/*
 * 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.
 */
/*
 * $Id$
 */
package org.apache.xpath;

import javax.xml.transform.ErrorListener;
import javax.xml.transform.TransformerException;

import org.apache.xalan.res.XSLMessages;
import org.apache.xml.dtm.DTM;
import org.apache.xml.dtm.DTMIterator;
import org.apache.xml.utils.XMLString;
import org.apache.xpath.objects.XNodeSet;
import org.apache.xpath.objects.XObject;
import org.apache.xpath.res.XPATHErrorResources;

import org.xml.sax.ContentHandler;

/**
 * This abstract class serves as the base for all expression objects.  An
 * Expression can be executed to return a {@link org.apache.xpath.objects.XObject},
 * normally has a location within a document or DOM, can send error and warning
 * events, and normally do not hold state and are meant to be immutable once
 * construction has completed.  An exception to the immutibility rule is iterators
 * and walkers, which must be cloned in order to be used -- the original must
 * still be immutable.
 */
public abstract class Expression implements java.io.Serializable, ExpressionNode, XPathVisitable
{
    static final long serialVersionUID = 565665869777906902L;
  /**
   * The location where this expression was built from.  Need for diagnostic
   *  messages. May be null.
   *  @serial
   */
  private ExpressionNode m_parent;

  /**
   * Tell if this expression or it's subexpressions can traverse outside
   * the current subtree.
   *
   * @return true if traversal outside the context node's subtree can occur.
   */
  public boolean canTraverseOutsideSubtree()
  {
    return false;
  }
  
//  /**
//   * Set the location where this expression was built from.
//   *
//   *
//   * @param locator the location where this expression was built from, may be
//   *                null.
//   */
//  public void setSourceLocator(SourceLocator locator)
//  {
//    m_slocator = locator;
//  }

  /**
   * Execute an expression in the XPath runtime context, and return the
   * result of the expression.
   *
   *
   * @param xctxt The XPath runtime context.
   * @param currentNode The currentNode.
   *
   * @return The result of the expression in the form of a <code>XObject</code>.
   *
   * @throws javax.xml.transform.TransformerException if a runtime exception
   *         occurs.
   */
  public XObject execute(XPathContext xctxt, int currentNode)
          throws javax.xml.transform.TransformerException
  {

    // For now, the current node is already pushed.
    return execute(xctxt);
  }

  /**
   * Execute an expression in the XPath runtime context, and return the
   * result of the expression.
   *
   *
   * @param xctxt The XPath runtime context.
   * @param currentNode The currentNode.
   * @param dtm The DTM of the current node.
   * @param expType The expanded type ID of the current node.
   *
   * @return The result of the expression in the form of a <code>XObject</code>.
   *
   * @throws javax.xml.transform.TransformerException if a runtime exception
   *         occurs.
   */
  public XObject execute(
          XPathContext xctxt, int currentNode, DTM dtm, int expType)
            throws javax.xml.transform.TransformerException
  {

    // For now, the current node is already pushed.
    return execute(xctxt);
  }

  /**
   * Execute an expression in the XPath runtime context, and return the
   * result of the expression.
   *
   *
   * @param xctxt The XPath runtime context.
   *
   * @return The result of the expression in the form of a <code>XObject</code>.
   *
   * @throws javax.xml.transform.TransformerException if a runtime exception
   *         occurs.
   */
  public abstract XObject execute(XPathContext xctxt)
    throws javax.xml.transform.TransformerException;

  /**
   * Execute an expression in the XPath runtime context, and return the
   * result of the expression, but tell that a "safe" object doesn't have 
   * to be returned.  The default implementation just calls execute(xctxt).
   *
   *
   * @param xctxt The XPath runtime context.
   * @param destructiveOK true if a "safe" object doesn't need to be returned.
   *
   * @return The result of the expression in the form of a <code>XObject</code>.
   *
   * @throws javax.xml.transform.TransformerException if a runtime exception
   *         occurs.
   */
  public XObject execute(XPathContext xctxt, boolean destructiveOK)
    throws javax.xml.transform.TransformerException
  {
  	return execute(xctxt);
  }


