//tabstop=4
//*****************************************************************************/
// Project: jpl
//
// File:    $Id: Variable.java,v 1.2 2004/09/21 14:30:27 jan Exp $
// Date:    $Date: 2004/09/21 14:30:27 $
// Author:  Fred Dushin <fadushin@syr.edu>
//          
//
// Description:
//    
//
// -------------------------------------------------------------------------
// Copyright (c) 2004 Paul Singleton
// Copyright (c) 1998 Fred Dushin
//                    All rights reserved.
// 
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Library Public License
// as published by the Free Software Foundation; either version 2
// 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 Library Public License for more details.
//*****************************************************************************/
package jpl;

import java.util.Iterator;
import java.util.Map;

import jpl.fli.Prolog;
import jpl.fli.term_t;

//----------------------------------------------------------------------/
// Variable
/**
 * This class supports Java representations of Prolog variables.<p>
 * 
 * A jpl.Variable instance is equivalent to a variable in a fragment of Prolog source text:
 * it is *not* a "live" variable within a Prolog stack or heap.
 * A corresponding Prolog variable is created only upon opening
 * a Query whose goal refers to a Variable (and then only temporarily).
 * 
 * <hr><i>
 * Copyright (C) 2004  Paul Singleton<p>
 * Copyright (C) 1998  Fred Dushin<p>
 * 
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.<p>
 * 
 * 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 Library Public License for more details.<p>
 * </i><hr>
 * @author  Fred Dushin <fadushin@syr.edu>
 * @version $Revision: 1.2 $
 */
// Implementation notes:  
// 
//----------------------------------------------------------------------/
public class Variable extends Term {

	//==================================================================/
	//  Attributes
	//==================================================================/

	private static long n = 0; // the integral part of the next automatic variable name to be allocated

	public final String name; // the name of this Variable

	protected transient term_t term_ = null; // defined between Query.open() and Query.get2()
	protected transient int index; // only used by (redundant?) 

	//==================================================================/
	//  Constructors
	//==================================================================/

	/**
	 * Create a new Variable with 'name' (which must not be null or ""),
	 * and may one day be constrained to comply with traditional Prolog syntax.
	 * 
	 * @param   name  the source name of this Variable
	 */
	public Variable(String name) {
		if (name == null) {
			throw new JPLException("constructor jpl.Variable(name): name cannot be null");
		}
		if (!isValidName(name)) {
			throw new JPLException("constructor jpl.Variable(name): name cannot be empty String");
		}
		this.name = name;
	}

	//==================================================================/
	//  Constructors (deprecated)
	//==================================================================/

	/**
	 * Create a new Variable with new sequential name of the form "_261".
	 * 
	 * @deprecated	use Variable(String name) instead
	 */
	public Variable() {
		this.name = "_" + Long.toString(n++); // e.g. _0, _1 etc.
	}

	//==================================================================/
	//  Methods (common)
	//==================================================================/

	/**
	 * returns the lexical name of this Variable
	 * 
	 * @return the lexical name of this Variable
	 */
	public final String name() {
		return this.name;
	}

	/**
	 * Returns a Prolog source text representation of this Variable
	 * 
	 * @return  a Prolog source text representation of this Variable
	 */
	public String toString() {
		return this.name;
	}

	/**
	 * A Variable is equal to another if their names are the same and they are not anonymous.
	 * 
	 * @param   obj  The Object to compare.
	 * @return  true if the Object is a Variable and the above condition apply.
	 */
	public final boolean equals(Object obj) {
		return obj instanceof Variable && !this.name.equals("_") && this.name.equals(((Variable) obj).name);
	}

	public final int type() {
		return Prolog.VARIABLE;
	}

	public String typeName() {
		return "Variable";
	}

	//==================================================================/
	//  Methods (private)
	//==================================================================/

	/**
	 * Tests the lexical validity of s as a variable's name
	 * 
	 * @return the lexical validity of s as a variable's name
	 * @deprecated
	 */
	private boolean isValidName(String s) {
		if (s == null) {
			throw new java.lang.NullPointerException(); // JPL won't call it this way
		}
		int len = s.length();
		if (len == 0) {
			throw new JPLException("invalid variable name");
		}
		char c = s.charAt(0);
		if (!(c == '_' || c >= 'A' && c <= 'Z')) {
			return false;
		}
		for (int i = 1; i < len; i++) {
			c = s.charAt(i);
			if (!(c == '_' || c >= 'A' && c <= 'Z' || c >= 'a' && c <= 'z' || c >= '0' && c <= '9')) {
				return false;
			}
		}
		return true;
	}

