//==============================================================================
//	
//	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.*;
import parser.ast.*;

public class SamplerRewardCumulCont extends SamplerDouble
{
	private double timeBound;
	private int rewardStructIndex;

	/**
	 * Construct a sampler for a (continuous-time) cumulative reward property.
	 * Passed in ExpressionTemporal should be a property of this type.
	 * Reward structure index should also be specified.
	 * All constants should have already been evaluated/replaced.
	 */
	public SamplerRewardCumulCont(ExpressionTemporal expr, int rewardStructIndex) throws PrismException
	{
		// Make sure expression is of the correct type
		// Then extract other required info
		if (expr.getOperator() != ExpressionTemporal.R_C)
			throw new PrismException("Error creating Sampler");
		
		timeBound = expr.getUpperBound().evaluateDouble();
		this.rewardStructIndex = rewardStructIndex;
		// Initialise sampler info
		reset();
		resetStats();
	}

	@Override
	public boolean update(Path path, TransitionList transList) throws PrismLangException
	{
		// If the answer is already known we should do nothing
		if (valueKnown)
			return true;
		
		// As soon as time bound exceeded, compute reward total
		if (path.getTotalTime() >= timeBound) {
			valueKnown = true;
			value = path.getTotalCumulativeReward(rewardStructIndex);
			// Compute excess time, i.e. how long ago time bound was reached
			double excessTime = path.getTotalTime() - timeBound;
			// If this is > 0 (very likely, unless time bound = 0),
			// need to subtract reward accumulated in excess time and transition reward
			// Note that this cannot be the case for the first state of path,
			// so the call to getTimeInPreviousState() is safe.
			if (excessTime > 0) {
				// Note: Time so far > 0 so cannot be first state,
				// so safe to look at previous state.
				value -= path.getPreviousStateReward(rewardStructIndex) * excessTime;
				value -= path.getPreviousTransitionReward(rewardStructIndex);
			}
		}
		
		return valueKnown;
	}
	
	@Override
	public boolean needsBoundedNumSteps()
	{
		// Always bounded (although we don't know the exact num steps, just the time bound)
		return true;
	}
}
