package com.exigen.ie.constrainer.impl;
import java.io.Serializable;
import com.exigen.ie.constrainer.ChoicePointLabel;
import com.exigen.ie.constrainer.Goal;
import com.exigen.ie.tools.FastStack;
/**
* An implementation of the goal stack.
*/
public final class GoalStack implements Serializable
{
private FastStack _exeStack;
private FastStack _choicePointStack;
private int _undoStackSize;
private UndoStack _undoStack;
/**
* Constructor with given undo stack.
*/
public GoalStack(UndoStack undoStack)
{
init(new FastStack(), undoStack);
}
/**
* Constructor with given main goal and undo stack.
*/
public GoalStack(Goal mainGoal, UndoStack undoStack)
{
FastStack initialGoals = new FastStack();
initialGoals.push(mainGoal);
init(initialGoals, undoStack);
}
/**
* Constructor with given initial goals and undo stack.
*/
public GoalStack(FastStack initialGoals, UndoStack undoStack)
{
init((FastStack)initialGoals.clone(), undoStack);
}
/**
* Initialize this goal stack.
*/
final void init(FastStack initialGoals, UndoStack undoStack)
{
_exeStack = initialGoals;
_choicePointStack = new FastStack();
_undoStack = undoStack;
_undoStackSize = undoStack.size();
}
/**
* Pushes the goal onto the execution stack.
*/
final public void pushGoal(Goal goal)
{
_exeStack.push(goal);
}
/**
* Pops the goal from the execution stack and returns it.
*/
final public Goal popGoal()
{
return (Goal)_exeStack.pop();
}
/**
* Returns true if the execution stack is empty.
*/
final public boolean empty()
{
return _exeStack.empty();
}
/**
* Sets a labeled choice point between two goals.
*/
final public void setChoicePoint(Goal g1, Goal g2, ChoicePointLabel label)
{
ChoicePoint cp = new ChoicePoint(g2, label, _exeStack, _undoStack.size());
_choicePointStack.push(cp);
pushGoal(g1);
}
/**
* Backtracks the state of this goal stack to the latest labeled choice point.
*
* If the required choice point is found:
* - it restores the state of the execution stack
* - the required choice point is returned
* If the required choice point is not found:
* - the goal stack is empty
* - null is returned
*/
final public ChoicePoint backtrackStack(ChoicePointLabel label)
{
while(!_choicePointStack.empty())
{
ChoicePoint cp = (ChoicePoint)_choicePointStack.pop();
// the condition "the required label found"
if(label == null || label.equals(cp.label()))
{
_exeStack = cp.exeStack();
return cp;
}
}
// no choice point found -> clear the state of the goal stack
_exeStack.clear();
return null;
}
/**
* Backtracks this goal stack and the undo stack
* to the most recent labeled choice point.
*
* If the required choice point was found:
* - goal stack is restored to the state when the choice point was created
* - undo stack is restored to the state when the choice point was created
* - choice point goal is pushed onto goal stack
* - true is returned
*
* If the required choice point was not found:
* - goal stack will be empty
* - undo stack is restored to the state when the goal stack was created
* - false is returned
*/
final public boolean backtrack(ChoicePointLabel label)
{
ChoicePoint cp = backtrackStack(label);
if(cp != null)
{
_undoStack.backtrack( cp.undoStackSize() );
pushGoal( cp.goal() );
return true;
}
else
{
_undoStack.backtrack( undoStackSize() );
return false;
}
}
/**
* Returns the current choice point.
*/
final public ChoicePoint currentChoicePoint()
{
if(_choicePointStack.empty())
throw new RuntimeException("No current choice point");
return (ChoicePoint)_choicePointStack.peek();
}
/**
* A placeholder for the information about the choice point.
*/
static public class ChoicePoint implements java.io.Serializable
{
Goal _goal;
ChoicePointLabel _label;
FastStack _exeStack;
int _undoStackSize;
public ChoicePoint(Goal goal, ChoicePointLabel label, FastStack exeStack, int undoStackSize)
{
_goal = goal;
_label = label;
_exeStack = (FastStack)exeStack.clone();
_undoStackSize = undoStackSize;
}
final public Goal goal()
{
return _goal;
}
final public ChoicePointLabel label()
{
return _label;
}
final public FastStack exeStack()
{
return _exeStack;
}
final public int undoStackSize()
{
return _undoStackSize;
}
} // ~ChoicePoint
final public int undoStackSize()
{
return _undoStackSize;
}
public String toString()
{
return "GoalStack: "
+"\n\tExecutionStack: " + _exeStack
+"\n\tChiocePointStack: " + _choicePointStack
;
}
} // ~GoalStack