CharacterNGramStringIterable.java

/*
 * Copyright 2011
 * Ubiquitous Knowledge Processing (UKP) Lab
 * Technische Universität Darmstadt
 *
 * Licensed 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 de.tudarmstadt.ukp.dkpro.core.ngrams.util;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import org.apache.commons.lang.StringUtils;

/**
 * Creates a character NGram iterable from a list of tokens.
 * 
 *
 */
public class CharacterNGramStringIterable
    implements Iterable<String>
{
    List<String> nGramList;

    /**
     * @param token
     *            A token
     * @param minN
     *            the minimal n-gram length.
     * @param maxN
     *            the maximal n-gram length.
     */
    public CharacterNGramStringIterable(String token, int minN, int maxN)
    {
        this.nGramList = createNGramList(token, minN, maxN);
    }

    @Override
    public Iterator<String> iterator()
    {
        return nGramList.iterator();
    }

    private List<String> createNGramList(String token, int minN, int maxN)
    {
        if (minN > maxN) {
            throw new IllegalArgumentException("minN needs to be smaller or equal than maxN.");
        }

        List<String> nGrams = new ArrayList<String>();

        // fill character list
        List<String> charList = new ArrayList<String>();
        for (char c : token.toCharArray()) {
            charList.add(Character.toString(c));
        }

        for (int k = minN; k <= maxN; k++) {
            // if the number of tokens is less than k => break
            if (charList.size() < k) {
                break;
            }
            nGrams.addAll(getNGrams(charList, k));
        }

        return nGrams;
    }

    private List<String> getNGrams(List<String> tokenList, int k)
    {
        List<String> nGrams = new ArrayList<String>();

        int size = tokenList.size();
        for (int i = 0; i < (size + 1 - k); i++) {
            nGrams.add(StringUtils.join(tokenList.subList(i, i + k), ""));
        }

        return nGrams;
    }
}