/*
 * $Id: ContentModel.java,v 1.1.1.1 2000/11/23 01:53:33 edwingo Exp $
 *
 * The Apache Software License, Version 1.1
 *
 *
 * Copyright (c) 2000 The Apache Software Foundation.  All rights 
 * reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer. 
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution,
 *    if any, must include the following acknowledgment:  
 *       "This product includes software developed by the
 *        Apache Software Foundation (http://www.apache.org/)."
 *    Alternately, this acknowledgment may appear in the software itself,
 *    if and wherever such third-party acknowledgments normally appear.
 *
 * 4. The names "Crimson" and "Apache Software Foundation" must
 *    not be used to endorse or promote products derived from this
 *    software without prior written permission. For written 
 *    permission, please contact apache@apache.org.
 *
 * 5. Products derived from this software may not be called "Apache",
 *    nor may "Apache" appear in their name, without prior written
 *    permission of the Apache Software Foundation.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR
 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
 * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation and was
 * originally based on software copyright (c) 1999, Sun Microsystems, Inc., 
 * http://www.sun.com.  For more information on the Apache Software 
 * Foundation, please see <http://www.apache.org/>.
 */

package org.apache.crimson.parser;

import java.util.Vector;
import java.util.Enumeration;
import java.io.*;


/**
 * A representation of a "children" content model.  These are basically a
 * regular expression; other content models are simpler.  There is an
 * SGML compatibility restriction on DTDs that such content models be
 * deterministic, which in this sense just means that backtracking isn't
 * needed to validate against it.
 *
 * <P> At the moment, for expediency, nondeterministic models are neither
 * tested for nor are they handled reasonably.  This could be done after
 * each element's content model is fully parsed.
 *
 * <P> The most efficient way to do this would be to compile each content
 * model pattern into a deterministic finite automaton (no stack) and
 * just walk the DFA's graph ... but for now, these aren't compiled.
 *
 * @author Arthur van Hoff
 * @author David Brownell
 * @version 	$Revision: 1.1.1.1 $ 
 */
final class ContentModel
{
    /**
     * Type. Either '*', '?', '+'; or connectives ',', '|'; or
     * zero for content that's an element.
     */
    public char type;

    /**
     * The content. Either an Element name, or a ContentModel.
     */
    public Object content;

    /**
     * The next content model (in a ',' or '|' connective expression).
     * "next" has a list of connectives of the same type.
     */
    public ContentModel next;

    //
    // Cache mapping element names --> TRUE or FALSE based on whether
    // they can be 'first' in this content model or not.  NOTE:  it'd
    // be nice to have a lower cost cache, e.g. numbering elements and
    // using byte arrays.
    //
    private SimpleHashtable	cache = new SimpleHashtable ();


    /**
     * Create a content model for an element.
     */
    public ContentModel (String element) {
	this.type = 0;
	this.content = element;
    }

    /**
     * Create a content model of a particular type.
     * Normally used to specify a frequency, or to start a connective.
     */
    public ContentModel (char type, ContentModel content) {
	this.type = type;
	this.content = content;
    }

    /**
     * Return true if the content model could
     * match an empty input stream.
     */
    public boolean empty () {
	// if it matters, this could cache as a simple boolean!

	switch (type) {
	  case '*':
	  case '?':
	    return true;

	  case '+':
	  case 0:
	    return false;

	  case '|':
	    if (content instanceof ContentModel
		    && ((ContentModel)content).empty ()) {
		return true;
	    }
	    for (ContentModel m = (ContentModel)next;
		    m != null;
		    m = m.next) {
		if (m.empty ())
		    return true;
	    }
	    return false;

	  case ',':
	    if (content instanceof ContentModel) {
		if (!((ContentModel)content).empty ()) {
		    return false;
		}
	    } else {
		return false;
	    }
	    for (ContentModel m = (ContentModel)next;
		    m != null;
		    m = m.next) {
		if (!m.empty ())
		    return false;
	    }
	    return true;

	  default:
	    throw new InternalError ();
	}
    }


    /**
     * Return true if the token could potentially be the
     * first token in the input stream.
     */
    public boolean first (String token) {
	Boolean		b = (Boolean) cache.get (token);
	boolean		retval;

	if (b != null)
	    return b.booleanValue ();

	// if we had no cached result, compute it
	switch (type) {
	  case '*':
	  case '?':
	  case '+':
	  case 0:
	    if (content instanceof String)
		retval = (content == token);
	    else
		retval = ((ContentModel)content).first (token);
	    break;

	  case ',':
	    if (content instanceof String)
		retval = (content == token);
	    else if (((ContentModel)content).first (token))
		retval = true;
	    else if (!((ContentModel)content).empty ())
		retval = false;
	    else if (next != null)
		retval = ((ContentModel)next).first (token);
	    else
		retval = false;
	    break;

	  case '|':
	    if (content instanceof String && content == token)
		retval = true;
	    else if (((ContentModel)content).first (token))
		retval = true;
	    else if (next != null)
		retval = ((ContentModel)next).first (token);
	    else
		retval = false;
	    break;

	  default:
	    throw new InternalError ();
	}

	// store the result, so we can be faster next time
	if (retval)
	    cache.put (token, Boolean.TRUE);
	else
	    cache.put (token, Boolean.FALSE);

	return retval;
    }


    /**
     * Convert to a string (for debugging).
     *
    public String toString () {
	return toString (true);
    }

    private String contentString ()
    {
	if (content instanceof ContentModel)
	    return ((ContentModel)content).toString (false);
	else
	    return (String) content;
    }

    private String toString (boolean isOuter)
    {
	String	temp = contentString ();

	switch (type) {
	  case '*':
	  case '?':
	  case '+':
	    if (isOuter && temp.charAt (0) != '(')
		return "(" + temp + type + ")";
	    else
		return temp + type;

	  case 0:
	    if (isOuter)
		return "(" + temp + ")";
	    else
		return temp;

	  case ',':
	  case '|':
	    if (next == null)
		return temp;
	    for (ContentModel m = next; m != null; m = m.next)
		temp += type + m.contentString ();
	    return "(" + temp + ")";

	  default:
	    throw new InternalError ("foo");
	}
    }
    /**/
}
