TF, IDF 구현

 /** Implemented as sqrt(freq). */
  public float tf(float freq) {
    return (float)Math.sqrt(freq);
  }
   
  /** Implemented as log(numDocs/(docFreq+1)) + 1. */
  public float idf(int docFreq, int numDocs) {
    return (float)(Math.log(numDocs/(double)(docFreq+1)) + 1.0);
  }