/*
 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package javax.swing.text;

import java.util.*;
import java.io.*;
import java.awt.font.TextAttribute;
import java.text.Bidi;

import javax.swing.UIManager;
import javax.swing.undo.*;
import javax.swing.event.*;
import javax.swing.tree.TreeNode;

import sun.font.BidiUtils;
import sun.swing.SwingUtilities2;

/**
 * An implementation of the document interface to serve as a
 * basis for implementing various kinds of documents.  At this
 * level there is very little policy, so there is a corresponding
 * increase in difficulty of use.
 * <p>
 * This class implements a locking mechanism for the document.  It
 * allows multiple readers or one writer, and writers must wait until
 * all observers of the document have been notified of a previous
 * change before beginning another mutation to the document.  The
 * read lock is acquired and released using the <code>render</code>
 * method.  A write lock is acquired by the methods that mutate the
 * document, and are held for the duration of the method call.
 * Notification is done on the thread that produced the mutation,
 * and the thread has full read access to the document for the
 * duration of the notification, but other readers are kept out
 * until the notification has finished.  The notification is a
 * beans event notification which does not allow any further
 * mutations until all listeners have been notified.
 * <p>
 * Any models subclassed from this class and used in conjunction
 * with a text component that has a look and feel implementation
 * that is derived from BasicTextUI may be safely updated
 * asynchronously, because all access to the View hierarchy
 * is serialized by BasicTextUI if the document is of type
 * <code>AbstractDocument</code>.  The locking assumes that an
 * independent thread will access the View hierarchy only from
 * the DocumentListener methods, and that there will be only
 * one event thread active at a time.
 * <p>
 * If concurrency support is desired, there are the following
 * additional implications.  The code path for any DocumentListener
 * implementation and any UndoListener implementation must be threadsafe,
 * and not access the component lock if trying to be safe from deadlocks.
 * The <code>repaint</code> and <code>revalidate</code> methods
 * on JComponent are safe.
 * <p>
 * AbstractDocument models an implied break at the end of the document.
 * Among other things this allows you to position the caret after the last
 * character. As a result of this, <code>getLength</code> returns one less
 * than the length of the Content. If you create your own Content, be
 * sure and initialize it to have an additional character. Refer to
 * StringContent and GapContent for examples of this. Another implication
 * of this is that Elements that model the implied end character will have
 * an endOffset == (getLength() + 1). For example, in DefaultStyledDocument
 * <code>getParagraphElement(getLength()).getEndOffset() == getLength() + 1
 * </code>.
 * <p>
 * <strong>Warning:</strong>
 * Serialized objects of this class will not be compatible with
 * future Swing releases. The current serialization support is
 * appropriate for short term storage or RMI between applications running
 * the same version of Swing.  As of 1.4, support for long term storage
 * of all JavaBeans&trade;
 * has been added to the <code>java.beans</code> package.
 * Please see {@link java.beans.XMLEncoder}.
 *
 * @author  Timothy Prinzing
 */
public abstract class AbstractDocument implements Document, Serializable {

    /**
     * Constructs a new <code>AbstractDocument</code>, wrapped around some
     * specified content storage mechanism.
     *
     * @param data the content
     */
    protected AbstractDocument(Content data) {
        this(data, StyleContext.getDefaultStyleContext());
    }

    /**
     * Constructs a new <code>AbstractDocument</code>, wrapped around some
     * specified content storage mechanism.
     *
     * @param data the content
     * @param context the attribute context
     */
    protected AbstractDocument(Content data, AttributeContext context) {
        this.data = data;
        this.context = context;
        bidiRoot = new BidiRootElement();

        if (defaultI18NProperty == null) {
            // determine default setting for i18n support
            String o = java.security.AccessController.doPrivileged(
                new java.security.PrivilegedAction<String>() {
                    public String run() {
                        return System.getProperty(I18NProperty);
                    }
                }
            );
            if (o != null) {
                defaultI18NProperty = Boolean.valueOf(o);
            } else {
                defaultI18NProperty = Boolean.FALSE;
            }
        }
        putProperty( I18NProperty, defaultI18NProperty);

        //REMIND(bcb) This creates an initial bidi element to account for
        //the \n that exists by default in the content.  Doing it this way
        //seems to expose a little too much knowledge of the content given
        //to us by the sub-class.  Consider having the sub-class' constructor
        //make an initial call to insertUpdate.
        writeLock();
        try {
            Element[] p = new Element[1];
            p[0] = new BidiElement( bidiRoot, 0, 1, 0 );
            bidiRoot.replace(0,0,p);
        } finally {
            writeUnlock();
        }
    }

    /**
     * Supports managing a set of properties. Callers
     * can use the <code>documentProperties</code> dictionary
     * to annotate the document with document-wide properties.
     *
     * @return a non-<code>null</code> <code>Dictionary</code>
     * @see #setDocumentProperties
     */
    public Dictionary<Object,Object> getDocumentProperties() {
        if (documentProperties == null) {
            documentProperties = new Hashtable<Object, Object>(2);
        }
        return documentProperties;
    }

    /**
     * Replaces the document properties dictionary for this document.
     *
     * @param x the new dictionary
     * @see #getDocumentProperties
     */
    public void setDocumentProperties(Dictionary<Object,Object> x) {
        documentProperties = x;
    }

    /**
     * Notifies all listeners that have registered interest for
     * notification on this event type.  The event instance
     * is lazily created using the parameters passed into
     * the fire method.
     *
     * @param e the event
     * @see EventListenerList
     */
    protected void fireInsertUpdate(DocumentEvent e) {
        notifyingListeners = true;
        try {
            // Guaranteed to return a non-null array
            Object[] listeners = listenerList.getListenerList();
            // Process the listeners last to first, notifying
            // those that are interested in this event
            for (int i = listeners.length-2; i>=0; i-=2) {
                if (listeners[i]==DocumentListener.class) {
                    // Lazily create the event:
                    // if (e == null)
                    // e = new ListSelectionEvent(this, firstIndex, lastIndex);
                    ((DocumentListener)listeners[i+1]).insertUpdate(e);
                }
            }
        } finally {
            notifyingListeners = false;
        }
    }

    /**
     * Notifies all listeners that have registered interest for
     * notification on this event type.  The event instance
     * is lazily created using the parameters passed into
     * the fire method.
     *
     * @param e the event
     * @see EventListenerList
     */
    protected void fireChangedUpdate(DocumentEvent e) {
        notifyingListeners = true;
        try {
            // Guaranteed to return a non-null array
            Object[] listeners = listenerList.getListenerList();
            // Process the listeners last to first, notifying
            // those that are interested in this event
            for (int i = listeners.length-2; i>=0; i-=2) {
                if (listeners[i]==DocumentListener.class) {
                    // Lazily create the event:
                    // if (e == null)
                    // e = new ListSelectionEvent(this, firstIndex, lastIndex);
                    ((DocumentListener)listeners[i+1]).changedUpdate(e);
                }
            }
        } finally {
            notifyingListeners = false;
        }
    }

