/// <summary>
/// Process and check the complex goal for loop detection
/// </summary>
/// <param name="currentGoal">The complex goal to be checked and processed</param>
/// <param name="currentBeliefs">The current set of beliefs</param>
/// <param name="previousStates">The list of previous states</param>
/// <param name="itemsToProcess">The stack of items to process</param>
/// <returns>True if the complex goal does not contain loops and is processed, false otherwise</returns>
private static bool DecomposeAndCheckComplexGoal(ComplexGoal currentGoal, List <Fact> currentBeliefs, List <State> previousStates, Stack <IStackItem> itemsToProcess)
{
//only if the complex goal is not fulfilled, do we need to process and check it
if (!currentGoal.IsFulFilled(currentBeliefs))
{
//if a loop is detected, we end execution, else we add this state to the list of states
if (CheckForLoop(currentBeliefs, currentGoal, previousStates))
{
return(false);
}
else
{
previousStates.Add(new State {
CurrentComplexGoal = currentGoal, CurrentBeliefs = currentBeliefs
});
}
//we repush the complex goal in the items to process so it can be checked again after all subgoals have been fulfilled
//this is done to avoid that the solution to a subgoal 'destroys' another fulfilled subgoal
itemsToProcess.Push(currentGoal);
//all subgoals are pushed on the stack
foreach (var simpleGoal in currentGoal.Goals)
{
itemsToProcess.Push(simpleGoal);
}
}
return(true);
}
/// <summary>
/// Check for a loop in the execution of our planner so that we do not introduce infinite loops.
/// We test this by testing if we encountered the same current beliefs and the same complex goal somewhere before.
/// If so, this indicated that we're stuck in a loop and we need to exit the current choice of actions to find a proper path
/// of steps/actions that lead to the fulfillment of all given goals.
/// </summary>
/// <param name="currentBeliefs">The set of facts we current believe to be true</param>
/// <param name="goal">The complex goal currently being processed</param>
/// <param name="states">The list of previous states we encountered</param>
/// <returns>True if a state can be found that contains the same given goal and where the set of beliefs of that state
/// completely matches the current set of beliefs.</returns>
private static bool CheckForLoop(List <Fact> currentBeliefs, ComplexGoal goal, List <State> states)
{
foreach (var state in states)
{
if (goal.IsSame(state.CurrentComplexGoal) && state.CurrentBeliefs.Count() == currentBeliefs.Count() &&
state.CurrentBeliefs.All(x => currentBeliefs.Any(y => y.IsSameAs(x))))
{
return(true);
}
}
return(false);
}
/// <summary>
/// Check to see if a complex goal is the same as another
/// </summary>
/// <param name="goal">The other complex goal to test for equality</param>
/// <returns>True if they have the same amount of subgoals and if every subgoal
/// in its ordered list is the same goal in the ordered list of the other complex goal</returns>
internal bool IsSame(ComplexGoal goal)
{
//not same amount of goals so can't be equal
if (goal.Goals.Count() != Goals.Count())
{
return(false);
}
//test if all subgoals (ordered) match (return false if one set does not match)
for (var i = 0; i < goal.Goals.Count(); i++)
{
if (!goal.Goals[i].Fact.IsSameAs(Goals[i].Fact))
{
return(false);
}
}
return(true);
}
/// <summary>
/// Fulfill a simple goal when needed. This is done by finding an action that has a more general version of the goal in its add list.
/// If we find an action, we instantiate it and add it along with its preconditions to the stack (preconditions first).
/// Some loopdetection is performed to see if we haven't already added a precondition of the action as this indicates a loop.
/// Multiple actions are possible so if choosing one action ends in an unplannable situation (due to a loop or no choice of actions further down the line)
/// we simply backtrack to the previous choice of actions and choose another. When looking for an optimal path, this backtracking is done for every choice so
/// we can guarantee that we make the most optimal choice at any point of the execution.
/// </summary>
/// <param name="simpleGoal">The goal we wish to see fulfilled by choosing an action</param>
/// <param name="plan">The current plan</param>
/// <param name="optimal">A flag indicating if we're looking for the optimal plan or not</param>
/// <param name="itemsToProcess">The stack of items to process containing goals and actions</param>
/// <param name="previousStates">A list of previous states to be used in loop detection</param>
/// <param name="currentBeliefs">The current set of beliefs</param>
/// <param name="preconditions">A list of preconditions used in loop detection</param>
/// <param name="actions">A list of executable actions of the agent</param>
/// <returns>True if we find a plan that fulfills the goal and all remaining items in the stack, false otherwise</returns>
private static bool FulFillSimpleGoal <TAction>(SimpleGoal simpleGoal, Plan <TAction> plan, bool optimal, Stack <IStackItem> itemsToProcess,
List <State> previousStates, List <Fact> currentBeliefs, List <Fact> preconditions, List <Action> actions) where TAction : Action
{
//only look for an action when the goal hasn't been achieved yet
if (!simpleGoal.IsFulFilled(currentBeliefs))
{
//add the fact of the goal in the list of visited preconditions
preconditions.Add(simpleGoal.Fact);
//look for possible actions to execute
var possibleActions = actions.Where(x => x.IsApplicableFor(simpleGoal, currentBeliefs));
var iter = possibleActions.GetEnumerator();
//keep a list of plans and beliefsets for each action that is possible (for optimization later)
var subPlansForAction = new List <Tuple <Plan <TAction>, List <Fact> > >();
while (iter.MoveNext())
{
var action = iter.Current;
//instantiate the action as much as possible
action = action.InstantiateFor(simpleGoal, currentBeliefs);
//perform loopdetection
var lstPre = action.Preconditions.ToList();
lstPre.Reverse();
var faulty = lstPre.Any(x => IsAlreadyACondition(x, preconditions));
if (!faulty)
{
//create the complex goal with the preconditions of the action
var complex = new ComplexGoal();
foreach (var precondition in lstPre)
{
complex.Goals.Add(new SimpleGoal(precondition, action));
}
//create a new stack but with action and complex goal added
var newStack = new Stack <IStackItem>(itemsToProcess.Reverse());
newStack.Push(action);
newStack.Push(complex);
//perform create plan for the new stack
(Plan <TAction> subPlanForAction, List <Fact> lstBeliefs) = CreatePlan <TAction>(newStack, currentBeliefs.ToList(), preconditions.ToList(),
previousStates.ToList(), actions, optimal);
//the chosen action gives a valid plan so add the plan (and beliefset) to the possible plans we can choose from
if (subPlanForAction != null)
{
subPlansForAction.Add(Tuple.Create(subPlanForAction, lstBeliefs));
if (optimal)
{
break;
}
}
}
}
//no plan found for this goal and current beliefset so return null
if (!subPlansForAction.Any())
{
return(false);
}
else
{
//we look for the minimal path (minimal number of steps) asuming that each step has an equal 'cost'
var minSteps = subPlansForAction.Min(x => x.Item1.Steps.Count());
var best = subPlansForAction.First(x => x.Item1.Steps.Count() == minSteps);
//add the steps of the optimal subplan to this plan and update the current set of beliefs
plan.Steps.AddRange(best.Item1.Steps);
currentBeliefs = best.Item2;
//clear the stack as we already processed all items in the stack when looking for all subplans
itemsToProcess.Clear();
}
}
return(true);
}
