Package opennlp.tools.util.featuregen

Source Code of opennlp.tools.util.featuregen.CharacterNgramFeatureGenerator

/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreemnets.  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 opennlp.tools.util.featuregen;

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

import opennlp.tools.ngram.NGramModel;
import opennlp.tools.util.StringList;

/**
* The {@link CharacterNgramFeatureGenerator} uses character ngrams to
* generate features about each token.
* The minimum and maximum length can be specified.
*/
public class CharacterNgramFeatureGenerator extends FeatureGeneratorAdapter {

  private final int minLength;
  private final int maxLength;

  public CharacterNgramFeatureGenerator(int minLength, int maxLength) {
    this.minLength = minLength;
    this.maxLength = maxLength;
  }

  /**
   * Initializes the current instance with min 2 length and max 5 length of ngrams.
   */
  public CharacterNgramFeatureGenerator() {
    this(2, 5);
  }

  public void createFeatures(List<String> features, String[] tokens, int index, String[] preds) {

    NGramModel model = new NGramModel();
    model.add(tokens[index], minLength, maxLength);

    for (Iterator<StringList> it = model.iterator(); it.hasNext();) {

      StringList tokenList = it.next();

      if (tokenList.size() > 0) {
        features.add("ng=" + tokenList.getToken(0).toLowerCase());
      }
    }
  }
}
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