    /**
     * Notifies all listeners that have registered interest for
     * notification on this event type.  The event instance
     * is lazily created using the parameters passed into
     * the fire method.
     *
     * @param e the event
     * @see EventListenerList
     */
    protected void fireRemoveUpdate(DocumentEvent e) {
        notifyingListeners = true;
        try {
            // Guaranteed to return a non-null array
            Object[] listeners = listenerList.getListenerList();
            // Process the listeners last to first, notifying
            // those that are interested in this event
            for (int i = listeners.length-2; i>=0; i-=2) {
                if (listeners[i]==DocumentListener.class) {
                    // Lazily create the event:
                    // if (e == null)
                    // e = new ListSelectionEvent(this, firstIndex, lastIndex);
                    ((DocumentListener)listeners[i+1]).removeUpdate(e);
                }
            }
        } finally {
            notifyingListeners = false;
        }
    }

    /**
     * Notifies all listeners that have registered interest for
     * notification on this event type.  The event instance
     * is lazily created using the parameters passed into
     * the fire method.
     *
     * @param e the event
     * @see EventListenerList
     */
    protected void fireUndoableEditUpdate(UndoableEditEvent e) {
        // Guaranteed to return a non-null array
        Object[] listeners = listenerList.getListenerList();
        // Process the listeners last to first, notifying
        // those that are interested in this event
        for (int i = listeners.length-2; i>=0; i-=2) {
            if (listeners[i]==UndoableEditListener.class) {
                // Lazily create the event:
                // if (e == null)
                // e = new ListSelectionEvent(this, firstIndex, lastIndex);
                ((UndoableEditListener)listeners[i+1]).undoableEditHappened(e);
            }
        }
    }

    /**
     * Returns an array of all the objects currently registered
     * as <code><em>Foo</em>Listener</code>s
     * upon this document.
     * <code><em>Foo</em>Listener</code>s are registered using the
     * <code>add<em>Foo</em>Listener</code> method.
     *
     * <p>
     * You can specify the <code>listenerType</code> argument
     * with a class literal, such as
     * <code><em>Foo</em>Listener.class</code>.
     * For example, you can query a
     * document <code>d</code>
     * for its document listeners with the following code:
     *
     * <pre>DocumentListener[] mls = (DocumentListener[])(d.getListeners(DocumentListener.class));</pre>
     *
     * If no such listeners exist, this method returns an empty array.
     *
     * @param listenerType the type of listeners requested; this parameter
     *          should specify an interface that descends from
     *          <code>java.util.EventListener</code>
     * @return an array of all objects registered as
     *          <code><em>Foo</em>Listener</code>s on this component,
     *          or an empty array if no such
     *          listeners have been added
     * @exception ClassCastException if <code>listenerType</code>
     *          doesn't specify a class or interface that implements
     *          <code>java.util.EventListener</code>
     *
     * @see #getDocumentListeners
     * @see #getUndoableEditListeners
     *
     * @since 1.3
     */
    public <T extends EventListener> T[] getListeners(Class<T> listenerType) {
        return listenerList.getListeners(listenerType);
    }

    /**
     * Gets the asynchronous loading priority.  If less than zero,
     * the document should not be loaded asynchronously.
     *
     * @return the asynchronous loading priority, or <code>-1</code>
     *   if the document should not be loaded asynchronously
     */
    public int getAsynchronousLoadPriority() {
        Integer loadPriority = (Integer)
            getProperty(AbstractDocument.AsyncLoadPriority);
        if (loadPriority != null) {
            return loadPriority.intValue();
        }
        return -1;
    }

    /**
     * Sets the asynchronous loading priority.
     * @param p the new asynchronous loading priority; a value
     *   less than zero indicates that the document should not be
     *   loaded asynchronously
     */
    public void setAsynchronousLoadPriority(int p) {
        Integer loadPriority = (p >= 0) ? Integer.valueOf(p) : null;
        putProperty(AbstractDocument.AsyncLoadPriority, loadPriority);
    }

    /**
     * Sets the <code>DocumentFilter</code>. The <code>DocumentFilter</code>
     * is passed <code>insert</code> and <code>remove</code> to conditionally
     * allow inserting/deleting of the text.  A <code>null</code> value
     * indicates that no filtering will occur.
     *
     * @param filter the <code>DocumentFilter</code> used to constrain text
     * @see #getDocumentFilter
     * @since 1.4
     */
    public void setDocumentFilter(DocumentFilter filter) {
        documentFilter = filter;
    }

    /**
     * Returns the <code>DocumentFilter</code> that is responsible for
     * filtering of insertion/removal. A <code>null</code> return value
     * implies no filtering is to occur.
     *
     * @since 1.4
     * @see #setDocumentFilter
     * @return the DocumentFilter
     */
    public DocumentFilter getDocumentFilter() {
        return documentFilter;
    }

    // --- Document methods -----------------------------------------

    /**
     * This allows the model to be safely rendered in the presence
     * of currency, if the model supports being updated asynchronously.
     * The given runnable will be executed in a way that allows it
     * to safely read the model with no changes while the runnable
     * is being executed.  The runnable itself may <em>not</em>
     * make any mutations.
     * <p>
     * This is implemented to acquire a read lock for the duration
     * of the runnables execution.  There may be multiple runnables
     * executing at the same time, and all writers will be blocked
     * while there are active rendering runnables.  If the runnable
     * throws an exception, its lock will be safely released.
     * There is no protection against a runnable that never exits,
     * which will effectively leave the document locked for it's
     * lifetime.
     * <p>
     * If the given runnable attempts to make any mutations in
     * this implementation, a deadlock will occur.  There is
     * no tracking of individual rendering threads to enable
     * detecting this situation, but a subclass could incur
     * the overhead of tracking them and throwing an error.
     * <p>
     * This method is thread safe, although most Swing methods
     * are not. Please see
     * <A HREF="https://docs.oracle.com/javase/tutorial/uiswing/concurrency/index.html">Concurrency
     * in Swing</A> for more information.
     *
     * @param r the renderer to execute
     */
    public void render(Runnable r) {
        readLock();
        try {
            r.run();
        } finally {
            readUnlock();
        }
    }

