gcc/libjava/classpath/javax/swing/text/AbstractDocument.java

/* AbstractDocument.java --
   Copyright (C) 2002, 2004, 2005  Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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General Public License for more details.

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02110-1301 USA.

Linking this library statically or dynamically with other modules is
making a combined work based on this library.  Thus, the terms and
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permission to link this library with independent modules to produce an
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package javax.swing.text;

import java.io.PrintStream;
import java.io.Serializable;
import java.util.Dictionary;
import java.util.Enumeration;
import java.util.EventListener;
import java.util.Hashtable;
import java.util.Vector;

import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.event.EventListenerList;
import javax.swing.event.UndoableEditEvent;
import javax.swing.event.UndoableEditListener;
import javax.swing.text.DocumentFilter;
import javax.swing.tree.TreeNode;
import javax.swing.undo.AbstractUndoableEdit;
import javax.swing.undo.CompoundEdit;
import javax.swing.undo.UndoableEdit;

/**
 * An abstract base implementation for the {@link Document} interface.
 * This class provides some common functionality for all <code>Element</code>s,
 * most notably it implements a locking mechanism to make document modification
 * thread-safe.
 *
 * @author original author unknown
 * @author Roman Kennke (roman@kennke.org)
 */
public abstract class AbstractDocument implements Document, Serializable
{
  /** The serialization UID (compatible with JDK1.5). */
  private static final long serialVersionUID = 6842927725919637215L;

  /**
   * Standard error message to indicate a bad location.
   */
  protected static final String BAD_LOCATION = "document location failure";

  /**
   * Standard name for unidirectional <code>Element</code>s.
   */
  public static final String BidiElementName = "bidi level";

  /**
   * Standard name for content <code>Element</code>s. These are usually
   * {@link LeafElement}s.
   */
  public static final String ContentElementName = "content";

  /**
   * Standard name for paragraph <code>Element</code>s. These are usually
   * {@link BranchElement}s.
   */
  public static final String ParagraphElementName = "paragraph";

  /**
   * Standard name for section <code>Element</code>s. These are usually
   * {@link DefaultStyledDocument.SectionElement}s.
   */
  public static final String SectionElementName = "section";

  /**
   * Attribute key for storing the element name.
   */
  public static final String ElementNameAttribute = "$ename";

  /**
   * The actual content model of this <code>Document</code>.
   */
  Content content;

  /**
   * The AttributeContext for this <code>Document</code>.
   */
  AttributeContext context;

  /**
   * The currently installed <code>DocumentFilter</code>.
   */
  DocumentFilter documentFilter;

  /**
   * The documents properties.
   */
  Dictionary properties;

  /**
   * Manages event listeners for this <code>Document</code>.
   */
  protected EventListenerList listenerList = new EventListenerList();
  
  /**
   * Stores the current writer thread.  Used for locking.
   */ 
  private Thread currentWriter = null;
  
  /**
   * The number of readers.  Used for locking.
   */
  private int numReaders = 0;
  
  /**
   * Tells if there are one or more writers waiting.
   */
  private int numWritersWaiting = 0;  

  /**
   * A condition variable that readers and writers wait on.
   */
  Object documentCV = new Object();

  /** An instance of a DocumentFilter.FilterBypass which allows calling
   * the insert, remove and replace method without checking for an installed
   * document filter.
   */
  DocumentFilter.FilterBypass bypass;
  
  /**
   * Creates a new <code>AbstractDocument</code> with the specified
   * {@link Content} model.
   *
   * @param doc the <code>Content</code> model to be used in this
   *        <code>Document<code>
   *
   * @see GapContent
   * @see StringContent
   */
  protected AbstractDocument(Content doc)
  {
    this(doc, StyleContext.getDefaultStyleContext());
  }

  /**
   * Creates a new <code>AbstractDocument</code> with the specified
   * {@link Content} model and {@link AttributeContext}.
   *
   * @param doc the <code>Content</code> model to be used in this
   *        <code>Document<code>
   * @param ctx the <code>AttributeContext</code> to use
   *
   * @see GapContent
   * @see StringContent
   */
  protected AbstractDocument(Content doc, AttributeContext ctx)
  {
    content = doc;
    context = ctx;
  }
  
  /** Returns the DocumentFilter.FilterBypass instance for this
   * document and create it if it does not exist yet.
   *  
   * @return This document's DocumentFilter.FilterBypass instance.
   */
  private DocumentFilter.FilterBypass getBypass()
  {
    if (bypass == null)
      bypass = new Bypass();
    
    return bypass;
  }

  /**
   * Returns the paragraph {@link Element} that holds the specified position.
   *
   * @param pos the position for which to get the paragraph element
   *
   * @return the paragraph {@link Element} that holds the specified position
   */
  public abstract Element getParagraphElement(int pos);

  /**
   * Returns the default root {@link Element} of this <code>Document</code>.
   * Usual <code>Document</code>s only have one root element and return this.
   * However, there may be <code>Document</code> implementations that
   * support multiple root elements, they have to return a default root element
   * here.
   *
   * @return the default root {@link Element} of this <code>Document</code>
   */
  public abstract Element getDefaultRootElement();

  /**
   * Creates and returns a branch element with the specified
   * <code>parent</code> and <code>attributes</code>. Note that the new
   * <code>Element</code> is linked to the parent <code>Element</code>
   * through {@link Element#getParentElement}, but it is not yet added
   * to the parent <code>Element</code> as child.
   *
   * @param parent the parent <code>Element</code> for the new branch element
   * @param attributes the text attributes to be installed in the new element
   *
   * @return the new branch <code>Element</code>
   *
   * @see BranchElement
   */
  protected Element createBranchElement(Element parent,
					AttributeSet attributes)
  {
    return new BranchElement(parent, attributes);
  }

  /**
   * Creates and returns a leaf element with the specified
   * <code>parent</code> and <code>attributes</code>. Note that the new
   * <code>Element</code> is linked to the parent <code>Element</code>
   * through {@link Element#getParentElement}, but it is not yet added
   * to the parent <code>Element</code> as child.
   *
   * @param parent the parent <code>Element</code> for the new branch element
   * @param attributes the text attributes to be installed in the new element
   *
   * @return the new branch <code>Element</code>
   *
   * @see LeafElement
   */
  protected Element createLeafElement(Element parent, AttributeSet attributes,
				      int start, int end)
  {
    return new LeafElement(parent, attributes, start, end);
  }

  /**
   * Creates a {@link Position} that keeps track of the location at the
   * specified <code>offset</code>.
   *
   * @param offset the location in the document to keep track by the new
   *        <code>Position</code>
   *
   * @return the newly created <code>Position</code>
   *
   * @throws BadLocationException if <code>offset</code> is not a valid
   *         location in the documents content model
   */
  public Position createPosition(final int offset) throws BadLocationException
  {
    return content.createPosition(offset);
  }

  /**
   * Notifies all registered listeners when the document model changes.
   *
   * @param event the <code>DocumentEvent</code> to be fired
   */
  protected void fireChangedUpdate(DocumentEvent event)
  {
    DocumentListener[] listeners = getDocumentListeners();

    for (int index = 0; index < listeners.length; ++index)
      listeners[index].changedUpdate(event);
  }

