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.*;
  import parser.ast.*;
  
  /**
   * Construct a sampler for a (continuous-time) bounded until property.
   * Passed in ExpressionTemporal should be a formula of this type.
   * All constants should have already been evaluated/replaced.
   */
  public class SamplerBoundedUntilCont extends SamplerBoolean
  {
  	private Expression left;
  	private Expression right;
  	private double lb;
  	private double ub;
  
  	/**
  	 * Construct a sampler for a (continuous) time-bounded until formula.
  	 * Passed in ExpressionTemporal should be a property of this type.
  	 * All constants should have already been evaluated/replaced.
  	 */
  	public SamplerBoundedUntilCont(ExpressionTemporal expr) throws PrismException
  	{
  		// Make sure expression is of the correct type
  		// Then extract other required info
  		if (expr.getOperator() != ExpressionTemporal.P_U)
  			throw new PrismException("Error creating Sampler");
  		left = expr.getOperand1();
  		right = expr.getOperand2();
  
  		lb = expr.getLowerBound() == null ? 0.0 : expr.getLowerBound().evaluateDouble();
  		if (lb < 0) {
  			throw new PrismException("Invalid lower bound " + lb + " in time-bounded until formula");
  		}
  
  		if (expr.getUpperBound() == null) {
  			ub = Double.POSITIVE_INFINITY;
  		} else {
  			ub = expr.getUpperBound().evaluateDouble();
  			if (ub < 0 || (ub == 0 && expr.upperBoundIsStrict())) {
  				String bound = (expr.upperBoundIsStrict() ? "<" : "<=") + ub;
  				throw new PrismException("Invalid upper bound " + bound + " in time-bounded until formula");
  			}
  			if (ub < lb) {
  				throw new PrismException("Upper bound must exceed lower bound in time-bounded until formula");
  			}
  		}
  
  		// 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;
  		
  		// For continuous-time bounded until, we may need to look back at previous state.
  		// So, treat first/subsequent calls to update() differently. 
  
  		// First path state:
  		if (path.size() == 0) {
  			// Initially, zero time has elapsed so to satisfy the until,
  			// we need a lower time bound of 0 and the RHS to be satisfied 
  			if (lb == 0.0) {
  				if (right.evaluateBoolean(path.getCurrentState())) {
  					valueKnown = true;
  					value = true;
  				}
  			}
  			// If LHS of the until violated, will never be true
  			else {
  				if (!left.evaluateBoolean(path.getCurrentState())) {
  					valueKnown = true;
  					value = false;
  				}
  			}
  		}
  		// Second and subsequent path states:
  		else {
  			double timeSoFar = path.getTotalTime();
  			// As soon as upper time bound exceeded, we can decide
  			if (timeSoFar > ub) {
  				// Upper time bound was exceeded (for first time) in *previous* state
  				// Had we reached the target (i.e. RHS of until)?
  				if (right.evaluateBoolean(path.getPreviousState())) {
  					valueKnown = true;
  					value = true;
  				}
  				// If not, it's too late now
  				else {
  					valueKnown = true;
  					value = false;
  				}
  			}
  			// Lower bound not yet exceeded but LHS of until violated
  			// (no need to check RHS because too early)
  			else if (timeSoFar <= lb) {
  				if (!left.evaluateBoolean(path.getCurrentState())) {
  					valueKnown = true;
  					value = false;
  				}
  			}
  			// Current time is between lower/upper bounds...
  			else {
  				// Have we reached the target (i.e. RHS of until)?
  				if (right.evaluateBoolean(path.getCurrentState())) {
  					valueKnown = true;
  					value = true;
  				}
  				// Or, if not, have we violated the LHS of the until?
  				else if (!left.evaluateBoolean(path.getCurrentState())) {
  					valueKnown = true;
  					value = false;
  				}
  				// If the lower bound was exceeded for the first time in the
  				// previous state, and that one satisfies the RHS of the until...
  				else if (timeSoFar - path.getTimeInPreviousState() <= lb) {
  					if (right.evaluateBoolean(path.getPreviousState())) {
  						valueKnown = true;
  						value = true;
  					}
  				}
  				// Or, if we are now at a deadlock
  				else if (transList != null && transList.isDeadlock()) {
  					valueKnown = true;
  					value = false;
  				}
  				// Otherwise, don't know
  			}
  		}
  		
  		return valueKnown;
  	}
  	
  	@Override
  	public boolean needsBoundedNumSteps()
  	{
  		// Bounded if there is a non-finite upper-bound (although we don't know the exact num steps, just the time bound)
  		return ub < Double.POSITIVE_INFINITY;
  	}
  }
  