/*
 * @(#)$Id: ExpressionWithChildState.java,v 1.7 2001/05/01 18:13:02 Bear Exp $
 *
 * Copyright 2001 Sun Microsystems, Inc. All Rights Reserved.
 * 
 * This software is the proprietary information of Sun Microsystems, Inc.  
 * Use is subject to license terms.
 * 
 */
package com.sun.msv.reader;

import com.sun.msv.grammar.Expression;
import com.sun.msv.util.StartTagInfo;

/**
 * State that parses Expression which contains other expressions.
 * 
 * @author <a href="mailto:kohsuke.kawaguchi@eng.sun.com">Kohsuke KAWAGUCHI</a>
 */
public abstract class ExpressionWithChildState
	extends ExpressionState implements ExpressionOwner {
	
	/**
	 * expression object that is being created.
	 * See {@link castPattern} and {@link annealPattern} methods
	 * for how will a pattern be created.
	 */
	protected Expression exp;

	protected void startSelf() {
		super.startSelf();
		exp = initialExpression();
	}
	
	/** sets initial pattern */
	protected Expression initialExpression() {
		return null;
	}
	
	/**
	 * computes default expression.
	 * this method is called before annealExpression when no child expression
	 * is given during parsing. return null to signal an error.
	 */
	protected Expression defaultExpression() {
		return null;
	}
	
	/** receives a Pattern object that is contained in this element. */
	public final void onEndChild( Expression childExpression ) {
		exp = castExpression( exp, childExpression );
	}
	
	protected final Expression makeExpression() {
		if( exp==null )
			exp = defaultExpression();
		
		if( exp==null ) {
			reader.reportError( GrammarReader.ERR_MISSING_CHILD_EXPRESSION );
			exp = Expression.nullSet;
			// recover by assuming some pattern.
		}
		return annealExpression(exp);
	}
	
	protected State createChildState( StartTagInfo tag ) {
		return reader.createExpressionChildState(this,tag);
	}

		
	/**
	 * combines half-made expression and newly found child expression into the expression.
	 * 
	 * <p>
	 * Say this container has three child expression exp1,exp2, and exp3.
	 * Then, the expression of this state will be made by the following method
	 * invocations.
	 * 
	 * <pre>
	 *   annealExpression(
	 *     castExpression(
	 *       castExpression(
	 *         castExpression(null,exp1), exp2), exp3 ) )
	 * </pre>
	 */
	protected abstract Expression castExpression( Expression halfCastedExpression, Expression newChildExpression );
	
	/**
	 * performs final wrap-up and returns a fully created Expression object
	 * that represents this element.
	 */
	protected Expression annealExpression( Expression exp ) {
		// default implementation do nothing.
		return exp;
	}
}