  /**
   * Notifies all registered listeners when content is inserted in the document
   * model.
   *
   * @param event the <code>DocumentEvent</code> to be fired
   */
  protected void fireInsertUpdate(DocumentEvent event)
  {
    DocumentListener[] listeners = getDocumentListeners();

    for (int index = 0; index < listeners.length; ++index)
      listeners[index].insertUpdate(event);
  }

  /**
   * Notifies all registered listeners when content is removed from the
   * document model.
   *
   * @param event the <code>DocumentEvent</code> to be fired
   */
  protected void fireRemoveUpdate(DocumentEvent event)
  {
    DocumentListener[] listeners = getDocumentListeners();

    for (int index = 0; index < listeners.length; ++index)
      listeners[index].removeUpdate(event);
  }

  /**
   * Notifies all registered listeners when an <code>UndoableEdit</code> has
   * been performed on this <code>Document</code>.
   *
   * @param event the <code>UndoableEditEvent</code> to be fired
   */
  protected void fireUndoableEditUpdate(UndoableEditEvent event)
  {
    UndoableEditListener[] listeners = getUndoableEditListeners();

    for (int index = 0; index < listeners.length; ++index)
      listeners[index].undoableEditHappened(event);
  }

  /**
   * Returns the asynchronous loading priority. Returns <code>-1</code> if this
   * document should not be loaded asynchronously.
   *
   * @return the asynchronous loading priority
   */
  public int getAsynchronousLoadPriority()
  {
    return 0;
  }

  /**
   * Returns the {@link AttributeContext} used in this <code>Document</code>.
   *
   * @return the {@link AttributeContext} used in this <code>Document</code>
   */
  protected final AttributeContext getAttributeContext()
  {
    return context;
  }

  /**
   * Returns the root element for bidirectional content.
   *
   * @return the root element for bidirectional content
   */
  public Element getBidiRootElement()
  {
    return null;
  }

  /**
   * Returns the {@link Content} model for this <code>Document</code>
   *
   * @return the {@link Content} model for this <code>Document</code>
   *
   * @see GapContent
   * @see StringContent
   */
  protected final Content getContent()
  {
    return content;
  }

  /**
   * Returns the thread that currently modifies this <code>Document</code>
   * if there is one, otherwise <code>null</code>. This can be used to
   * distinguish between a method call that is part of an ongoing modification
   * or if it is a separate modification for which a new lock must be aquired.
   *
   * @return the thread that currently modifies this <code>Document</code>
   *         if there is one, otherwise <code>null</code>
   */
  protected final Thread getCurrentWriter()
  {
    return currentWriter;
  }

  /**
   * Returns the properties of this <code>Document</code>.
   *
   * @return the properties of this <code>Document</code>
   */
  public Dictionary getDocumentProperties()
  {
    // FIXME: make me thread-safe
    if (properties == null)
      properties = new Hashtable();

    return properties;
  }

  /**
   * Returns a {@link Position} which will always mark the end of the
   * <code>Document</code>.
   *
   * @return a {@link Position} which will always mark the end of the
   *         <code>Document</code>
   */
  public final Position getEndPosition()
  {
    // FIXME: Properly implement this by calling Content.createPosition().
    return new Position() 
      {        
        public int getOffset() 
        { 
          return getLength(); 
        } 
      };
  }

  /**
   * Returns the length of this <code>Document</code>'s content.
   *
   * @return the length of this <code>Document</code>'s content
   */
  public int getLength()
  {
    // We return Content.getLength() -1 here because there is always an
    // implicit \n at the end of the Content which does count in Content
    // but not in Document.
    return content.length() - 1;
  }

  /**
   * Returns all registered listeners of a given listener type.
   *
   * @param listenerType the type of the listeners to be queried
   *
   * @return all registered listeners of the specified type
   */
  public EventListener[] getListeners(Class listenerType)
  {
    return listenerList.getListeners(listenerType);
  }

  /**
   * Returns a property from this <code>Document</code>'s property list.
   *
   * @param key the key of the property to be fetched
   *
   * @return the property for <code>key</code> or <code>null</code> if there
   *         is no such property stored
   */
  public final Object getProperty(Object key)
  {
    // FIXME: make me thread-safe
    Object value = null;
    if (properties != null)
      value = properties.get(key);

    return value;
  }

  /**
   * Returns all root elements of this <code>Document</code>. By default
   * this just returns the single root element returned by
   * {@link #getDefaultRootElement()}. <code>Document</code> implementations
   * that support multiple roots must override this method and return all roots
   * here.
   *
   * @return all root elements of this <code>Document</code>
   */
  public Element[] getRootElements()
  {
    Element[] elements = new Element[1];
    elements[0] = getDefaultRootElement();
    return elements;
  }

  /**
   * Returns a {@link Position} which will always mark the beginning of the
   * <code>Document</code>.
   *
   * @return a {@link Position} which will always mark the beginning of the
   *         <code>Document</code>
   */
  public final Position getStartPosition()
  {
    // FIXME: Properly implement this using Content.createPosition().
    return new Position() 
      {        
        public int getOffset() 
        { 
          return 0; 
        } 
      };
  }

  /**
   * Returns a piece of this <code>Document</code>'s content.
   *
   * @param offset the start offset of the content
   * @param length the length of the content
   *
   * @return the piece of content specified by <code>offset</code> and
   *         <code>length</code>
   *
   * @throws BadLocationException if <code>offset</code> or <code>offset +
   *         length</code> are invalid locations with this
   *         <code>Document</code>
   */
  public String getText(int offset, int length) throws BadLocationException
  {
    return content.getString(offset, length);
  }

  /**
   * Fetches a piece of this <code>Document</code>'s content and stores
   * it in the given {@link Segment}.
   *
   * @param offset the start offset of the content
   * @param length the length of the content
   * @param segment the <code>Segment</code> to store the content in
   *
   * @throws BadLocationException if <code>offset</code> or <code>offset +
   *         length</code> are invalid locations with this
   *         <code>Document</code>
   */
  public void getText(int offset, int length, Segment segment)
    throws BadLocationException
  {
    content.getChars(offset, length, segment);
  }

  /**
   * Inserts a String into this <code>Document</code> at the specified
   * position and assigning the specified attributes to it.
   * 
   * <p>If a {@link DocumentFilter} is installed in this document, the
   * corresponding method of the filter object is called.</p>
   * 
   * <p>The method has no effect when <code>text</code> is <code>null</code>
   * or has a length of zero.</p>
   * 
   *
   * @param offset the location at which the string should be inserted
   * @param text the content to be inserted
   * @param attributes the text attributes to be assigned to that string
   *
   * @throws BadLocationException if <code>offset</code> is not a valid
   *         location in this <code>Document</code>
   */
  public void insertString(int offset, String text, AttributeSet attributes)
    throws BadLocationException
  {
    // Bail out if we have a bogus insertion (Behavior observed in RI).
    if (text == null || text.length() == 0)
      return;
    
    if (documentFilter == null)
      insertStringImpl(offset, text, attributes);
    else
      documentFilter.insertString(getBypass(), offset, text, attributes);
  }

  void insertStringImpl(int offset, String text, AttributeSet attributes)
    throws BadLocationException
  {
    // Just return when no text to insert was given.
    if (text == null || text.length() == 0)
      return;
    DefaultDocumentEvent event =
      new DefaultDocumentEvent(offset, text.length(),
			       DocumentEvent.EventType.INSERT);

    try
      {
        writeLock();
        UndoableEdit undo = content.insertString(offset, text);
        if (undo != null)
          event.addEdit(undo);

        insertUpdate(event, attributes);

        fireInsertUpdate(event);
        if (undo != null)
          fireUndoableEditUpdate(new UndoableEditEvent(this, undo));
      }
    finally
      {
        writeUnlock();
      }
  }

