ubc.cs.JLog.Foundation.jProver Maven / Gradle / Ivy
/*
This file is part of JLog.
Created by Glendon Holst for Alan Mackworth and the
"Computational Intelligence: A Logical Approach" text.
Copyright 1998, 2000, 2002 by University of British Columbia and
Alan Mackworth.
This notice must remain in all files which belong to, or are derived
from JLog.
Check for further information
about JLog, or to contact the authors.
JLog 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.
JLog 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 JLog; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
URLs: or
*/
//#########################################################################
// jProver
//#########################################################################
package ubc.cs.JLog.Foundation;
import java.lang.*;
import java.util.*;
import ubc.cs.JLog.Terms.*;
/**
* This class implements a prolog proving engine. It uses two
* goal stacks to represent progress in the search tree. The goals goal
* stack stores goals that have yet to be evaluated. The proved goal
* stack stores goals that are already evaluated (i.e., the frontier).
*
* The proving process takes a goal from the goals stack and attempts
* to prove it. If successful, any subgoals of the goal are pushed onto the
* goals stack and the goal to be proved is pushed onto the
* proved stack.the is the default goal stack implementation.
*
* @author Glendon Holst
* @version %I%, %G%
*/
public class jProver
{
protected jKnowledgeBase database;
protected iGoalStack goals;
protected iGoalStack proved;
/**
* Construct a prover instance with a given jKnowledgeBase.
*
* @param kb the knowledge base which attempted proofs are based upon.
*/
public jProver(jKnowledgeBase kb)
{
database = kb;
goals = null;
proved = null;
};
/**
* Initiate a proof of the provided goal.
*
* @param goal the term to attempt to prove.
* @return true if the proof of goal succeeded,
* false otherwise.
*/
public boolean prove(jPredicateTerms goal)
{jVariableVector vars;
jUserGoal ug;
goals = createGoalsStack();
proved = createProvedStack();
proved.push(ug = new jUserGoal());
goal.consult(database);
vars = new jVariableVector();
goal.registerVariables(vars);
goal.addGoals(ug,vars.getVariables(),goals);
return internal_prove();
};
/**
* Attempts to prove the previous goal in a different way than before. Only call after
* the previous call to prove succeeded, and no call to retry
* have failed.
*
* @return true if the proof of goal succeeded,
* false otherwise.
*/
public boolean retry()
{
if (!goals.empty() || !internal_retry())
return false;
return internal_prove();
};
protected boolean internal_prove()
{
try
{
while (!goals.empty())
{
if (!goals.pop().prove(goals,proved))
{
while (!proved.pop().retry(goals,proved))
;
}
}
return true;
}
catch (EmptyStackException e)
{// this should only occur when the proved stack is empty
return false;
}
};
protected boolean internal_retry()
{
while (!proved.empty())
{
if (proved.pop().retry(goals,proved))
return true;
}
return false;
};
/**
* Construct a goal stack for proved. Subclasses can override this
* factory method to return goal stacks with different capabilities (but same interface).
*
* @return instance of iGoalStack.
*/
protected iGoalStack createProvedStack()
{
return new jGoalStack();
};
/**
* Construct a goal stack for goals. Subclasses can override this
* factory method to return goal stacks with different capabilities (but same interface).
*
* @return instance of iGoalStack.
*/
protected iGoalStack createGoalsStack()
{
return new jGoalStack();
};
};