/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements. See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership. The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied. See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.axiom.util.stax.debug;

import org.apache.axiom.util.stax.wrapper.XMLStreamReaderWrapper;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import javax.xml.namespace.QName;
import javax.xml.stream.XMLStreamConstants;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.XMLStreamReader;

import java.util.Stack;

/**
 * {@link XMLStreamReader} wrapper that performs some simple consistency checks on the events
 * returned by the wrapper reader. This is most useful for custom {@link XMLStreamReader}
 * implementations. Validating events can help find and correct errors when they occur. Otherwise
 * the errors may be caught much further downstream and hard to fix. In its current version, the
 * validator ensures that the start element events match the end element events.
 */
public class XMLStreamReaderValidator extends XMLStreamReaderWrapper {

    private static final Log log = LogFactory.getLog(XMLStreamReaderValidator.class);
    private static boolean IS_ADV_DEBUG_ENABLED = false;  // Turn this on to trace every event
    
    private boolean throwExceptions = false;   // Indicates whether OMException should be thrown if errors are disovered
    private Stack<QName> stack = new Stack<QName>(); // Stack keeps track of the nested element QName
    

    /**
     * @param delegate XMLStreamReader to validate
     * @param throwExceptions (true if exceptions should be thrown when errors are encountered)
     */
    public XMLStreamReaderValidator(XMLStreamReader delegate, boolean throwExceptions) {
        super(delegate);
        this.throwExceptions = throwExceptions;
    }

 
    @Override
    public int next() throws XMLStreamException {
        int event = super.next();
        trackEvent(event);
        return event;
    }
    
    @Override
    public String getElementText() throws XMLStreamException {
        String text = super.getElementText();
        trackEvent(END_ELEMENT);
        return text;
    }

    @Override
    public int nextTag() throws XMLStreamException {
        int event = super.nextTag();
        trackEvent(event);
        return event;
    }

    private void trackEvent(int event) throws XMLStreamException {
        logParserState();
        
        // Make sure that the start element and end element events match.
        // Mismatched events are a key indication that the delegate stream reader is 
        // broken or corrupted.
        switch (event) {
        case XMLStreamConstants.START_ELEMENT:
            stack.push(super.getName());
            break;
        case XMLStreamConstants.END_ELEMENT:
            QName delegateQName = super.getName();
            if (stack.isEmpty()) {
                reportError("An END_ELEMENT event for " + delegateQName + 
                        " was encountered, but the START_ELEMENT stack is empty.");
            } else {
                QName expectedQName = stack.pop();
                
                if (!expectedQName.equals(delegateQName)) {
                    reportError("An END_ELEMENT event for " + delegateQName + 
                            " was encountered, but this doesn't match the corresponding START_ELEMENT " + 
                            expectedQName + " event.");
                }
            }
            break;
        case XMLStreamConstants.END_DOCUMENT:
            if (!stack.isEmpty()) {
                reportError("An unexpected END_DOCUMENT event was encountered; element stack: " + stack);
            }
            break;
        default :
        
        }
    }
    
    private void reportError(String message) throws XMLStreamException {
        log.debug(message);
        if (throwExceptions) {
            throw new XMLStreamException(message);
        }
    }
    
    /**
     * Dump the current event of the delegate.
     */
    protected void logParserState() {
        if (IS_ADV_DEBUG_ENABLED) {
            int currentEvent = super.getEventType();
            
            switch (currentEvent) {
            case XMLStreamConstants.START_ELEMENT:
                log.trace("START_ELEMENT: ");
                log.trace("  QName: " + super.getName());
                break;
            case XMLStreamConstants.START_DOCUMENT:
                log.trace("START_DOCUMENT: ");
                break;
            case XMLStreamConstants.CHARACTERS:
                log.trace("CHARACTERS: ");
                log.trace(   "[" + super.getText() + "]");
                break;
            case XMLStreamConstants.CDATA:
                log.trace("CDATA: ");
                log.trace(   "[" + super.getText() + "]");
                break;
            case XMLStreamConstants.END_ELEMENT:
                log.trace("END_ELEMENT: ");
                log.trace("  QName: " + super.getName());
                break;
            case XMLStreamConstants.END_DOCUMENT:
                log.trace("END_DOCUMENT: ");
                break;
            case XMLStreamConstants.SPACE:
                log.trace("SPACE: ");
                log.trace(   "[" + super.getText() + "]");
                break;
            case XMLStreamConstants.COMMENT:
                log.trace("COMMENT: ");
                log.trace(   "[" + super.getText() + "]");
                break;
            case XMLStreamConstants.DTD:
                log.trace("DTD: ");
                log.trace(   "[" + super.getText() + "]");
                break;
            case XMLStreamConstants.PROCESSING_INSTRUCTION:
                log.trace("PROCESSING_INSTRUCTION: ");
                log.trace("   [" + super.getPITarget() + "][" +
                        super.getPIData() + "]");
                break;
            case XMLStreamConstants.ENTITY_REFERENCE:
                log.trace("ENTITY_REFERENCE: ");
                log.trace("    " + super.getLocalName() + "[" +
                        super.getText() + "]");
                break;
            default :
                log.trace("UNKNOWN_STATE: " + currentEvent);
            
            }
        }
    }
}