  /**
   * Called to indicate that text has been inserted into this
   * <code>Document</code>. The default implementation does nothing.
   * This method is executed within a write lock.
   *
   * @param chng the <code>DefaultDocumentEvent</code> describing the change
   * @param attr the attributes of the changed content
   */
  protected void insertUpdate(DefaultDocumentEvent chng, AttributeSet attr)
  {
    // Do nothing here. Subclasses may want to override this.
  }

  /**
   * Called after some content has been removed from this
   * <code>Document</code>. The default implementation does nothing.
   * This method is executed within a write lock.
   *
   * @param chng the <code>DefaultDocumentEvent</code> describing the change
   */
  protected void postRemoveUpdate(DefaultDocumentEvent chng)
  {
    // Do nothing here. Subclasses may want to override this.
  }

  /**
   * Stores a property in this <code>Document</code>'s property list.
   *
   * @param key the key of the property to be stored
   * @param value the value of the property to be stored
   */
  public final void putProperty(Object key, Object value)
  {
    // FIXME: make me thread-safe
    if (properties == null)
      properties = new Hashtable();

    properties.put(key, value);
  }

  /**
   * Blocks until a read lock can be obtained.  Must block if there is
   * currently a writer modifying the <code>Document</code>.
   */
  public final void readLock()
  {
    if (currentWriter != null && currentWriter.equals(Thread.currentThread()))
      return;
    synchronized (documentCV)
      {
        while (currentWriter != null || numWritersWaiting > 0)
          {
            try
              {
                documentCV.wait();
              }
            catch (InterruptedException ie)
              {
                throw new Error("interrupted trying to get a readLock");
              }
          }
          numReaders++;
      }
  }

  /**
   * Releases the read lock. If this was the only reader on this
   * <code>Document</code>, writing may begin now.
   */
  public final void readUnlock()
  {
    // Note we could have a problem here if readUnlock was called without a
    // prior call to readLock but the specs simply warn users to ensure that
    // balance by using a finally block:
    // readLock()
    // try
    // { 
    //   doSomethingHere 
    // }
    // finally
    // {
    //   readUnlock();
    // }
    
    // All that the JDK seems to check for is that you don't call unlock
    // more times than you've previously called lock, but it doesn't make
    // sure that the threads calling unlock were the same ones that called lock

    // If the current thread holds the write lock, and attempted to also obtain
    // a readLock, then numReaders hasn't been incremented and we don't need
    // to unlock it here.
    if (currentWriter == Thread.currentThread())
      return;

    // FIXME: the reference implementation throws a 
    // javax.swing.text.StateInvariantError here
    if (numReaders == 0)
      throw new IllegalStateException("document lock failure");
    
    synchronized (documentCV)
    {
      // If currentWriter is not null, the application code probably had a 
      // writeLock and then tried to obtain a readLock, in which case 
      // numReaders wasn't incremented
      if (currentWriter == null)
        {
          numReaders --;
          if (numReaders == 0 && numWritersWaiting != 0)
            documentCV.notify();
        }
    }
  }

  /**
   * Removes a piece of content from this <code>Document</code>.
   * 
   * <p>If a {@link DocumentFilter} is installed in this document, the
   * corresponding method of the filter object is called. The
   * <code>DocumentFilter</code> is called even if <code>length</code>
   * is zero. This is different from {@link #replace}.</p>
   * 
   * <p>Note: When <code>length</code> is zero or below the call is not
   * forwarded to the underlying {@link AbstractDocument.Content} instance
   * of this document and no exception is thrown.</p>
   * 
   * @param offset the start offset of the fragment to be removed
   * @param length the length of the fragment to be removed
   *
   * @throws BadLocationException if <code>offset</code> or
   *         <code>offset + length</code> or invalid locations within this
   *         document
   */
  public void remove(int offset, int length) throws BadLocationException
  {
    if (documentFilter == null)
      removeImpl(offset, length);
    else
      documentFilter.remove(getBypass(), offset, length);
  }
  
  void removeImpl(int offset, int length) throws BadLocationException
  {
    // Prevent some unneccessary method invocation (observed in the RI). 
    if (length <= 0)
      return;
    
    DefaultDocumentEvent event =
      new DefaultDocumentEvent(offset, length,
			       DocumentEvent.EventType.REMOVE);
    
    try
      {
        writeLock();
        
        // The order of the operations below is critical!        
        removeUpdate(event);
        UndoableEdit temp = content.remove(offset, length);
        
        postRemoveUpdate(event);
        fireRemoveUpdate(event);
      }
    finally
      {
        writeUnlock();
      } 
  }

  /**
   * Replaces a piece of content in this <code>Document</code> with
   * another piece of content.
   * 
   * <p>If a {@link DocumentFilter} is installed in this document, the
   * corresponding method of the filter object is called.</p>
   * 
   * <p>The method has no effect if <code>length</code> is zero (and
   * only zero) and, at the same time, <code>text</code> is
   * <code>null</code> or has zero length.</p>
   *
   * @param offset the start offset of the fragment to be removed
   * @param length the length of the fragment to be removed
   * @param text the text to replace the content with
   * @param attributes the text attributes to assign to the new content
   *
   * @throws BadLocationException if <code>offset</code> or
   *         <code>offset + length</code> or invalid locations within this
   *         document
   *
   * @since 1.4
   */
  public void replace(int offset, int length, String text,
                      AttributeSet attributes)
    throws BadLocationException
  {
    // Bail out if we have a bogus replacement (Behavior observed in RI).
    if (length == 0 
        && (text == null || text.length() == 0))
      return;
    
    if (documentFilter == null)
      {
        // It is important to call the methods which again do the checks
        // of the arguments and the DocumentFilter because subclasses may
        // have overridden these methods and provide crucial behavior
        // which would be skipped if we call the non-checking variants.
        // An example for this is PlainDocument where insertString can
        // provide a filtering of newlines.
        remove(offset, length);
        insertString(offset, text, attributes);
      }
    else
      documentFilter.replace(getBypass(), offset, length, text, attributes);
    
  }
  
  void replaceImpl(int offset, int length, String text,
		      AttributeSet attributes)
    throws BadLocationException
  {
    removeImpl(offset, length);
    insertStringImpl(offset, text, attributes);
  }

  /**
   * Adds a <code>DocumentListener</code> object to this document.
   *
   * @param listener the listener to add
   */
  public void addDocumentListener(DocumentListener listener)
  {
    listenerList.add(DocumentListener.class, listener);
  }

  /**
   * Removes a <code>DocumentListener</code> object from this document.
   *
   * @param listener the listener to remove
   */
  public void removeDocumentListener(DocumentListener listener)
  {
    listenerList.remove(DocumentListener.class, listener);
  }

  /**
   * Returns all registered <code>DocumentListener</code>s.
   *
   * @return all registered <code>DocumentListener</code>s
   */
  public DocumentListener[] getDocumentListeners()
  {
    return (DocumentListener[]) getListeners(DocumentListener.class);
  }