    /**
     * Returns the length of the data.  This is the number of
     * characters of content that represents the users data.
     *
     * @return the length &gt;= 0
     * @see Document#getLength
     */
    public int getLength() {
        return data.length() - 1;
    }

    /**
     * Adds a document listener for notification of any changes.
     *
     * @param listener the <code>DocumentListener</code> to add
     * @see Document#addDocumentListener
     */
    public void addDocumentListener(DocumentListener listener) {
        listenerList.add(DocumentListener.class, listener);
    }

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

    /**
     * Returns an array of all the document listeners
     * registered on this document.
     *
     * @return all of this document's <code>DocumentListener</code>s
     *         or an empty array if no document listeners are
     *         currently registered
     *
     * @see #addDocumentListener
     * @see #removeDocumentListener
     * @since 1.4
     */
    public DocumentListener[] getDocumentListeners() {
        return listenerList.getListeners(DocumentListener.class);
    }

    /**
     * Adds an undo listener for notification of any changes.
     * Undo/Redo operations performed on the <code>UndoableEdit</code>
     * will cause the appropriate DocumentEvent to be fired to keep
     * the view(s) in sync with the model.
     *
     * @param listener the <code>UndoableEditListener</code> to add
     * @see Document#addUndoableEditListener
     */
    public void addUndoableEditListener(UndoableEditListener listener) {
        listenerList.add(UndoableEditListener.class, listener);
    }

    /**
     * Removes an undo listener.
     *
     * @param listener the <code>UndoableEditListener</code> to remove
     * @see Document#removeDocumentListener
     */
    public void removeUndoableEditListener(UndoableEditListener listener) {
        listenerList.remove(UndoableEditListener.class, listener);
    }

    /**
     * Returns an array of all the undoable edit listeners
     * registered on this document.
     *
     * @return all of this document's <code>UndoableEditListener</code>s
     *         or an empty array if no undoable edit listeners are
     *         currently registered
     *
     * @see #addUndoableEditListener
     * @see #removeUndoableEditListener
     *
     * @since 1.4
     */
    public UndoableEditListener[] getUndoableEditListeners() {
        return listenerList.getListeners(UndoableEditListener.class);
    }

    /**
     * A convenience method for looking up a property value. It is
     * equivalent to:
     * <pre>
     * getDocumentProperties().get(key);
     * </pre>
     *
     * @param key the non-<code>null</code> property key
     * @return the value of this property or <code>null</code>
     * @see #getDocumentProperties
     */
    public final Object getProperty(Object key) {
        return getDocumentProperties().get(key);
    }


    /**
     * A convenience method for storing up a property value.  It is
     * equivalent to:
     * <pre>
     * getDocumentProperties().put(key, value);
     * </pre>
     * If <code>value</code> is <code>null</code> this method will
     * remove the property.
     *
     * @param key the non-<code>null</code> key
     * @param value the property value
     * @see #getDocumentProperties
     */
    public final void putProperty(Object key, Object value) {
        if (value != null) {
            getDocumentProperties().put(key, value);
        } else {
            getDocumentProperties().remove(key);
        }
        if( key == TextAttribute.RUN_DIRECTION
            && Boolean.TRUE.equals(getProperty(I18NProperty)) )
        {
            //REMIND - this needs to flip on the i18n property if run dir
            //is rtl and the i18n property is not already on.
            writeLock();
            try {
                DefaultDocumentEvent e
                    = new DefaultDocumentEvent(0, getLength(),
                                               DocumentEvent.EventType.INSERT);
                updateBidi( e );
            } finally {
                writeUnlock();
            }
        }
    }

    /**
     * Removes some content from the document.
     * Removing content causes a write lock to be held while the
     * actual changes are taking place.  Observers are notified
     * of the change on the thread that called this method.
     * <p>
     * This method is thread safe, although most Swing methods
     * are not. Please see
     * <A HREF="https://docs.oracle.com/javase/tutorial/uiswing/concurrency/index.html">Concurrency
     * in Swing</A> for more information.
     *
     * @param offs the starting offset &gt;= 0
     * @param len the number of characters to remove &gt;= 0
     * @exception BadLocationException  the given remove position is not a valid
     *   position within the document
     * @see Document#remove
     */
    public void remove(int offs, int len) throws BadLocationException {
        DocumentFilter filter = getDocumentFilter();

        writeLock();
        try {
            if (filter != null) {
                filter.remove(getFilterBypass(), offs, len);
            }
            else {
                handleRemove(offs, len);
            }
        } finally {
            writeUnlock();
        }
    }

    /**
     * Performs the actual work of the remove. It is assumed the caller
     * will have obtained a <code>writeLock</code> before invoking this.
     */
    void handleRemove(int offs, int len) throws BadLocationException {
        if (len > 0) {
            if (offs < 0 || (offs + len) > getLength()) {
                throw new BadLocationException("Invalid remove",
                                               getLength() + 1);
            }
            DefaultDocumentEvent chng =
                    new DefaultDocumentEvent(offs, len, DocumentEvent.EventType.REMOVE);

            boolean isComposedTextElement;
            // Check whether the position of interest is the composed text
            isComposedTextElement = Utilities.isComposedTextElement(this, offs);

            removeUpdate(chng);
            UndoableEdit u = data.remove(offs, len);
            if (u != null) {
                chng.addEdit(u);
            }
            postRemoveUpdate(chng);
            // Mark the edit as done.
            chng.end();
            fireRemoveUpdate(chng);
            // only fire undo if Content implementation supports it
            // undo for the composed text is not supported for now
            if ((u != null) && !isComposedTextElement) {
                fireUndoableEditUpdate(new UndoableEditEvent(this, chng));
            }
        }
    }

    /**
     * Deletes the region of text from <code>offset</code> to
     * <code>offset + length</code>, and replaces it with <code>text</code>.
     * It is up to the implementation as to how this is implemented, some
     * implementations may treat this as two distinct operations: a remove
     * followed by an insert, others may treat the replace as one atomic
     * operation.
     *
     * @param offset index of child element
     * @param length length of text to delete, may be 0 indicating don't
     *               delete anything
     * @param text text to insert, <code>null</code> indicates no text to insert
     * @param attrs AttributeSet indicating attributes of inserted text,
     *              <code>null</code>
     *              is legal, and typically treated as an empty attributeset,
     *              but exact interpretation is left to the subclass
     * @exception BadLocationException the given position is not a valid
     *            position within the document
     * @since 1.4
     */
    public void replace(int offset, int length, String text,
                        AttributeSet attrs) throws BadLocationException {
        if (length == 0 && (text == null || text.length() == 0)) {
            return;
        }
        DocumentFilter filter = getDocumentFilter();

        writeLock();
        try {
            if (filter != null) {
                filter.replace(getFilterBypass(), offset, length, text,
                               attrs);
            }
            else {
                if (length > 0) {
                    remove(offset, length);
                }
                if (text != null && text.length() > 0) {
                    insertString(offset, text, attrs);
                }
            }
        } finally {
            writeUnlock();
        }
    }

