ExecutionContext.java

/*
 * tuProlog - Copyright (C) 2001-2002  aliCE team at deis.unibo.it
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */
package alice.tuprolog;

import java.util.ArrayList;
import java.util.List;

/**
 * 
 * @author Alex Benini
 */
public class ExecutionContext {
	
	private int id;
	int depth;
	
	Struct currentGoal;
	
	ExecutionContext fatherCtx;
	SubGoalId fatherGoalId;
	
	Struct clause;
	Struct headClause;
	SubGoalStore goalsToEval;
	OneWayList<List<Var>> trailingVars;
	OneWayList<List<Var>> fatherVarsList;
	
	ChoicePointContext choicePointAfterCut;
		
	boolean haveAlternatives;
	
	ExecutionContext(int id) {
		this.id = id;
	}
	
	public int getId() {
		return id;
	}
	
	@Override
	public String toString() {
		return "	     id: " + id + "\n" +
		"     currentGoal:  " + currentGoal + "\n" +
		"     clause:       " + clause + "\n" +
		"     subGoalStore: " + goalsToEval + "\n" +
		"     trailingVars: " + trailingVars + "\n";
	}
	
	/*
	 * Methods for spyListeners
	 */
	
	public int getDepth() {
		return depth;
	}
	
	public Struct getCurrentGoal() {
		return currentGoal;
	}
	
	public SubGoalId getFatherGoalId() {
		return fatherGoalId;
	}
	
	public Struct getClause() {
		return clause;
	}
	
	public Struct getHeadClause() {
		return headClause;
	}
	
	public SubGoalStore getSubGoalStore() {
		return goalsToEval;
	}
	
	public List<List<Var>> getTrailingVars() {
		List<List<Var>> l = new ArrayList<List<Var>>();
		OneWayList<List<Var>> t = trailingVars;
		while (t != null) {
			l.add(t.getHead());
			t = t.getTail();
		}
		return l;		
	}
	
	/**
	 * Save the state of the parent context to later bring the ExectutionContext
	 * objects tree in a consistent state after a backtracking step.
	 */
	void saveParentState() {
		if (fatherCtx != null) {
			fatherGoalId = fatherCtx.goalsToEval.getCurrentGoalId();
			fatherVarsList = fatherCtx.trailingVars;
		}
	}
	
	/**
	 * If no open alternatives and no other term to execute ->
	 * current context no more needed ->
	 * reused to execute g subgoal =>
	 * got TAIL RECURSION OPTIMIZATION!
	 */
	void performTailRecursionOptimization(Engine e) {
		fatherCtx =
			(!haveAlternatives && !goalsToEval.haveSubGoals()) ?
					e.currentContext.fatherCtx :
					e.currentContext;
	}
	
}