  /**
   * Adds an {@link UndoableEditListener} to this <code>Document</code>.
   *
   * @param listener the listener to add
   */
  public void addUndoableEditListener(UndoableEditListener listener)
  {
    listenerList.add(UndoableEditListener.class, listener);
  }

  /**
   * Removes an {@link UndoableEditListener} from this <code>Document</code>.
   *
   * @param listener the listener to remove
   */
  public void removeUndoableEditListener(UndoableEditListener listener)
  {
    listenerList.remove(UndoableEditListener.class, listener);
  }

  /**
   * Returns all registered {@link UndoableEditListener}s.
   *
   * @return all registered {@link UndoableEditListener}s
   */
  public UndoableEditListener[] getUndoableEditListeners()
  {
    return (UndoableEditListener[]) getListeners(UndoableEditListener.class);
  }

  /**
   * Called before some content gets removed from this <code>Document</code>.
   * The default implementation does nothing but may be overridden by
   * subclasses to modify the <code>Document</code> structure in response
   * to a remove request. The method is executed within a write lock.
   *
   * @param chng the <code>DefaultDocumentEvent</code> describing the change
   */
  protected void removeUpdate(DefaultDocumentEvent chng)
  {
    // Do nothing here. Subclasses may wish to override this.
  }

  /**
   * Called to render this <code>Document</code> visually. It obtains a read
   * lock, ensuring that no changes will be made to the <code>document</code>
   * during the rendering process. It then calls the {@link Runnable#run()}
   * method on <code>runnable</code>. This method <em>must not</em> attempt
   * to modifiy the <code>Document</code>, since a deadlock will occur if it
   * tries to obtain a write lock. When the {@link Runnable#run()} method
   * completes (either naturally or by throwing an exception), the read lock
   * is released. Note that there is nothing in this method related to
   * the actual rendering. It could be used to execute arbitrary code within
   * a read lock.
   *
   * @param runnable the {@link Runnable} to execute
   */
  public void render(Runnable runnable)
  {
    readLock();
    try
    {
      runnable.run();
    }
    finally
    {
      readUnlock();
    }
  }

  /**
   * Sets the asynchronous loading priority for this <code>Document</code>.
   * A value of <code>-1</code> indicates that this <code>Document</code>
   * should be loaded synchronously.
   *
   * @param p the asynchronous loading priority to set
   */
  public void setAsynchronousLoadPriority(int p)
  {
    // TODO: Implement this properly.
  }

  /**
   * Sets the properties of this <code>Document</code>.
   *
   * @param p the document properties to set
   */
  public void setDocumentProperties(Dictionary p)
  {
    // FIXME: make me thread-safe
    properties = p;
  }

  /**
   * Blocks until a write lock can be obtained.  Must wait if there are 
   * readers currently reading or another thread is currently writing.
   */
  protected final void writeLock()
  {
    if (currentWriter != null && currentWriter.equals(Thread.currentThread()))
      return;
    synchronized (documentCV)
      {
        numWritersWaiting++;
        while (numReaders > 0)
          {
            try
              {
                documentCV.wait();
              }
            catch (InterruptedException ie)
              {
                throw new Error("interruped while trying to obtain write lock");
              }
          }
        numWritersWaiting --;
        currentWriter = Thread.currentThread();
      }
  }

  /**
   * Releases the write lock. This allows waiting readers or writers to
   * obtain the lock.
   */
  protected final void writeUnlock()
  {
    synchronized (documentCV)
    {
        if (Thread.currentThread().equals(currentWriter))
          {
            currentWriter = null;
            documentCV.notifyAll();
          }
    }
  }

  /**
   * Returns the currently installed {@link DocumentFilter} for this
   * <code>Document</code>.
   *
   * @return the currently installed {@link DocumentFilter} for this
   *         <code>Document</code>
   *
   * @since 1.4
   */
  public DocumentFilter getDocumentFilter()
  {
    return documentFilter;
  }

  /**
   * Sets the {@link DocumentFilter} for this <code>Document</code>.
   *
   * @param filter the <code>DocumentFilter</code> to set
   *
   * @since 1.4
   */
  public void setDocumentFilter(DocumentFilter filter)
  {
    this.documentFilter = filter;
  }

  /**
   * Dumps diagnostic information to the specified <code>PrintStream</code>.
   *
   * @param out the stream to write the diagnostic information to
   */
  public void dump(PrintStream out)
  {
    ((AbstractElement) getDefaultRootElement()).dump(out, 0);
  }

  /**
   * Defines a set of methods for managing text attributes for one or more
   * <code>Document</code>s.
   *
   * Replicating {@link AttributeSet}s throughout a <code>Document</code> can
   * be very expensive. Implementations of this interface are intended to
   * provide intelligent management of <code>AttributeSet</code>s, eliminating
   * costly duplication.
   *
   * @see StyleContext
   */
  public interface AttributeContext
  {
    /**
     * Returns an {@link AttributeSet} that contains the attributes
     * of <code>old</code> plus the new attribute specified by
     * <code>name</code> and <code>value</code>.
     *
     * @param old the attribute set to be merged with the new attribute
     * @param name the name of the attribute to be added
     * @param value the value of the attribute to be added
     *
     * @return the old attributes plus the new attribute
     */
    AttributeSet addAttribute(AttributeSet old, Object name, Object value);

    /**
     * Returns an {@link AttributeSet} that contains the attributes
     * of <code>old</code> plus the new attributes in <code>attributes</code>.
     *
     * @param old the set of attributes where to add the new attributes
     * @param attributes the attributes to be added
     *
     * @return an {@link AttributeSet} that contains the attributes
     *         of <code>old</code> plus the new attributes in
     *         <code>attributes</code>
     */
    AttributeSet addAttributes(AttributeSet old, AttributeSet attributes);

    /**
     * Returns an empty {@link AttributeSet}.
     *
     * @return  an empty {@link AttributeSet}
     */
    AttributeSet getEmptySet();

    /**
     * Called to indicate that the attributes in <code>attributes</code> are
     * no longer used.
     *
     * @param attributes the attributes are no longer used
     */
    void reclaim(AttributeSet attributes);

    /**
     * Returns a {@link AttributeSet} that has the attribute with the specified
     * <code>name</code> removed from <code>old</code>.
     *
     * @param old the attribute set from which an attribute is removed
     * @param name the name of the attribute to be removed
     *
     * @return the attributes of <code>old</code> minus the attribute
     *         specified by <code>name</code>
     */
    AttributeSet removeAttribute(AttributeSet old, Object name);

    /**
     * Removes all attributes in <code>attributes</code> from <code>old</code>
     * and returns the resulting <code>AttributeSet</code>.
     *
     * @param old the set of attributes from which to remove attributes
     * @param attributes the attributes to be removed from <code>old</code>
     *
     * @return the attributes of <code>old</code> minus the attributes in
     *         <code>attributes</code>
     */
    AttributeSet removeAttributes(AttributeSet old, AttributeSet attributes);

    /**
     * Removes all attributes specified by <code>names</code> from
     * <code>old</code> and returns the resulting <code>AttributeSet</code>.
     *
     * @param old the set of attributes from which to remove attributes
     * @param names the names of the attributes to be removed from
     *        <code>old</code>
     *
     * @return the attributes of <code>old</code> minus the attributes in
     *         <code>attributes</code>
     */
    AttributeSet removeAttributes(AttributeSet old, Enumeration names);
  }