    /**
     * Inserts some content into the document.
     * Inserting content causes a write lock to be held while the
     * actual changes are taking place, followed by notification
     * to the observers on the thread that grabbed the write lock.
     * <p>
     * This method is thread safe, although most Swing methods
     * are not. Please see
     * <A HREF="https://docs.oracle.com/javase/tutorial/uiswing/concurrency/index.html">Concurrency
     * in Swing</A> for more information.
     *
     * @param offs the starting offset &gt;= 0
     * @param str the string to insert; does nothing with null/empty strings
     * @param a the attributes for the inserted content
     * @exception BadLocationException  the given insert position is not a valid
     *   position within the document
     * @see Document#insertString
     */
    public void insertString(int offs, String str, AttributeSet a) throws BadLocationException {
        if ((str == null) || (str.length() == 0)) {
            return;
        }
        DocumentFilter filter = getDocumentFilter();

        writeLock();

        try {
            if (filter != null) {
                filter.insertString(getFilterBypass(), offs, str, a);
            } else {
                handleInsertString(offs, str, a);
            }
        } finally {
            writeUnlock();
        }
    }

    /**
     * Performs the actual work of inserting the text; it is assumed the
     * caller has obtained a write lock before invoking this.
     */
    private void handleInsertString(int offs, String str, AttributeSet a)
            throws BadLocationException {
        if ((str == null) || (str.length() == 0)) {
            return;
        }
        UndoableEdit u = data.insertString(offs, str);
        DefaultDocumentEvent e =
            new DefaultDocumentEvent(offs, str.length(), DocumentEvent.EventType.INSERT);
        if (u != null) {
            e.addEdit(u);
        }

        // see if complex glyph layout support is needed
        if( getProperty(I18NProperty).equals( Boolean.FALSE ) ) {
            // if a default direction of right-to-left has been specified,
            // we want complex layout even if the text is all left to right.
            Object d = getProperty(TextAttribute.RUN_DIRECTION);
            if ((d != null) && (d.equals(TextAttribute.RUN_DIRECTION_RTL))) {
                putProperty( I18NProperty, Boolean.TRUE);
            } else {
                char[] chars = str.toCharArray();
                if (SwingUtilities2.isComplexLayout(chars, 0, chars.length)) {
                    putProperty( I18NProperty, Boolean.TRUE);
                }
            }
        }

        insertUpdate(e, a);
        // Mark the edit as done.
        e.end();
        fireInsertUpdate(e);
        // only fire undo if Content implementation supports it
        // undo for the composed text is not supported for now
        if (u != null && (a == null || !a.isDefined(StyleConstants.ComposedTextAttribute))) {
            fireUndoableEditUpdate(new UndoableEditEvent(this, e));
        }
    }

    /**
     * Gets a sequence of text from the document.
     *
     * @param offset the starting offset &gt;= 0
     * @param length the number of characters to retrieve &gt;= 0
     * @return the text
     * @exception BadLocationException  the range given includes a position
     *   that is not a valid position within the document
     * @see Document#getText
     */
    public String getText(int offset, int length) throws BadLocationException {
        if (length < 0) {
            throw new BadLocationException("Length must be positive", length);
        }
        String str = data.getString(offset, length);
        return str;
    }

    /**
     * Fetches the text contained within the given portion
     * of the document.
     * <p>
     * If the partialReturn property on the txt parameter is false, the
     * data returned in the Segment will be the entire length requested and
     * may or may not be a copy depending upon how the data was stored.
     * If the partialReturn property is true, only the amount of text that
     * can be returned without creating a copy is returned.  Using partial
     * returns will give better performance for situations where large
     * parts of the document are being scanned.  The following is an example
     * of using the partial return to access the entire document:
     *
     * <pre>
     * &nbsp; int nleft = doc.getDocumentLength();
     * &nbsp; Segment text = new Segment();
     * &nbsp; int offs = 0;
     * &nbsp; text.setPartialReturn(true);
     * &nbsp; while (nleft &gt; 0) {
     * &nbsp;     doc.getText(offs, nleft, text);
     * &nbsp;     // do something with text
     * &nbsp;     nleft -= text.count;
     * &nbsp;     offs += text.count;
     * &nbsp; }
     * </pre>
     *
     * @param offset the starting offset &gt;= 0
     * @param length the number of characters to retrieve &gt;= 0
     * @param txt the Segment object to retrieve the text into
     * @exception BadLocationException  the range given includes a position
     *   that is not a valid position within the document
     */
    public void getText(int offset, int length, Segment txt) throws BadLocationException {
        if (length < 0) {
            throw new BadLocationException("Length must be positive", length);
        }
        data.getChars(offset, length, txt);
    }

    /**
     * Returns a position that will track change as the document
     * is altered.
     * <p>
     * This method is thread safe, although most Swing methods
     * are not. Please see
     * <A HREF="https://docs.oracle.com/javase/tutorial/uiswing/concurrency/index.html">Concurrency
     * in Swing</A> for more information.
     *
     * @param offs the position in the model &gt;= 0
     * @return the position
     * @exception BadLocationException  if the given position does not
     *   represent a valid location in the associated document
     * @see Document#createPosition
     */
    public synchronized Position createPosition(int offs) throws BadLocationException {
        return data.createPosition(offs);
    }

    /**
     * Returns a position that represents the start of the document.  The
     * position returned can be counted on to track change and stay
     * located at the beginning of the document.
     *
     * @return the position
     */
    public final Position getStartPosition() {
        Position p;
        try {
            p = createPosition(0);
        } catch (BadLocationException bl) {
            p = null;
        }
        return p;
    }

    /**
     * Returns a position that represents the end of the document.  The
     * position returned can be counted on to track change and stay
     * located at the end of the document.
     *
     * @return the position
     */
    public final Position getEndPosition() {
        Position p;
        try {
            p = createPosition(data.length());
        } catch (BadLocationException bl) {
            p = null;
        }
        return p;
    }

    /**
     * Gets all root elements defined.  Typically, there
     * will only be one so the default implementation
     * is to return the default root element.
     *
     * @return the root element
     */
    public Element[] getRootElements() {
        Element[] elems = new Element[2];
        elems[0] = getDefaultRootElement();
        elems[1] = getBidiRootElement();
        return elems;
    }

