/* Author: Jeff Dalton <J.Dalton@ed.ac.uk>
 * Updated: Sun Dec  2 03:26:18 2001 by Jeff Dalton
 */

package ix.util.lisp;

import java.io.Serializable;
import java.util.*;
import ix.util.*;


/**
 * The root class for Lisp symbols. <p>
 *
 * The correct way to create a new symbol is to call 
 *    <tt>Symbol.intern(String name)</tt>.
 * It will return an instance of Symbol or of an appropriate subclass,
 * depending on the first character of the name. 
 *
 */

// At least that's how things are for now.  It's not yet clear what's best.

public class Symbol implements LispObject, Comparable, Serializable {

    protected static Hashtable obTable = new Hashtable();

    public String name;

    protected Symbol(String name) {
	this.name = name;
	Debug.assert(obTable.get(name) == null);
	obTable.put(name, this);
    }

    public static Symbol intern(String s) {
	Symbol sym = (Symbol)obTable.get(s);
	if (sym != null)
	    return sym;
	else if (s.length() == 0)
	    return new Symbol(s);
	else {
	    switch (s.charAt(0)) {
	      case ':' : return new Keyword(s);
	      case '?' : return new ItemVar(s);
	      default  : return new Symbol(s);
	    }
	}
    }

    public int compareTo(Object o) {
	if (o instanceof Symbol)
	    return name.compareTo(((Symbol)o).name);
	else
	    throw new ClassCastException
		("Cannot compare symbol " + this + " to " + o);
    }

    protected Object readResolve() throws java.io.ObjectStreamException {
	// Deserialization magic to avoid multiple (visible)
	// instances of Symbols that should be eq.  The instances
	// are created (it seems) but then this method is called ...
	return intern(this.name);
    }

    public String toString() {
	return name;
    }

}