  /**
   * A sequence of data that can be edited. This is were the actual content
   * in <code>AbstractDocument</code>'s is stored.
   */
  public interface Content
  {
    /**
     * Creates a {@link Position} that keeps track of the location at
     * <code>offset</code>.
     *
     * @return a {@link Position} that keeps track of the location at
     *         <code>offset</code>.
     *
     * @throw BadLocationException if <code>offset</code> is not a valid
     *        location in this <code>Content</code> model
     */
    Position createPosition(int offset) throws BadLocationException;

    /**
     * Returns the length of the content.
     *
     * @return the length of the content
     */
    int length();

    /**
     * Inserts a string into the content model.
     *
     * @param where the offset at which to insert the string
     * @param str the string to be inserted
     *
     * @return an <code>UndoableEdit</code> or <code>null</code> if undo is
     *         not supported by this <code>Content</code> model
     *
     * @throws BadLocationException if <code>where</code> is not a valid
     *         location in this <code>Content</code> model
     */
    UndoableEdit insertString(int where, String str)
      throws BadLocationException;

    /**
     * Removes a piece of content from the content model.
     *
     * @param where the offset at which to remove content
     * @param nitems the number of characters to be removed
     *
     * @return an <code>UndoableEdit</code> or <code>null</code> if undo is
     *         not supported by this <code>Content</code> model
     *
     * @throws BadLocationException if <code>where</code> is not a valid
     *         location in this <code>Content</code> model
     */
    UndoableEdit remove(int where, int nitems) throws BadLocationException;

    /**
     * Returns a piece of content.
     *
     * @param where the start offset of the requested fragment
     * @param len the length of the requested fragment
     *
     * @return the requested fragment
     * @throws BadLocationException if <code>offset</code> or
     *         <code>offset + len</code>is not a valid
     *         location in this <code>Content</code> model
     */
    String getString(int where, int len) throws BadLocationException;

    /**
     * Fetches a piece of content and stores it in <code>txt</code>.
     *
     * @param where the start offset of the requested fragment
     * @param len the length of the requested fragment
     * @param txt the <code>Segment</code> where to fragment is stored into
     *
     * @throws BadLocationException if <code>offset</code> or
     *         <code>offset + len</code>is not a valid
     *         location in this <code>Content</code> model
     */
    void getChars(int where, int len, Segment txt) throws BadLocationException;
  }

  /**
   * An abstract base implementation of the {@link Element} interface.
   */
  public abstract class AbstractElement
    implements Element, MutableAttributeSet, TreeNode, Serializable
  {
    /** The serialization UID (compatible with JDK1.5). */
    private static final long serialVersionUID = 1712240033321461704L;

    /** The number of characters that this Element spans. */
    int count;

    /** The starting offset of this Element. */
    int offset;

    /** The attributes of this Element. */
    AttributeSet attributes;

    /** The parent element. */
    Element element_parent;

    /** The parent in the TreeNode interface. */
    TreeNode tree_parent;

    /** The children of this element. */
    Vector tree_children;

    /**
     * Creates a new instance of <code>AbstractElement</code> with a
     * specified parent <code>Element</code> and <code>AttributeSet</code>.
     *
     * @param p the parent of this <code>AbstractElement</code>
     * @param s the attributes to be assigned to this
     *        <code>AbstractElement</code>
     */
    public AbstractElement(Element p, AttributeSet s)
    {
      element_parent = p;
      AttributeContext ctx = getAttributeContext();
      attributes = ctx.getEmptySet();
      if (s != null)
        attributes = ctx.addAttributes(attributes, s);
    }

    /**
     * Returns the child nodes of this <code>Element</code> as an
     * <code>Enumeration</code> of {@link TreeNode}s.
     *
     * @return the child nodes of this <code>Element</code> as an
     *         <code>Enumeration</code> of {@link TreeNode}s
     */
    public abstract Enumeration children();

    /**
     * Returns <code>true</code> if this <code>AbstractElement</code>
     * allows children.
     *
     * @return <code>true</code> if this <code>AbstractElement</code>
     *         allows children
     */
    public abstract boolean getAllowsChildren();

    /**
     * Returns the child of this <code>AbstractElement</code> at
     * <code>index</code>.
     *
     * @param index the position in the child list of the child element to
     *        be returned
     *
     * @return the child of this <code>AbstractElement</code> at
     *         <code>index</code>
     */
    public TreeNode getChildAt(int index)
    {
      return (TreeNode) tree_children.get(index);
    }

    /**
     * Returns the number of children of this <code>AbstractElement</code>.
     *
     * @return the number of children of this <code>AbstractElement</code>
     */
    public int getChildCount()
    {
      return tree_children.size();
    }

    /**
     * Returns the index of a given child <code>TreeNode</code> or
     * <code>-1</code> if <code>node</code> is not a child of this
     * <code>AbstractElement</code>.
     *
     * @param node the node for which the index is requested
     *
     * @return the index of a given child <code>TreeNode</code> or
     *         <code>-1</code> if <code>node</code> is not a child of this
     *         <code>AbstractElement</code>
     */
    public int getIndex(TreeNode node)
    {
      return tree_children.indexOf(node);
    }

    /**
     * Returns the parent <code>TreeNode</code> of this
     * <code>AbstractElement</code> or <code>null</code> if this element
     * has no parent.
     *
     * @return the parent <code>TreeNode</code> of this
     *         <code>AbstractElement</code> or <code>null</code> if this
     *         element has no parent
     */
    public TreeNode getParent()
    {
      return tree_parent;
    }

    /**
     * Returns <code>true</code> if this <code>AbstractElement</code> is a
     * leaf element, <code>false</code> otherwise.
     *
     * @return <code>true</code> if this <code>AbstractElement</code> is a
     *         leaf element, <code>false</code> otherwise
     */
    public abstract boolean isLeaf();

    /**
     * Adds an attribute to this element.
     *
     * @param name the name of the attribute to be added
     * @param value the value of the attribute to be added
     */
    public void addAttribute(Object name, Object value)
    {
      attributes = getAttributeContext().addAttribute(attributes, name, value);
    }

    /**
     * Adds a set of attributes to this element.
     *
     * @param attrs the attributes to be added to this element
     */
    public void addAttributes(AttributeSet attrs)
    {
      attributes = getAttributeContext().addAttributes(attributes, attrs);
    }

    /**
     * Removes an attribute from this element.
     *
     * @param name the name of the attribute to be removed
     */
    public void removeAttribute(Object name)
    {
      attributes = getAttributeContext().removeAttribute(attributes, name);
    }

    /**
     * Removes a set of attributes from this element.
     *
     * @param attrs the attributes to be removed
     */
    public void removeAttributes(AttributeSet attrs)
    {
      attributes = getAttributeContext().removeAttributes(attributes, attrs);
    }

    /**
     * Removes a set of attribute from this element.
     *
     * @param names the names of the attributes to be removed
     */
    public void removeAttributes(Enumeration names)
    {
      attributes = getAttributeContext().removeAttributes(attributes, names);
    }

    /**
     * Sets the parent attribute set against which the element can resolve
     * attributes that are not defined in itself.
     *
     * @param parent the resolve parent to set
     */
    public void setResolveParent(AttributeSet parent)
    {
      attributes = getAttributeContext().addAttribute(attributes,
                                                      ResolveAttribute,
                                                      parent);
    }