    /**
     * Returns the root element that views should be based upon
     * unless some other mechanism for assigning views to element
     * structures is provided.
     *
     * @return the root element
     * @see Document#getDefaultRootElement
     */
    public abstract Element getDefaultRootElement();

    // ---- local methods -----------------------------------------

    /**
     * Returns the <code>FilterBypass</code>. This will create one if one
     * does not yet exist.
     */
    private DocumentFilter.FilterBypass getFilterBypass() {
        if (filterBypass == null) {
            filterBypass = new DefaultFilterBypass();
        }
        return filterBypass;
    }

    /**
     * Returns the root element of the bidirectional structure for this
     * document.  Its children represent character runs with a given
     * Unicode bidi level.
     */
    public Element getBidiRootElement() {
        return bidiRoot;
    }

    /**
     * Returns true if the text in the range <code>p0</code> to
     * <code>p1</code> is left to right.
     */
    static boolean isLeftToRight(Document doc, int p0, int p1) {
        if (Boolean.TRUE.equals(doc.getProperty(I18NProperty))) {
            if (doc instanceof AbstractDocument) {
                AbstractDocument adoc = (AbstractDocument) doc;
                Element bidiRoot = adoc.getBidiRootElement();
                int index = bidiRoot.getElementIndex(p0);
                Element bidiElem = bidiRoot.getElement(index);
                if (bidiElem.getEndOffset() >= p1) {
                    AttributeSet bidiAttrs = bidiElem.getAttributes();
                    return ((StyleConstants.getBidiLevel(bidiAttrs) % 2) == 0);
                }
            }
        }
        return true;
    }

    /**
     * Get the paragraph element containing the given position.  Sub-classes
     * must define for themselves what exactly constitutes a paragraph.  They
     * should keep in mind however that a paragraph should at least be the
     * unit of text over which to run the Unicode bidirectional algorithm.
     *
     * @param pos the starting offset &gt;= 0
     * @return the element */
    public abstract Element getParagraphElement(int pos);


    /**
     * Fetches the context for managing attributes.  This
     * method effectively establishes the strategy used
     * for compressing AttributeSet information.
     *
     * @return the context
     */
    protected final AttributeContext getAttributeContext() {
        return context;
    }

    /**
     * Updates document structure as a result of text insertion.  This
     * will happen within a write lock.  If a subclass of
     * this class reimplements this method, it should delegate to the
     * superclass as well.
     *
     * @param chng a description of the change
     * @param attr the attributes for the change
     */
    protected void insertUpdate(DefaultDocumentEvent chng, AttributeSet attr) {
        if( getProperty(I18NProperty).equals( Boolean.TRUE ) )
            updateBidi( chng );

        // Check if a multi byte is encountered in the inserted text.
        if (chng.type == DocumentEvent.EventType.INSERT &&
                        chng.getLength() > 0 &&
                        !Boolean.TRUE.equals(getProperty(MultiByteProperty))) {
            Segment segment = SegmentCache.getSharedSegment();
            try {
                getText(chng.getOffset(), chng.getLength(), segment);
                segment.first();
                do {
                    if ((int)segment.current() > 255) {
                        putProperty(MultiByteProperty, Boolean.TRUE);
                        break;
                    }
                } while (segment.next() != Segment.DONE);
            } catch (BadLocationException ble) {
                // Should never happen
            }
            SegmentCache.releaseSharedSegment(segment);
        }
    }

    /**
     * Updates any document structure as a result of text removal.  This
     * method is called before the text is actually removed from the Content.
     * This will happen within a write lock. If a subclass
     * of this class reimplements this method, it should delegate to the
     * superclass as well.
     *
     * @param chng a description of the change
     */
    protected void removeUpdate(DefaultDocumentEvent chng) {
    }

    /**
     * Updates any document structure as a result of text removal.  This
     * method is called after the text has been removed from the Content.
     * This will happen within a write lock. If a subclass
     * of this class reimplements this method, it should delegate to the
     * superclass as well.
     *
     * @param chng a description of the change
     */
    protected void postRemoveUpdate(DefaultDocumentEvent chng) {
        if( getProperty(I18NProperty).equals( Boolean.TRUE ) )
            updateBidi( chng );
    }


    /**
     * Update the bidi element structure as a result of the given change
     * to the document.  The given change will be updated to reflect the
     * changes made to the bidi structure.
     *
     * This method assumes that every offset in the model is contained in
     * exactly one paragraph.  This method also assumes that it is called
     * after the change is made to the default element structure.
     */
    void updateBidi( DefaultDocumentEvent chng ) {

        // Calculate the range of paragraphs affected by the change.
        int firstPStart;
        int lastPEnd;
        if( chng.type == DocumentEvent.EventType.INSERT
            || chng.type == DocumentEvent.EventType.CHANGE )
        {
            int chngStart = chng.getOffset();
            int chngEnd =  chngStart + chng.getLength();
            firstPStart = getParagraphElement(chngStart).getStartOffset();
            lastPEnd = getParagraphElement(chngEnd).getEndOffset();
        } else if( chng.type == DocumentEvent.EventType.REMOVE ) {
            Element paragraph = getParagraphElement( chng.getOffset() );
            firstPStart = paragraph.getStartOffset();
            lastPEnd = paragraph.getEndOffset();
        } else {
            throw new Error("Internal error: unknown event type.");
        }
        //System.out.println("updateBidi: firstPStart = " + firstPStart + " lastPEnd = " + lastPEnd );


        // Calculate the bidi levels for the affected range of paragraphs.  The
        // levels array will contain a bidi level for each character in the
        // affected text.
        byte levels[] = calculateBidiLevels( firstPStart, lastPEnd );


        Vector<Element> newElements = new Vector<Element>();

        // Calculate the first span of characters in the affected range with
        // the same bidi level.  If this level is the same as the level of the
        // previous bidi element (the existing bidi element containing
        // firstPStart-1), then merge in the previous element.  If not, but
        // the previous element overlaps the affected range, truncate the
        // previous element at firstPStart.
        int firstSpanStart = firstPStart;
        int removeFromIndex = 0;
        if( firstSpanStart > 0 ) {
            int prevElemIndex = bidiRoot.getElementIndex(firstPStart-1);
            removeFromIndex = prevElemIndex;
            Element prevElem = bidiRoot.getElement(prevElemIndex);
            int prevLevel=StyleConstants.getBidiLevel(prevElem.getAttributes());
            //System.out.println("createbidiElements: prevElem= " + prevElem  + " prevLevel= " + prevLevel + "level[0] = " + levels[0]);
            if( prevLevel==levels[0] ) {
                firstSpanStart = prevElem.getStartOffset();
            } else if( prevElem.getEndOffset() > firstPStart ) {
                newElements.addElement(new BidiElement(bidiRoot,
                                                       prevElem.getStartOffset(),
                                                       firstPStart, prevLevel));
            } else {
                removeFromIndex++;
            }
        }

        int firstSpanEnd = 0;
        while((firstSpanEnd<levels.length) && (levels[firstSpanEnd]==levels[0]))
            firstSpanEnd++;


