/*
 *  Copyright (c) 2012, The University of Sheffield. See the file
 *  COPYRIGHT.txt in the software or at http://gate.ac.uk/gate/COPYRIGHT.txt
 *
 *  This file is part of GATE (see http://gate.ac.uk/), and is free
 *  software, licenced under the GNU Library General Public License,
 *  Version 2, June 1991 (in the distribution as file licence.html,
 *  and also available at http://gate.ac.uk/gate/licence.html).
 *
 *  $Id: IdfCalculation.java 16296 2012-11-20 12:17:11Z adamfunk $
 */
package gate.termraider.bank.modes;

public enum IdfCalculation {
  Natural,
  Logarithmic,
  LogarithmicPlus1;
  
  
  public static double calculate(IdfCalculation mode, int rawDF, int corpusSize) {
    double df = (double) rawDF;
    double n = (double) corpusSize;
    
    if (mode == Logarithmic) {
      return logarithm(n / df);
    }
    
    if (mode == LogarithmicPlus1) {
      return 1.0 + logarithm(n / df);
    }
    
    // must be Natural
    return 1.0 / df;
  }

  public static final double logBase = 2.0;
  private static double conversion;
  
  static {
    conversion = Math.log10(logBase);
  }
  
  public static double logarithm(double input) {
    return Math.log10(input) / conversion;
  }
  
}