    /**
     * Returns <code>true</code> if this element contains the specified
     * attribute.
     *
     * @param name the name of the attribute to check
     * @param value the value of the attribute to check
     *
     * @return <code>true</code> if this element contains the specified
     *         attribute
     */
    public boolean containsAttribute(Object name, Object value)
    {
      return attributes.containsAttribute(name, value);
    }

    /**
     * Returns <code>true</code> if this element contains all of the
     * specified attributes.
     *
     * @param attrs the attributes to check
     *
     * @return <code>true</code> if this element contains all of the
     *         specified attributes
     */
    public boolean containsAttributes(AttributeSet attrs)
    {
      return attributes.containsAttributes(attrs);
    }

    /**
     * Returns a copy of the attributes of this element.
     *
     * @return a copy of the attributes of this element
     */
    public AttributeSet copyAttributes()
    {
      return attributes.copyAttributes();
    }

    /**
     * Returns the attribute value with the specified key. If this attribute
     * is not defined in this element and this element has a resolving
     * parent, the search goes upward to the resolve parent chain.
     *
     * @param key the key of the requested attribute
     *
     * @return the attribute value for <code>key</code> of <code>null</code>
     *         if <code>key</code> is not found locally and cannot be resolved
     *         in this element's resolve parents
     */
    public Object getAttribute(Object key)
    {
      Object result = attributes.getAttribute(key);
      if (result == null)
        {
          AttributeSet resParent = getResolveParent();
          if (resParent != null)
            result = resParent.getAttribute(key);
        }
      return result;
    }

    /**
     * Returns the number of defined attributes in this element.
     *
     * @return the number of defined attributes in this element
     */
    public int getAttributeCount()
    {
      return attributes.getAttributeCount();
    }

    /**
     * Returns the names of the attributes of this element.
     *
     * @return the names of the attributes of this element
     */
    public Enumeration getAttributeNames()
    {
      return attributes.getAttributeNames();
    }

    /**
     * Returns the resolve parent of this element.
     * This is taken from the AttributeSet, but if this is null,
     * this method instead returns the Element's parent's 
     * AttributeSet
     *
     * @return the resolve parent of this element
     *
     * @see #setResolveParent(AttributeSet)
     */
    public AttributeSet getResolveParent()
    {
      return attributes.getResolveParent();
    }

    /**
     * Returns <code>true</code> if an attribute with the specified name
     * is defined in this element, <code>false</code> otherwise.
     *
     * @param attrName the name of the requested attributes
     *
     * @return <code>true</code> if an attribute with the specified name
     *         is defined in this element, <code>false</code> otherwise
     */
    public boolean isDefined(Object attrName)
    {
      return attributes.isDefined(attrName);
    }

    /**
     * Returns <code>true</code> if the specified <code>AttributeSet</code>
     * is equal to this element's <code>AttributeSet</code>, <code>false</code>
     * otherwise.
     *
     * @param attrs the attributes to compare this element to
     *
     * @return <code>true</code> if the specified <code>AttributeSet</code>
     *         is equal to this element's <code>AttributeSet</code>,
     *         <code>false</code> otherwise
     */
    public boolean isEqual(AttributeSet attrs) 
    {
      return attributes.isEqual(attrs);
    }

    /**
     * Returns the attributes of this element.
     *
     * @return the attributes of this element
     */
    public AttributeSet getAttributes()
    {
      return this;
    }

    /**
     * Returns the {@link Document} to which this element belongs.
     *
     * @return the {@link Document} to which this element belongs
     */
    public Document getDocument()
    {
      return AbstractDocument.this;
    }

    /**
     * Returns the child element at the specified <code>index</code>.
     *
     * @param index the index of the requested child element
     *
     * @return the requested element
     */
    public abstract Element getElement(int index);

    /**
     * Returns the name of this element.
     *
     * @return the name of this element
     */
    public String getName()
    {
      return (String) getAttribute(NameAttribute);
    }

    /**
     * Returns the parent element of this element.
     *
     * @return the parent element of this element
     */
    public Element getParentElement()
    {
      return element_parent;
    }

    /**
     * Returns the offset inside the document model that is after the last
     * character of this element.
     *
     * @return the offset inside the document model that is after the last
     *         character of this element
     */
    public abstract int getEndOffset();

    /**
     * Returns the number of child elements of this element.
     *
     * @return the number of child elements of this element
     */
    public abstract int getElementCount();

    /**
     * Returns the index of the child element that spans the specified
     * offset in the document model.
     *
     * @param offset the offset for which the responsible element is searched
     *
     * @return the index of the child element that spans the specified
     *         offset in the document model
     */
    public abstract int getElementIndex(int offset);

    /**
     * Returns the start offset if this element inside the document model.
     *
     * @return the start offset if this element inside the document model
     */
    public abstract int getStartOffset();

    /**
     * Prints diagnostic output to the specified stream.
     *
     * @param stream the stream to write to
     * @param indent the indentation level
     */
    public void dump(PrintStream stream, int indent)
    {
      StringBuffer b = new StringBuffer();
      for (int i = 0; i < indent; ++i)
        b.append(' ');
      b.append('<');
      b.append(getName());
      // Dump attributes if there are any.
      if (getAttributeCount() > 0)
        {
          b.append('\n');
          Enumeration attNames = getAttributeNames();
          while (attNames.hasMoreElements())
            {
              for (int i = 0; i < indent + 2; ++i)
                b.append(' ');
              Object attName = attNames.nextElement();
              b.append(attName);
              b.append('=');
              Object attribute = getAttribute(attName);
              b.append(attribute);
              b.append('\n');
            }
        }
      b.append(">\n");

      // Dump element content for leaf elements.
      if (isLeaf())
        {
          for (int i = 0; i < indent + 2; ++i)
            b.append(' ');
          int start = getStartOffset();
          int end = getEndOffset();
          b.append('[');
          b.append(start);
          b.append(',');
          b.append(end);
          b.append("][");
          try
            {
              b.append(getDocument().getText(start, end - start));
            }
          catch (BadLocationException ex)
            {
              AssertionError err = new AssertionError("BadLocationException "
                                                      + "must not be thrown "
                                                      + "here.");
              err.initCause(ex);
	      throw err;
            }
          b.append("]\n");
        }
      stream.print(b.toString());

      // Dump child elements if any.
      int count = getElementCount();
      for (int i = 0; i < count; ++i)
        {
          Element el = getElement(i);
          if (el instanceof AbstractElement)
            ((AbstractElement) el).dump(stream, indent + 2);
        }
    }
  }

  /**
   * An implementation of {@link Element} to represent composite
   * <code>Element</code>s that contain other <code>Element</code>s.
   */
  public class BranchElement extends AbstractElement
  {
    /** The serialization UID (compatible with JDK1.5). */
    private static final long serialVersionUID = -6037216547466333183L;

    /** The child elements of this BranchElement. */
    private Element[] children = new Element[0];

    /**
     * The cached startOffset value. This is used in the case when a
     * BranchElement (temporarily) has no child elements.
     */
    private int startOffset;

    /**
     * The cached endOffset value. This is used in the case when a
     * BranchElement (temporarily) has no child elements.
     */
    private int endOffset;

