code: //==============================================================================
  //	
  //	Copyright (c) 2002-
  //	Authors:
  //	* Dave Parker <david.parker@comlab.ox.ac.uk> (University of Oxford)
  //	
  //------------------------------------------------------------------------------
  //	
  //	This file is part of PRISM.
  //	
  //	PRISM is free software; you can redistribute it and/or modify
  //	it under the terms of the GNU General Public License as published by
  //	the Free Software Foundation; either version 2 of the License, or
  //	(at your option) any later version.
  //	
  //	PRISM is distributed in the hope that it will be useful,
  //	but WITHOUT ANY WARRANTY; without even the implied warranty of
  //	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  //	GNU General Public License for more details.
  //	
  //	You should have received a copy of the GNU General Public License
  //	along with PRISM; if not, write to the Free Software Foundation,
  //	Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  //	
  //==============================================================================
  
  package simulator.sampler;
  
  import simulator.*;
  import prism.PrismException;
  import prism.PrismLangException;
  
  /**
   * Samplers for properties that associate a simulation path with a Boolean value.
   */
  public abstract class SamplerBoolean extends Sampler
  {
  	// Value of current path
  	protected boolean value;
  	// Whether the actual value should be the negation of 'value'
  	protected boolean negated = false;
  	// Stats over all paths
  	protected int numSamples;
  	protected int numTrue;
  
  	@Override
  	public void reset()
  	{
  		valueKnown = false;
  		value = false;
  	}
  
  	@Override
  	public void resetStats()
  	{
  		numSamples = 0;
  		numTrue = 0;
  	}
  
  	@Override
  	public abstract boolean update(Path path, TransitionList transList) throws PrismLangException;
  
  	@Override
  	public void updateStats()
  	{
  		numSamples++;
  		// XOR: value && !negated || !value && negated 
  		if (value != negated)
  			numTrue++;
  	}
  
  	@Override
  	public Object getCurrentValue()
  	{
  		// XOR: value && !negated || !value && negated 
  		return new Boolean(value != negated);
  	}
  
  	@Override
  	public double getMeanValue()
  	{
  		return numTrue / (double) numSamples;
  	}
  
  	@Override
  	public double getVariance()
  	{
  		// Estimator to the variance (see p.24 of Vincent Nimal's MSc thesis)
  		if (numSamples <= 1) {
  			return 0.0;
  		} else {
  			return (numTrue * ((double) numSamples - numTrue) / ( numSamples * (numSamples - 1.0))); 
  		}
  		
  		// An alternative, below, would be to use the empirical mean
  		// (this is not equivalent (or unbiased) but, asymptotically, is the same)
  		//double mean = numTrue / (double) numSamples;
  		//return mean * (1.0 - mean);
  	}
  
  	@Override
  	public double getLikelihoodRatio(double p1, double p0) throws PrismException
  	{
  		// See Sec 5.3 of Vincent Nimal's MSc thesis for details
  		return Math.pow(p1 / p0, numTrue) * Math.pow((1 - p1) / (1 - p0), numSamples - numTrue);
  	}
  
  	/**
  	 * Negate the meaning of this sampler.
  	 */
  	public boolean negate()
  	{
  		return negated = !negated;
  	}
  
  	/**
  	 * Is this sampler negated?
  	 */
  	public boolean getNegated()
  	{
  		return negated;
  	}
  }
  