        // Calculate the last span of characters in the affected range with
        // the same bidi level.  If this level is the same as the level of the
        // next bidi element (the existing bidi element containing lastPEnd),
        // then merge in the next element.  If not, but the next element
        // overlaps the affected range, adjust the next element to start at
        // lastPEnd.
        int lastSpanEnd = lastPEnd;
        Element newNextElem = null;
        int removeToIndex = bidiRoot.getElementCount() - 1;
        if( lastSpanEnd <= getLength() ) {
            int nextElemIndex = bidiRoot.getElementIndex( lastPEnd );
            removeToIndex = nextElemIndex;
            Element nextElem = bidiRoot.getElement( nextElemIndex );
            int nextLevel = StyleConstants.getBidiLevel(nextElem.getAttributes());
            if( nextLevel == levels[levels.length-1] ) {
                lastSpanEnd = nextElem.getEndOffset();
            } else if( nextElem.getStartOffset() < lastPEnd ) {
                newNextElem = new BidiElement(bidiRoot, lastPEnd,
                                              nextElem.getEndOffset(),
                                              nextLevel);
            } else {
                removeToIndex--;
            }
        }

        int lastSpanStart = levels.length;
        while( (lastSpanStart>firstSpanEnd)
               && (levels[lastSpanStart-1]==levels[levels.length-1]) )
            lastSpanStart--;


        // If the first and last spans are contiguous and have the same level,
        // merge them and create a single new element for the entire span.
        // Otherwise, create elements for the first and last spans as well as
        // any spans in between.
        if((firstSpanEnd==lastSpanStart)&&(levels[0]==levels[levels.length-1])){
            newElements.addElement(new BidiElement(bidiRoot, firstSpanStart,
                                                   lastSpanEnd, levels[0]));
        } else {
            // Create an element for the first span.
            newElements.addElement(new BidiElement(bidiRoot, firstSpanStart,
                                                   firstSpanEnd+firstPStart,
                                                   levels[0]));
            // Create elements for the spans in between the first and last
            for( int i=firstSpanEnd; i<lastSpanStart; ) {
                //System.out.println("executed line 872");
                int j;
                for( j=i;  (j<levels.length) && (levels[j] == levels[i]); j++ );
                newElements.addElement(new BidiElement(bidiRoot, firstPStart+i,
                                                       firstPStart+j,
                                                       (int)levels[i]));
                i=j;
            }
            // Create an element for the last span.
            newElements.addElement(new BidiElement(bidiRoot,
                                                   lastSpanStart+firstPStart,
                                                   lastSpanEnd,
                                                   levels[levels.length-1]));
        }

        if( newNextElem != null )
            newElements.addElement( newNextElem );


        // Calculate the set of existing bidi elements which must be
        // removed.
        int removedElemCount = 0;
        if( bidiRoot.getElementCount() > 0 ) {
            removedElemCount = removeToIndex - removeFromIndex + 1;
        }
        Element[] removedElems = new Element[removedElemCount];
        for( int i=0; i<removedElemCount; i++ ) {
            removedElems[i] = bidiRoot.getElement(removeFromIndex+i);
        }

        Element[] addedElems = new Element[ newElements.size() ];
        newElements.copyInto( addedElems );

        // Update the change record.
        ElementEdit ee = new ElementEdit( bidiRoot, removeFromIndex,
                                          removedElems, addedElems );
        chng.addEdit( ee );

        // Update the bidi element structure.
        bidiRoot.replace( removeFromIndex, removedElems.length, addedElems );
    }


    /**
     * Calculate the levels array for a range of paragraphs.
     */
    private byte[] calculateBidiLevels( int firstPStart, int lastPEnd ) {

        byte levels[] = new byte[ lastPEnd - firstPStart ];
        int  levelsEnd = 0;
        Boolean defaultDirection = null;
        Object d = getProperty(TextAttribute.RUN_DIRECTION);
        if (d instanceof Boolean) {
            defaultDirection = (Boolean) d;
        }

        // For each paragraph in the given range of paragraphs, get its
        // levels array and add it to the levels array for the entire span.
        for(int o=firstPStart; o<lastPEnd; ) {
            Element p = getParagraphElement( o );
            int pStart = p.getStartOffset();
            int pEnd = p.getEndOffset();

            // default run direction for the paragraph.  This will be
            // null if there is no direction override specified (i.e.
            // the direction will be determined from the content).
            Boolean direction = defaultDirection;
            d = p.getAttributes().getAttribute(TextAttribute.RUN_DIRECTION);
            if (d instanceof Boolean) {
                direction = (Boolean) d;
            }

            //System.out.println("updateBidi: paragraph start = " + pStart + " paragraph end = " + pEnd);

            // Create a Bidi over this paragraph then get the level
            // array.
            Segment seg = SegmentCache.getSharedSegment();
            try {
                getText(pStart, pEnd-pStart, seg);
            } catch (BadLocationException e ) {
                throw new Error("Internal error: " + e.toString());
            }
            // REMIND(bcb) we should really be using a Segment here.
            Bidi bidiAnalyzer;
            int bidiflag = Bidi.DIRECTION_DEFAULT_LEFT_TO_RIGHT;
            if (direction != null) {
                if (TextAttribute.RUN_DIRECTION_LTR.equals(direction)) {
                    bidiflag = Bidi.DIRECTION_LEFT_TO_RIGHT;
                } else {
                    bidiflag = Bidi.DIRECTION_RIGHT_TO_LEFT;
                }
            }
            bidiAnalyzer = new Bidi(seg.array, seg.offset, null, 0, seg.count,
                    bidiflag);
            BidiUtils.getLevels(bidiAnalyzer, levels, levelsEnd);
            levelsEnd += bidiAnalyzer.getLength();

            o =  p.getEndOffset();
            SegmentCache.releaseSharedSegment(seg);
        }

        // REMIND(bcb) remove this code when debugging is done.
        if( levelsEnd != levels.length )
            throw new Error("levelsEnd assertion failed.");

        return levels;
    }

    /**
     * Gives a diagnostic dump.
     *
     * @param out the output stream
     */
    public void dump(PrintStream out) {
        Element root = getDefaultRootElement();
        if (root instanceof AbstractElement) {
            ((AbstractElement)root).dump(out, 0);
        }
        bidiRoot.dump(out,0);
    }