    /**
     * Creates a new <code>BranchElement</code> with the specified
     * parent and attributes.
     *
     * @param parent the parent element of this <code>BranchElement</code>
     * @param attributes the attributes to set on this
     *        <code>BranchElement</code>
     */
    public BranchElement(Element parent, AttributeSet attributes)
    {
      super(parent, attributes);
      startOffset = -1;
      endOffset = -1;
    }

    /**
     * Returns the children of this <code>BranchElement</code>.
     *
     * @return the children of this <code>BranchElement</code>
     */
    public Enumeration children()
    {
      if (children.length == 0)
        return null;

      Vector tmp = new Vector();

      for (int index = 0; index < children.length; ++index)
	tmp.add(children[index]);
      
      return tmp.elements();
    }

    /**
     * Returns <code>true</code> since <code>BranchElements</code> allow
     * child elements.
     *
     * @return <code>true</code> since <code>BranchElements</code> allow
     *         child elements
     */
    public boolean getAllowsChildren()
    {
      return true;
    }

    /**
     * Returns the child element at the specified <code>index</code>.
     *
     * @param index the index of the requested child element
     *
     * @return the requested element
     */
    public Element getElement(int index)
    {
      if (index < 0 || index >= children.length)
	return null;

      return children[index];
    }

    /**
     * Returns the number of child elements of this element.
     *
     * @return the number of child elements of this element
     */
    public int getElementCount()
    {
      return children.length;
    }

    /**
     * Returns the index of the child element that spans the specified
     * offset in the document model.
     *
     * @param offset the offset for which the responsible element is searched
     *
     * @return the index of the child element that spans the specified
     *         offset in the document model
     */
    public int getElementIndex(int offset)
    {
      // If offset is less than the start offset of our first child,
      // return 0
      if (offset < getStartOffset())
        return 0;

      // XXX: There is surely a better algorithm
      // as beginning from first element each time.
      for (int index = 0; index < children.length - 1; ++index)
        {
          Element elem = children[index];

          if ((elem.getStartOffset() <= offset)
               && (offset < elem.getEndOffset()))
            return index;
          // If the next element's start offset is greater than offset
          // then we have to return the closest Element, since no Elements
          // will contain the offset
          if (children[index + 1].getStartOffset() > offset)
            {
              if ((offset - elem.getEndOffset()) > (children[index + 1].getStartOffset() - offset))
                return index + 1;
              else
                return index;
            }
        }

      // If offset is greater than the index of the last element, return
      // the index of the last element.
      return getElementCount() - 1;
    }

    /**
     * Returns the offset inside the document model that is after the last
     * character of this element.
     * This is the end offset of the last child element. If this element
     * has no children, this method throws a <code>NullPointerException</code>.
     *
     * @return the offset inside the document model that is after the last
     *         character of this element
     *
     * @throws NullPointerException if this branch element has no children
     */
    public int getEndOffset()
    {
      if (children.length == 0)
        {
          if (endOffset == -1)
            throw new NullPointerException("BranchElement has no children.");
        }
      else
        endOffset = children[children.length - 1].getEndOffset();

      return endOffset;
    }

    /**
     * Returns the name of this element. This is {@link #ParagraphElementName}
     * in this case.
     *
     * @return the name of this element
     */
    public String getName()
    {
      return ParagraphElementName;
    }

    /**
     * Returns the start offset of this element inside the document model.
     * This is the start offset of the first child element. If this element
     * has no children, this method throws a <code>NullPointerException</code>.
     *
     * @return the start offset of this element inside the document model
     *
     * @throws NullPointerException if this branch element has no children and
     *         no startOffset value has been cached
     */
    public int getStartOffset()
    {
      if (children.length == 0)
        {
          if (startOffset == -1)
            throw new NullPointerException("BranchElement has no children.");
        }
      else
        startOffset = children[0].getStartOffset();

      return startOffset;
    }

    /**
     * Returns <code>false</code> since <code>BranchElement</code> are no
     * leafes.
     *
     * @return <code>false</code> since <code>BranchElement</code> are no
     *         leafes
     */
    public boolean isLeaf()
    {
      return false;
    }

    /**
     * Returns the <code>Element</code> at the specified <code>Document</code>
     * offset.
     *
     * @return the <code>Element</code> at the specified <code>Document</code>
     *         offset
     *
     * @see #getElementIndex(int)
     */
    public Element positionToElement(int position)
    {
      // XXX: There is surely a better algorithm
      // as beginning from first element each time.
      for (int index = 0; index < children.length; ++index)
        {
	  Element elem = children[index];

	  if ((elem.getStartOffset() <= position)
	      && (position < elem.getEndOffset()))
	    return elem;
        }

      return null;
    }

    /**
     * Replaces a set of child elements with a new set of child elemens.
     *
     * @param offset the start index of the elements to be removed
     * @param length the number of elements to be removed
     * @param elements the new elements to be inserted
     */
    public void replace(int offset, int length, Element[] elements)
    {
      Element[] target = new Element[children.length - length
				     + elements.length];
      System.arraycopy(children, 0, target, 0, offset);
      System.arraycopy(elements, 0, target, offset, elements.length);
      System.arraycopy(children, offset + length, target,
		       offset + elements.length,
		       children.length - offset - length);
      children = target;
    }

    /**
     * Returns a string representation of this element.
     *
     * @return a string representation of this element
     */
    public String toString()
    {
      return ("BranchElement(" + getName() + ") "
	      + getStartOffset() + "," + getEndOffset() + "\n");
    }
  }

  /**
   * Stores the changes when a <code>Document</code> is beeing modified.
   */
  public class DefaultDocumentEvent extends CompoundEdit
    implements DocumentEvent
  {
    /** The serialization UID (compatible with JDK1.5). */
    private static final long serialVersionUID = 5230037221564563284L;

    /** The starting offset of the change. */
    private int offset;

    /** The length of the change. */
    private int length;

    /** The type of change. */
    private DocumentEvent.EventType type;

    /**
     * Maps <code>Element</code> to their change records.
     */
    Hashtable changes;

    /**
     * Indicates if this event has been modified or not. This is used to
     * determine if this event is thrown.
     */
    boolean modified;

    /**
     * Creates a new <code>DefaultDocumentEvent</code>.
     *
     * @param offset the starting offset of the change
     * @param length the length of the change
     * @param type the type of change
     */
    public DefaultDocumentEvent(int offset, int length,
				DocumentEvent.EventType type)
    {
      this.offset = offset;
      this.length = length;
      this.type = type;
      changes = new Hashtable();
      modified = false;
    }

    /**
     * Adds an UndoableEdit to this <code>DocumentEvent</code>. If this
     * edit is an instance of {@link ElementEdit}, then this record can
     * later be fetched by calling {@link #getChange}.
     *
     * @param edit the undoable edit to add
     */
    public boolean addEdit(UndoableEdit edit)
    {
      // XXX - Fully qualify ElementChange to work around gcj bug #2499.
      if (edit instanceof DocumentEvent.ElementChange)
        {
          modified = true;
          DocumentEvent.ElementChange elEdit =
            (DocumentEvent.ElementChange) edit;
          changes.put(elEdit.getElement(), elEdit);
        }
      return super.addEdit(edit);
    }

    /**
     * Returns the document that has been modified.
     *
     * @return the document that has been modified
     */
    public Document getDocument()
    {
      return AbstractDocument.this;
    }

    /**
     * Returns the length of the modification.
     *
     * @return the length of the modification
     */
    public int getLength()
    {
      return length;
    }