  /**
   * Evaluate expression to a number.
   *
   *
   * @param xctxt The XPath runtime context.
   * @return The expression evaluated as a double.
   *
   * @throws javax.xml.transform.TransformerException
   */
  public double num(XPathContext xctxt)
          throws javax.xml.transform.TransformerException
  {
    return execute(xctxt).num();
  }

  /**
   * Evaluate expression to a boolean.
   *
   *
   * @param xctxt The XPath runtime context.
   * @return false
   *
   * @throws javax.xml.transform.TransformerException
   */
  public boolean bool(XPathContext xctxt)
          throws javax.xml.transform.TransformerException
  {
    return execute(xctxt).bool();
  }

  /**
   * Cast result object to a string.
   *
   *
   * @param xctxt The XPath runtime context.
   * @return The string this wraps or the empty string if null
   *
   * @throws javax.xml.transform.TransformerException
   */
  public XMLString xstr(XPathContext xctxt)
          throws javax.xml.transform.TransformerException
  {
    return execute(xctxt).xstr();
  }

  /**
   * Tell if the expression is a nodeset expression.  In other words, tell
   * if you can execute {@link #asNode(XPathContext) asNode} without an exception.
   * @return true if the expression can be represented as a nodeset.
   */
  public boolean isNodesetExpr()
  {
    return false;
  }

  /**
   * Return the first node out of the nodeset, if this expression is
   * a nodeset expression.
   * @param xctxt The XPath runtime context.
   * @return the first node out of the nodeset, or DTM.NULL.
   *
   * @throws javax.xml.transform.TransformerException
   */
  public int asNode(XPathContext xctxt)
          throws javax.xml.transform.TransformerException
  {
  	DTMIterator iter = execute(xctxt).iter();
    return iter.nextNode();
  }

  /**
   * Given an select expression and a context, evaluate the XPath
   * and return the resulting iterator.
   *
   * @param xctxt The execution context.
   * @param contextNode The node that "." expresses.
   *
   *
   * @return A valid DTMIterator.
   * @throws TransformerException thrown if the active ProblemListener decides
   * the error condition is severe enough to halt processing.
   *
   * @throws javax.xml.transform.TransformerException
   * @xsl.usage experimental
   */
  public DTMIterator asIterator(XPathContext xctxt, int contextNode)
          throws javax.xml.transform.TransformerException
  {

    try
    {
      xctxt.pushCurrentNodeAndExpression(contextNode, contextNode);

      return execute(xctxt).iter();
    }
    finally
    {
      xctxt.popCurrentNodeAndExpression();
    }
  }
  
  /**
   * Given an select expression and a context, evaluate the XPath
   * and return the resulting iterator, but do not clone.
   *
   * @param xctxt The execution context.
   * @param contextNode The node that "." expresses.
   *
   *
   * @return A valid DTMIterator.
   * @throws TransformerException thrown if the active ProblemListener decides
   * the error condition is severe enough to halt processing.
   *
   * @throws javax.xml.transform.TransformerException
   * @xsl.usage experimental
   */
  public DTMIterator asIteratorRaw(XPathContext xctxt, int contextNode)
          throws javax.xml.transform.TransformerException
  {

    try
    {
      xctxt.pushCurrentNodeAndExpression(contextNode, contextNode);

      XNodeSet nodeset = (XNodeSet)execute(xctxt);
      return nodeset.iterRaw();
    }
    finally
    {
      xctxt.popCurrentNodeAndExpression();
    }
  }


  /**
   * Execute an expression in the XPath runtime context, and return the
   * result of the expression.
   *
   *
   * @param xctxt The XPath runtime context.
   * NEEDSDOC @param handler
   *
   * @return The result of the expression in the form of a <code>XObject</code>.
   *
   * @throws javax.xml.transform.TransformerException if a runtime exception
   *         occurs.
   * @throws org.xml.sax.SAXException
   */
  public void executeCharsToContentHandler(
          XPathContext xctxt, ContentHandler handler)
            throws javax.xml.transform.TransformerException,
                   org.xml.sax.SAXException
  {