    /**
     * Gets the content for the document.
     *
     * @return the content
     */
    protected final Content getContent() {
        return data;
    }

    /**
     * Creates a document leaf element.
     * Hook through which elements are created to represent the
     * document structure.  Because this implementation keeps
     * structure and content separate, elements grow automatically
     * when content is extended so splits of existing elements
     * follow.  The document itself gets to decide how to generate
     * elements to give flexibility in the type of elements used.
     *
     * @param parent the parent element
     * @param a the attributes for the element
     * @param p0 the beginning of the range &gt;= 0
     * @param p1 the end of the range &gt;= p0
     * @return the new element
     */
    protected Element createLeafElement(Element parent, AttributeSet a, int p0, int p1) {
        return new LeafElement(parent, a, p0, p1);
    }

    /**
     * Creates a document branch element, that can contain other elements.
     *
     * @param parent the parent element
     * @param a the attributes
     * @return the element
     */
    protected Element createBranchElement(Element parent, AttributeSet a) {
        return new BranchElement(parent, a);
    }

    // --- Document locking ----------------------------------

    /**
     * Fetches the current writing thread if there is one.
     * This can be used to distinguish whether a method is
     * being called as part of an existing modification or
     * if a lock needs to be acquired and a new transaction
     * started.
     *
     * @return the thread actively modifying the document
     *  or <code>null</code> if there are no modifications in progress
     */
    protected synchronized final Thread getCurrentWriter() {
        return currWriter;
    }

    /**
     * Acquires a lock to begin mutating the document this lock
     * protects.  There can be no writing, notification of changes, or
     * reading going on in order to gain the lock.  Additionally a thread is
     * allowed to gain more than one <code>writeLock</code>,
     * as long as it doesn't attempt to gain additional <code>writeLock</code>s
     * from within document notification.  Attempting to gain a
     * <code>writeLock</code> from within a DocumentListener notification will
     * result in an <code>IllegalStateException</code>.  The ability
     * to obtain more than one <code>writeLock</code> per thread allows
     * subclasses to gain a writeLock, perform a number of operations, then
     * release the lock.
     * <p>
     * Calls to <code>writeLock</code>
     * must be balanced with calls to <code>writeUnlock</code>, else the
     * <code>Document</code> will be left in a locked state so that no
     * reading or writing can be done.
     *
     * @exception IllegalStateException thrown on illegal lock
     *  attempt.  If the document is implemented properly, this can
     *  only happen if a document listener attempts to mutate the
     *  document.  This situation violates the bean event model
     *  where order of delivery is not guaranteed and all listeners
     *  should be notified before further mutations are allowed.
     */
    protected synchronized final void writeLock() {
        try {
            while ((numReaders > 0) || (currWriter != null)) {
                if (Thread.currentThread() == currWriter) {
                    if (notifyingListeners) {
                        // Assuming one doesn't do something wrong in a
                        // subclass this should only happen if a
                        // DocumentListener tries to mutate the document.
                        throw new IllegalStateException(
                                      "Attempt to mutate in notification");
                    }
                    numWriters++;
                    return;
                }
                wait();
            }
            currWriter = Thread.currentThread();
            numWriters = 1;
        } catch (InterruptedException e) {
            throw new Error("Interrupted attempt to acquire write lock");
        }
    }

    /**
     * Releases a write lock previously obtained via <code>writeLock</code>.
     * After decrementing the lock count if there are no outstanding locks
     * this will allow a new writer, or readers.
     *
     * @see #writeLock
     */
    protected synchronized final void writeUnlock() {
        if (--numWriters <= 0) {
            numWriters = 0;
            currWriter = null;
            notifyAll();
        }
    }

    /**
     * Acquires a lock to begin reading some state from the
     * document.  There can be multiple readers at the same time.
     * Writing blocks the readers until notification of the change
     * to the listeners has been completed.  This method should
     * be used very carefully to avoid unintended compromise
     * of the document.  It should always be balanced with a
     * <code>readUnlock</code>.
     *
     * @see #readUnlock
     */
    public synchronized final void readLock() {
        try {
            while (currWriter != null) {
                if (currWriter == Thread.currentThread()) {
                    // writer has full read access.... may try to acquire
                    // lock in notification
                    return;
                }
                wait();
            }
            numReaders += 1;
        } catch (InterruptedException e) {
            throw new Error("Interrupted attempt to acquire read lock");
        }
    }

    /**
     * Does a read unlock.  This signals that one
     * of the readers is done.  If there are no more readers
     * then writing can begin again.  This should be balanced
     * with a readLock, and should occur in a finally statement
     * so that the balance is guaranteed.  The following is an
     * example.
     * <pre><code>
     * &nbsp;   readLock();
     * &nbsp;   try {
     * &nbsp;       // do something
     * &nbsp;   } finally {
     * &nbsp;       readUnlock();
     * &nbsp;   }
     * </code></pre>
     *
     * @see #readLock
     */
    public synchronized final void readUnlock() {
        if (currWriter == Thread.currentThread()) {
            // writer has full read access.... may try to acquire
            // lock in notification
            return;
        }
        if (numReaders <= 0) {
            throw new StateInvariantError(BAD_LOCK_STATE);
        }
        numReaders -= 1;
        notify();
    }

    // --- serialization ---------------------------------------------

    private void readObject(ObjectInputStream s)
      throws ClassNotFoundException, IOException
    {
        s.defaultReadObject();
        listenerList = new EventListenerList();

        // Restore bidi structure
        //REMIND(bcb) This creates an initial bidi element to account for
        //the \n that exists by default in the content.
        bidiRoot = new BidiRootElement();
        try {
            writeLock();
            Element[] p = new Element[1];
            p[0] = new BidiElement( bidiRoot, 0, 1, 0 );
            bidiRoot.replace(0,0,p);
        } finally {
            writeUnlock();
        }
        // At this point bidi root is only partially correct. To fully
        // restore it we need access to getDefaultRootElement. But, this
        // is created by the subclass and at this point will be null. We
        // thus use registerValidation.
        s.registerValidation(new ObjectInputValidation() {
            public void validateObject() {
                try {
                    writeLock();
                    DefaultDocumentEvent e = new DefaultDocumentEvent
                                   (0, getLength(),
                                    DocumentEvent.EventType.INSERT);
                    updateBidi( e );
                }
                finally {
                    writeUnlock();
                }
            }
        }, 0);
    }

