ngramtokenizer.java

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package org.apache.lucene.analysis.ngram;

/**
 * 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.
 */

import org.apache.lucene.analysis.Token;
import org.apache.lucene.analysis.Tokenizer;

import java.io.IOException;
import java.io.Reader;

/**
 * Tokenizes the input into n-grams of the given size(s).
 * @author Otis Gospodnetic
 */
public class NGramTokenizer extends Tokenizer {
    public static final int DEFAULT_MIN_NGRAM_SIZE = 1;
    public static final int DEFAULT_MAX_NGRAM_SIZE = 2;

    private int minGram, maxGram;
    private int gramSize;
    private int pos = 0;
    private int inLen;
    private String inStr;
    private boolean started = false;

    /**
     * Creates NGramTokenizer with given min and max n-grams.
     * @param input Reader holding the input to be tokenized
     * @param minGram the smallest n-gram to generate
     * @param maxGram the largest n-gram to generate
     */
    public NGramTokenizer(Reader input, int minGram, int maxGram) {
        super(input);
        if (minGram < 1) {
            throw new IllegalArgumentException("minGram must be greater than zero");
        }
        if (minGram > maxGram) {
            throw new IllegalArgumentException("minGram must not be greater than maxGram");
        }
        this.minGram = minGram;
        this.maxGram = maxGram;
    }
    /**
     * Creates NGramTokenizer with default min and max n-grams.
     * @param input Reader holding the input to be tokenized
     */
    public NGramTokenizer(Reader input) {
        this(input, DEFAULT_MIN_NGRAM_SIZE, DEFAULT_MAX_NGRAM_SIZE);
    }

    /** Returns the next token in the stream, or null at EOS. */
    public final Token next() throws IOException {
        if (!started) {
            started = true;
            gramSize = minGram;
            char[] chars = new char[1024];
            input.read(chars);
            inStr = new String(chars).trim();  // remove any trailing empty strings 
            inLen = inStr.length();
        }

        if (pos+gramSize > inLen) {            // if we hit the end of the string
            pos = 0;                           // reset to beginning of string
            gramSize++;                        // increase n-gram size
            if (gramSize > maxGram)            // we are done
                return null;
            if (pos+gramSize > inLen)
                return null;
        }
        String gram = inStr.substring(pos, pos+gramSize);
        int oldPos = pos;
        pos++;
        return new Token(gram, oldPos, oldPos+gramSize);
    }
}