    /**
     * Returns the start offset of the modification.
     *
     * @return the start offset of the modification
     */
    public int getOffset()
    {
      return offset;
    }

    /**
     * Returns the type of the modification.
     *
     * @return the type of the modification
     */
    public DocumentEvent.EventType getType()
    {
      return type;
    }

    /**
     * Returns the changes for an element.
     *
     * @param elem the element for which the changes are requested
     *
     * @return the changes for <code>elem</code> or <code>null</code> if
     *         <code>elem</code> has not been changed
     */
    public DocumentEvent.ElementChange getChange(Element elem)
    {
      // XXX - Fully qualify ElementChange to work around gcj bug #2499.
      return (DocumentEvent.ElementChange) changes.get(elem);
    }
    
    /**
     * Returns a String description of the change event.  This returns the
     * toString method of the Vector of edits.
     */
    public String toString()
    {
      return edits.toString();
    }
  }
  
  /**
   * An implementation of {@link DocumentEvent.ElementChange} to be added
   * to {@link DefaultDocumentEvent}s.
   */
  public static class ElementEdit extends AbstractUndoableEdit
    implements DocumentEvent.ElementChange
  {
    /** The serial version UID of ElementEdit. */
    private static final long serialVersionUID = -1216620962142928304L;

    /**
     * The changed element.
     */
    private Element elem;

    /**
     * The index of the change.
     */
    private int index;

    /**
     * The removed elements.
     */
    private Element[] removed;

    /**
     * The added elements.
     */
    private Element[] added;
    
    /**
     * Creates a new <code>ElementEdit</code>.
     *
     * @param elem the changed element
     * @param index the index of the change
     * @param removed the removed elements
     * @param added the added elements
     */
    public ElementEdit(Element elem, int index,
		       Element[] removed, Element[] added)
    {
      this.elem = elem;
      this.index = index;
      this.removed = removed;
      this.added = added;
    }

    /**
     * Returns the added elements.
     *
     * @return the added elements
     */
    public Element[] getChildrenAdded()
    {
      return added;
    }

    /**
     * Returns the removed elements.
     *
     * @return the removed elements
     */
    public Element[] getChildrenRemoved()
    {
      return removed;
    }

    /**
     * Returns the changed element.
     *
     * @return the changed element
     */
    public Element getElement()
    {
      return elem;
    }

    /**
     * Returns the index of the change.
     *
     * @return the index of the change
     */
    public int getIndex()
    {
      return index;
    }
  }

  /**
   * An implementation of {@link Element} that represents a leaf in the
   * document structure. This is used to actually store content.
   */
  public class LeafElement extends AbstractElement
  {
    /** The serialization UID (compatible with JDK1.5). */
    private static final long serialVersionUID = -8906306331347768017L;

    /**
     * Manages the start offset of this element.
     */
    private Position startPos;

    /**
     * Manages the end offset of this element.
     */
    private Position endPos;

    /**
     * This gets possible added to the startOffset when a startOffset
     * outside the document range is requested.
     */
    private int startDelta;

    /**
     * This gets possible added to the endOffset when a endOffset
     * outside the document range is requested.
     */
    private int endDelta;
    
    /**
     * Creates a new <code>LeafElement</code>.
     *
     * @param parent the parent of this <code>LeafElement</code>
     * @param attributes the attributes to be set
     * @param start the start index of this element inside the document model
     * @param end the end index of this element inside the document model
     */
    public LeafElement(Element parent, AttributeSet attributes, int start,
                       int end)
    {
      super(parent, attributes);
      int len = content.length();
      startDelta = 0;
      if (start > len)
        startDelta = start - len;
      endDelta = 0;
      if (end > len)
        endDelta = end - len;
      try
	    {
		  startPos = createPosition(start - startDelta);
		  endPos = createPosition(end - endDelta);
		}
	  catch (BadLocationException ex)
	    {
	      AssertionError as;
	      as = new AssertionError("BadLocationException thrown "
				      + "here. start=" + start
				      + ", end=" + end
				      + ", length=" + getLength());
	      as.initCause(ex);
	      throw as;
	    }
    }

    /**
     * Returns <code>null</code> since <code>LeafElement</code>s cannot have
     * children.
     *
     * @return <code>null</code> since <code>LeafElement</code>s cannot have
     *         children
     */
    public Enumeration children()
    {
      return null;
    }

    /**
     * Returns <code>false</code> since <code>LeafElement</code>s cannot have
     * children.
     *
     * @return <code>false</code> since <code>LeafElement</code>s cannot have
     *         children
     */
    public boolean getAllowsChildren()
    {
      return false;
    }

    /**
     * Returns <code>null</code> since <code>LeafElement</code>s cannot have
     * children.
     *
     * @return <code>null</code> since <code>LeafElement</code>s cannot have
     *         children
     */
    public Element getElement(int index)
    {
      return null;
    }

    /**
     * Returns <code>0</code> since <code>LeafElement</code>s cannot have
     * children.
     *
     * @return <code>0</code> since <code>LeafElement</code>s cannot have
     *         children
     */
    public int getElementCount()
    {
      return 0;
    }

    /**
     * Returns <code>-1</code> since <code>LeafElement</code>s cannot have
     * children.
     *
     * @return <code>-1</code> since <code>LeafElement</code>s cannot have
     *         children
     */
    public int getElementIndex(int offset)
    {
      return -1;
    }

    /**
     * Returns the end offset of this <code>Element</code> inside the
     * document.
     *
     * @return the end offset of this <code>Element</code> inside the
     *         document
     */
    public int getEndOffset()
    {
      return endPos.getOffset() + endDelta;
    }

    /**
     * Returns the name of this <code>Element</code>. This is
     * {@link #ContentElementName} in this case.
     *
     * @return the name of this <code>Element</code>
     */
    public String getName()
    {
      String name = super.getName();
      if (name == null)
        name = ContentElementName;
      return name;
    }

    /**
     * Returns the start offset of this <code>Element</code> inside the
     * document.
     *
     * @return the start offset of this <code>Element</code> inside the
     *         document
     */
    public int getStartOffset()
    {
      return startPos.getOffset() + startDelta;
    }

    /**
     * Returns <code>true</code>.
     *
     * @return <code>true</code>
     */
    public boolean isLeaf()
    {
      return true;
    }

    /**
     * Returns a string representation of this <code>Element</code>.
     *
     * @return a string representation of this <code>Element</code>
     */
    public String toString()
    {
      return ("LeafElement(" + getName() + ") "
	      + getStartOffset() + "," + getEndOffset() + "\n");
    }
  }
  
  /** A class whose methods delegate to the insert, remove and replace methods
   * of this document which do not check for an installed DocumentFilter.
   */
  class Bypass extends DocumentFilter.FilterBypass
  {

    public Document getDocument()
    {
      return AbstractDocument.this;
    }

    public void insertString(int offset, String string, AttributeSet attr)
    throws BadLocationException
    {
      AbstractDocument.this.insertStringImpl(offset, string, attr);
    }

    public void remove(int offset, int length)
    throws BadLocationException
    {
      AbstractDocument.this.removeImpl(offset, length);
    }

    public void replace(int offset, int length, String string,
                        AttributeSet attrs)
    throws BadLocationException
    {
      AbstractDocument.this.replaceImpl(offset, length, string, attrs);
    }
    
  }
  
}