
            if (next == 'N') {
                return CHARS_NN;
            }
            return CHARS_C;
        default:
            break;
        }

        // 4. SCH -> SSS
        if (curr == 'S' && next == 'C' && aNext == 'H') {
            return CHARS_SSS;
        }

        // PH -> FF
        if (curr == 'P' && next == 'H') {
            return CHARS_FF;
        }

        // 5. H -> If previous or next is a non vowel, previous.
        if (curr == 'H' && (!isVowel(prev) || !isVowel(next))) {
            return new char[] { prev };
        }

        // 6. W -> If previous is vowel, previous.
        if (curr == 'W' && isVowel(prev)) {
            return new char[] { prev };
        }

        return new char[] { curr };
    }

    /** Indicates the strict mode. */
    private final boolean strict;

    /**
     * Creates an instance of the {@link Nysiis} encoder with strict mode (original form),
     * i.e. encoded strings have a maximum length of 6.
     */
    public Nysiis() {
        this(true);
    }

    /**
     * Create an instance of the {@link Nysiis} encoder with the specified strict mode:
     *
     * <ul>
     *  <li>{@code true}: encoded strings have a maximum length of 6</li>
     *  <li>{@code false}: encoded strings may have arbitrary length</li>
     * </ul>
     *
     * @param strict
     *            the strict mode.
     */
    public Nysiis(final boolean strict) {
        this.strict = strict;
    }

    /**
     * Encodes an Object using the NYSIIS algorithm. This method is provided in order to satisfy the requirements of the Encoder interface, and will throw an
     * {@link EncoderException} if the supplied object is not of type {@link String}.
     *
     * @param obj Object to encode.
     * @return An object (or a {@link String}) containing the NYSIIS code which corresponds to the given String.
     * @throws EncoderException         if the parameter supplied is not of a {@link String}.
     * @throws IllegalArgumentException if a character is not mapped.
     */
    @Override
    public Object encode(final Object obj) throws EncoderException {
        if (!(obj instanceof String)) {
            throw new EncoderException("Parameter supplied to Nysiis encode is not of type java.lang.String");
        }
        return nysiis((String) obj);
    }

    /**
     * Encodes a String using the NYSIIS algorithm.
     *
     * @param str A String object to encode.
     * @return A Nysiis code corresponding to the String supplied.
     * @throws IllegalArgumentException if a character is not mapped.
     */
    @Override
    public String encode(final String str) {
        return nysiis(str);
    }

    /**
     * Indicates the strict mode for this {@link Nysiis} encoder.
     *
     * @return {@code true} if the encoder is configured for strict mode, {@code false} otherwise
     */
    public boolean isStrict() {
        return this.strict;
    }

    /**
     * Retrieves the NYSIIS code for a given String object.
     *
     * @param str String to encode using the NYSIIS algorithm.
     * @return A NYSIIS code for the String supplied.
     */
    public String nysiis(String str) {
        if (str == null) {
            return null;
        }

        // Use the same clean rules as Soundex
        str = SoundexUtils.clean(str);

        if (str.isEmpty()) {
            return str;
        }

        // Translate first characters of name:
        // MAC -> MCC, KN -> NN, K -> C, PH | PF -> FF, SCH -> SSS
        str = PAT_MAC.matcher(str).replaceFirst("MCC");
        str = PAT_KN.matcher(str).replaceFirst("NN");
        str = PAT_K.matcher(str).replaceFirst("C");
        str = PAT_PH_PF.matcher(str).replaceFirst("FF");
        str = PAT_SCH.matcher(str).replaceFirst("SSS");

        // Translate last characters of name:
        // EE -> Y, IE -> Y, DT | RT | RD | NT | ND -> D
        str = PAT_EE_IE.matcher(str).replaceFirst("Y");
        str = PAT_DT_ETC.matcher(str).replaceFirst("D");

        // First character of key = first character of name.
        final StringBuilder key = new StringBuilder(str.length());
        key.append(str.charAt(0));

        // Transcode remaining characters, incrementing by one character each time
        final char[] chars = str.toCharArray();
        final int len = chars.length;

        for (int i = 1; i < len; i++) {
            final char next = i < len - 1 ? chars[i + 1] : SPACE;
            final char aNext = i < len - 2 ? chars[i + 2] : SPACE;
            final char[] transcoded = transcodeRemaining(chars[i - 1], chars[i], next, aNext);
            System.arraycopy(transcoded, 0, chars, i, transcoded.length);

            // only append the current char to the key if it is different from the last one
            if (chars[i] != chars[i - 1]) {
                key.append(chars[i]);
            }
        }

        if (key.length() > 1) {
            char lastChar = key.charAt(key.length() - 1);

            // If last character is S, remove it.
            if (lastChar == 'S') {
                key.deleteCharAt(key.length() - 1);
                lastChar = key.charAt(key.length() - 1);
            }

            if (key.length() > 2) {
                final char last2Char = key.charAt(key.length() - 2);
                // If last characters are AY, replace with Y.
                if (last2Char == 'A' && lastChar == 'Y') {
                    key.deleteCharAt(key.length() - 2);
                }
            }

            // If last character is A, remove it.
            if (lastChar == 'A') {
                key.deleteCharAt(key.length() - 1);
            }
        }

        final String string = key.toString();
        return isStrict() ? string.substring(0, Math.min(TRUE_LENGTH, string.length())) : string;
    }

}