	//==================================================================/
	//  Methods (deprecated)
	//==================================================================/

	/**
	 * The (nonexistent) args of this Variable
	 * 
	 * @return the (nonexistent) args of this Variable
	 * @deprecated
	 */
	public Term[] args() {
		return new Term[] {
		};
	}

	/**
	 * Returns a debug-friendly String representation of an Atom.
	 * 
	 * @return  a debug-friendly String representation of an Atom
	 * @deprecated
	 */
	public String debugString() {
		return "(Variable " + toString() + ")";
	}

	//==================================================================/
	//  Converting JPL Terms to Prolog terms
	//==================================================================/

	/**
	 * To put a Variable, we must check whether a (non-anonymous) variable with the same name
	 * has already been put in the Term.  If one has, then the corresponding Prolog variable has
	 * been stashed in the varnames_to_vars Map, keyed by the Variable name, so we can look
	 * it up and reuse it (this way, the sharing of variables in the Prolog term 
	 * reflects the sharing of Variable names in the Term.
	 * Otherwise, if this Variable name has not
	 * already been seen in the Term, then we put a new Prolog variable and add it into the Map
	 * (keyed by this Variable name).
	 * 
	 * @param   varnames_to_vars  A Map from variable names to Prolog variables.
	 * @param   term              A (previously created) term_t which is to be
	 *                            set to a (new or reused) Prolog variable.
	 */
	protected final void put(Map varnames_to_vars, term_t term) {
		term_t var;
		// if this var is anonymous or as yet unseen, put a new Prolog variable
		if (this.name.equals("_") || (var = (term_t) varnames_to_vars.get(this.name)) == null) {
			this.term_ = term;
			this.index = varnames_to_vars.size(); // i.e. first var in is #0 etc.
			Prolog.put_variable(term);
			if (!this.name.equals("_")) {
				varnames_to_vars.put(this.name, term);
			}
		} else {
			this.term_ = var;
			Prolog.put_term(term, var);
		}
	}

	//==================================================================/
	//  Converting Prolog terms to JPL Terms
	//==================================================================/

	/**
	 * Converts a term_t (known to refer to a Prolog variable) to a Variable.
	 * If the variable has already been seen (and hence converted),
	 * return its corresponding Variable from the map,
	 * else create a new Variable, stash it in the map (keyed by the Prolog variable),
	 * and return it.
	 *
	 * @param   vars_to_Vars      a map from Prolog to JPL variables
	 * @param   var               The term_t (known to be a variable) to convert
	 * @return                    A new or reused Variable
	 */
	protected static Term getTerm(Map vars_to_Vars, term_t var) {

		for (Iterator i = vars_to_Vars.keySet().iterator(); i.hasNext();) {
			term_t varX = (term_t) i.next(); // a previously seen Prolog variable
			if (Prolog.compare(varX, var) == 0) { // identical Prolog variables?
				return (Term) vars_to_Vars.get(varX); // return the associated JPL Variable
			}
		}
		// otherwise, the Prolog variable in term has not been seen before
		Variable Var = new Variable(); // allocate a new (sequentially named) Variable to represent it
		Var.term_ = var; // this should become redundant...
		vars_to_Vars.put(var, Var); // use Hashtable(var,null), but only need set(var)
		return Var;
	}

	//==================================================================/
	//  Computing Substitutions
	//==================================================================/

	/**
	 * If this Variable instance is not an anonymous or (in dont-tell-me mode) a dont-tell-me variable, and its binding is not already in the varnames_to_Terms Map,
	 * put the result of converting the term_t to which this variable
	 * has been unified to a Term in the Map, keyed on this Variable's name.
	 * 
	 * @param   varnames_to_Terms  A Map of bindings from variable names to JPL Terms.
	 * @param   vars_to_Vars       A Map from Prolog variables to JPL Variables.
	 */
	protected final void getSubst(Map varnames_to_Terms, Map vars_to_Vars) {
		// NB a Variable.name cannot be "" i.e. of 0 length
		// if (!(this.name.charAt(0) == '_') && varnames_to_Terms.get(this.name) == null) {
		if (tellThem() && varnames_to_Terms.get(this.name) == null) {
			varnames_to_Terms.put(this.name, Term.getTerm(vars_to_Vars, this.term_));
		}
	}

	// whether, according to prevailing policy and theis Variable's name,
	// any binding should be returned
	// (yes, unless it's anonymous or we're in dont-tell-me mode and its a dont-tell-me variable)
	private final boolean tellThem() {
		return !(this.name.equals("_") || jpl.JPL.modeDontTellMe && this.name.charAt(0) == '_');
		// return !this.name.equals("_");
	}
}

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