Lucene에서 TF, IDF 구하는 소스

package org.apache.lucene.search;

/**
 * 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.
 */

/** Expert: Default scoring implementation. */
public class DefaultSimilarity extends Similarity {
  /** Implemented as <code>1/sqrt(numTerms)</code>. */
  public float lengthNorm(String fieldName, int numTerms) {
    return (float)(1.0 / Math.sqrt(numTerms));
  }
 
  /** Implemented as <code>1/sqrt(sumOfSquaredWeights)</code>. */
  public float queryNorm(float sumOfSquaredWeights) {
    return (float)(1.0 / Math.sqrt(sumOfSquaredWeights));
  }

  /** Implemented as <code>sqrt(freq)</code>. */
  public float tf(float freq) {
    return (float)Math.sqrt(freq);
  }
   
  /** Implemented as <code>1 / (distance + 1)</code>. */
  public float sloppyFreq(int distance) {
    return 1.0f / (distance + 1);
  }
   
  /** Implemented as <code>log(numDocs/(docFreq+1)) + 1</code>. */
  public float idf(int docFreq, int numDocs) {
    return (float)(Math.log(numDocs/(double)(docFreq+1)) + 1.0);
  }
   
  /** Implemented as <code>overlap / maxOverlap</code>. */
  public float coord(int overlap, int maxOverlap) {
    return overlap / (float)maxOverlap;
  }
}