    XObject obj = execute(xctxt);

    obj.dispatchCharactersEvents(handler);
    obj.detach();
  }

  /**
   * Tell if this expression returns a stable number that will not change during 
   * iterations within the expression.  This is used to determine if a proximity 
   * position predicate can indicate that no more searching has to occur.
   * 
   *
   * @return true if the expression represents a stable number.
   */
  public boolean isStableNumber()
  {
    return false;
  }

  /**
   * This function is used to fixup variables from QNames to stack frame
   * indexes at stylesheet build time.
   * @param vars List of QNames that correspond to variables.  This list
   * should be searched backwards for the first qualified name that
   * corresponds to the variable reference qname.  The position of the
   * QName in the vector from the start of the vector will be its position
   * in the stack frame (but variables above the globalsTop value will need
   * to be offset to the current stack frame).
   * NEEDSDOC @param globalsSize
   */
  public abstract void fixupVariables(java.util.Vector vars, int globalsSize);
  
  /**
   * Compare this object with another object and see 
   * if they are equal, include the sub heararchy.
   * 
   * @param expr Another expression object.
   * @return true if this objects class and the expr
   * object's class are the same, and the data contained 
   * within both objects are considered equal.
   */
  public abstract boolean deepEquals(Expression expr);
  
  /**
   * This is a utility method to tell if the passed in 
   * class is the same class as this.  It is to be used by
   * the deepEquals method.  I'm bottlenecking it here 
   * because I'm not totally confident that comparing the 
   * class objects is the best way to do this.
   * @return true of the passed in class is the exact same 
   * class as this class.
   */
  protected final boolean isSameClass(Expression expr)
  {
  	if(null == expr)
  	  return false;
  	  
  	return (getClass() == expr.getClass());
  }

  /**
   * Warn the user of an problem.
   *
   * @param xctxt The XPath runtime context.
   * @param msg An error msgkey that corresponds to one of the conststants found
   *            in {@link org.apache.xpath.res.XPATHErrorResources}, which is
   *            a key for a format string.
   * @param args An array of arguments represented in the format string, which
   *             may be null.
   *
   * @throws TransformerException if the current ErrorListoner determines to
   *                              throw an exception.
   *
   * @throws javax.xml.transform.TransformerException
   */
  public void warn(XPathContext xctxt, String msg, Object[] args)
          throws javax.xml.transform.TransformerException
  {

    java.lang.String fmsg = XSLMessages.createXPATHWarning(msg, args);

    if (null != xctxt)
    {
      ErrorListener eh = xctxt.getErrorListener();

      // TO DO: Need to get stylesheet Locator from here.
      eh.warning(new TransformerException(fmsg, xctxt.getSAXLocator()));
    }
  }

  /**
   * Tell the user of an assertion error, and probably throw an
   * exception.
   *
   * @param b  If false, a runtime exception will be thrown.
   * @param msg The assertion message, which should be informative.
   *
   * @throws RuntimeException if the b argument is false.
   *
   * @throws javax.xml.transform.TransformerException
   */
  public void assertion(boolean b, java.lang.String msg)
  {

    if (!b)
    {
      java.lang.String fMsg = XSLMessages.createXPATHMessage(
        XPATHErrorResources.ER_INCORRECT_PROGRAMMER_ASSERTION,
        new Object[]{ msg });

      throw new RuntimeException(fMsg);
    }
  }

  /**
   * Tell the user of an error, and probably throw an
   * exception.
   *
   * @param xctxt The XPath runtime context.
   * @param msg An error msgkey that corresponds to one of the constants found
   *            in {@link org.apache.xpath.res.XPATHErrorResources}, which is
   *            a key for a format string.
   * @param args An array of arguments represented in the format string, which
   *             may be null.
   *
   * @throws TransformerException if the current ErrorListoner determines to
   *                              throw an exception.
   *
   * @throws javax.xml.transform.TransformerException
   */
  public void error(XPathContext xctxt, String msg, Object[] args)
          throws javax.xml.transform.TransformerException
  {

    java.lang.String fmsg = XSLMessages.createXPATHMessage(msg, args);

    if (null != xctxt)
    {
      ErrorListener eh = xctxt.getErrorListener();
      TransformerException te = new TransformerException(fmsg, this);

      eh.fatalError(te);
    }
  }
  
  /**
   * Get the first non-Expression parent of this node.
   * @return null or first ancestor that is not an Expression.
   */
  public ExpressionNode getExpressionOwner()
  {
  	ExpressionNode parent = exprGetParent();
  	while((null != parent) && (parent instanceof Expression))
  		parent = parent.exprGetParent();
  	return parent;
  }
  
  //=============== ExpressionNode methods ================
  
  /** This pair of methods are used to inform the node of its
    parent. */
  public void exprSetParent(ExpressionNode n)
  {
  	assertion(n != this, "Can not parent an expression to itself!");
  	m_parent = n;
  }
  
  public ExpressionNode exprGetParent()
  {
  	return m_parent;
  }

  /** This method tells the node to add its argument to the node's
    list of children.  */
  public void exprAddChild(ExpressionNode n, int i)
  {
  	assertion(false, "exprAddChild method not implemented!");
  }

  /** This method returns a child node.  The children are numbered
     from zero, left to right. */
  public ExpressionNode exprGetChild(int i)
  {
  	return null;
  }

  /** Return the number of children the node has. */
  public int exprGetNumChildren()
  {
  	return 0;
  }
  
  //=============== SourceLocator methods ================

  /**
   * Return the public identifier for the current document event.
   *
   * <p>The return value is the public identifier of the document
   * entity or of the external parsed entity in which the markup that
   * triggered the event appears.</p>
   *
   * @return A string containing the public identifier, or
   *         null if none is available.
   * @see #getSystemId
   */
  public String getPublicId()
  {
  	if(null == m_parent)
  	  return null;
  	return m_parent.getPublicId();
  }

  /**
   * Return the system identifier for the current document event.
   *
   * <p>The return value is the system identifier of the document
   * entity or of the external parsed entity in which the markup that
   * triggered the event appears.</p>
   *
   * <p>If the system identifier is a URL, the parser must resolve it
   * fully before passing it to the application.</p>
   *
   * @return A string containing the system identifier, or null
   *         if none is available.
   * @see #getPublicId
   */
  public String getSystemId()
  {
  	if(null == m_parent)
  	  return null;
  	return m_parent.getSystemId();
  }

  /**
   * Return the line number where the current document event ends.
   *
   * <p><strong>Warning:</strong> The return value from the method
   * is intended only as an approximation for the sake of error
   * reporting; it is not intended to provide sufficient information
   * to edit the character content of the original XML document.</p>
   *
   * <p>The return value is an approximation of the line number
   * in the document entity or external parsed entity where the
   * markup that triggered the event appears.</p>
   *
   * @return The line number, or -1 if none is available.
   * @see #getColumnNumber
   */
  public int getLineNumber()
  {
  	if(null == m_parent)
  	  return 0;
  	return m_parent.getLineNumber();
  }

  /**
   * Return the character position where the current document event ends.
   *
   * <p><strong>Warning:</strong> The return value from the method
   * is intended only as an approximation for the sake of error
   * reporting; it is not intended to provide sufficient information
   * to edit the character content of the original XML document.</p>
   *
   * <p>The return value is an approximation of the column number
   * in the document entity or external parsed entity where the
   * markup that triggered the event appears.</p>
   *
   * @return The column number, or -1 if none is available.
   * @see #getLineNumber
   */
  public int getColumnNumber()
  {
  	if(null == m_parent)
  	  return 0;
  	return m_parent.getColumnNumber();
  }
}