    // ----- member variables ------------------------------------------

    private transient int numReaders;
    private transient Thread currWriter;
    /**
     * The number of writers, all obtained from <code>currWriter</code>.
     */
    private transient int numWriters;
    /**
     * True will notifying listeners.
     */
    private transient boolean notifyingListeners;

    private static Boolean defaultI18NProperty;

    /**
     * Storage for document-wide properties.
     */
    private Dictionary<Object,Object> documentProperties = null;

    /**
     * The event listener list for the document.
     */
    protected EventListenerList listenerList = new EventListenerList();

    /**
     * Where the text is actually stored, and a set of marks
     * that track change as the document is edited are managed.
     */
    private Content data;

    /**
     * Factory for the attributes.  This is the strategy for
     * attribute compression and control of the lifetime of
     * a set of attributes as a collection.  This may be shared
     * with other documents.
     */
    private AttributeContext context;

    /**
     * The root of the bidirectional structure for this document.  Its children
     * represent character runs with the same Unicode bidi level.
     */
    private transient BranchElement bidiRoot;

    /**
     * Filter for inserting/removing of text.
     */
    private DocumentFilter documentFilter;

    /**
     * Used by DocumentFilter to do actual insert/remove.
     */
    private transient DocumentFilter.FilterBypass filterBypass;

    private static final String BAD_LOCK_STATE = "document lock failure";

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

    /**
     * Name of elements used to represent paragraphs
     */
    public static final String ParagraphElementName = "paragraph";

    /**
     * Name of elements used to represent content
     */
    public static final String ContentElementName = "content";

    /**
     * Name of elements used to hold sections (lines/paragraphs).
     */
    public static final String SectionElementName = "section";

    /**
     * Name of elements used to hold a unidirectional run
     */
    public static final String BidiElementName = "bidi level";

    /**
     * Name of the attribute used to specify element
     * names.
     */
    public static final String ElementNameAttribute = "$ename";

    /**
     * Document property that indicates whether internationalization
     * functions such as text reordering or reshaping should be
     * performed. This property should not be publicly exposed,
     * since it is used for implementation convenience only.  As a
     * side effect, copies of this property may be in its subclasses
     * that live in different packages (e.g. HTMLDocument as of now),
     * so those copies should also be taken care of when this property
     * needs to be modified.
     */
    static final String I18NProperty = "i18n";

    /**
     * Document property that indicates if a character has been inserted
     * into the document that is more than one byte long.  GlyphView uses
     * this to determine if it should use BreakIterator.
     */
    static final Object MultiByteProperty = "multiByte";

    /**
     * Document property that indicates asynchronous loading is
     * desired, with the thread priority given as the value.
     */
    static final String AsyncLoadPriority = "load priority";

    /**
     * Interface to describe a sequence of character content that
     * can be edited.  Implementations may or may not support a
     * history mechanism which will be reflected by whether or not
     * mutations return an UndoableEdit implementation.
     * @see AbstractDocument
     */
    public interface Content {

        /**
         * Creates a position within the content that will
         * track change as the content is mutated.
         *
         * @param offset the offset in the content &gt;= 0
         * @return a Position
         * @exception BadLocationException for an invalid offset
         */
        public Position createPosition(int offset) throws BadLocationException;

        /**
         * Current length of the sequence of character content.
         *
         * @return the length &gt;= 0
         */
        public int length();

        /**
         * Inserts a string of characters into the sequence.
         *
         * @param where   offset into the sequence to make the insertion &gt;= 0
         * @param str     string to insert
         * @return  if the implementation supports a history mechanism,
         *    a reference to an <code>Edit</code> implementation will be returned,
         *    otherwise returns <code>null</code>
         * @exception BadLocationException  thrown if the area covered by
         *   the arguments is not contained in the character sequence
         */
        public UndoableEdit insertString(int where, String str) throws BadLocationException;

        /**
         * Removes some portion of the sequence.
         *
         * @param where   The offset into the sequence to make the
         *   insertion &gt;= 0.
         * @param nitems  The number of items in the sequence to remove &gt;= 0.
         * @return  If the implementation supports a history mechanism,
         *    a reference to an Edit implementation will be returned,
         *    otherwise null.
         * @exception BadLocationException  Thrown if the area covered by
         *   the arguments is not contained in the character sequence.
         */
        public UndoableEdit remove(int where, int nitems) throws BadLocationException;

        /**
         * Fetches a string of characters contained in the sequence.
         *
         * @param where   Offset into the sequence to fetch &gt;= 0.
         * @param len     number of characters to copy &gt;= 0.
         * @return the string
         * @exception BadLocationException  Thrown if the area covered by
         *   the arguments is not contained in the character sequence.
         */
        public String getString(int where, int len) throws BadLocationException;

        /**
         * Gets a sequence of characters and copies them into a Segment.
         *
         * @param where the starting offset &gt;= 0
         * @param len the number of characters &gt;= 0
         * @param txt the target location to copy into
         * @exception BadLocationException  Thrown if the area covered by
         *   the arguments is not contained in the character sequence.
         */
        public void getChars(int where, int len, Segment txt) throws BadLocationException;
    }

    /**
     * An interface that can be used to allow MutableAttributeSet
     * implementations to use pluggable attribute compression
     * techniques.  Each mutation of the attribute set can be
     * used to exchange a previous AttributeSet instance with
     * another, preserving the possibility of the AttributeSet
     * remaining immutable.  An implementation is provided by
     * the StyleContext class.
     *
     * The Element implementations provided by this class use
     * this interface to provide their MutableAttributeSet
     * implementations, so that different AttributeSet compression
     * techniques can be employed.  The method
     * <code>getAttributeContext</code> should be implemented to
     * return the object responsible for implementing the desired
     * compression technique.
     *
     * @see StyleContext
     */
    public interface AttributeContext {

        /**
         * Adds an attribute to the given set, and returns
         * the new representative set.
         *
         * @param old the old attribute set
         * @param name the non-null attribute name
         * @param value the attribute value
         * @return the updated attribute set
         * @see MutableAttributeSet#addAttribute
         */
        public AttributeSet addAttribute(AttributeSet old, Object name, Object value);

        /**
         * Adds a set of attributes to the element.
         *
         * @param old the old attribute set
         * @param attr the attributes to add
         * @return the updated attribute set
         * @see MutableAttributeSet#addAttribute
         */
        public AttributeSet addAttributes(AttributeSet old, AttributeSet attr);

        /**
         * Removes an attribute from the set.
         *
         * @param old the old attribute set
         * @param name the non-null attribute name
         * @return the updated attribute set
         * @see MutableAttributeSet#removeAttribute
         */
        public AttributeSet removeAttribute(AttributeSet old, Object name);

        /**
         * Removes a set of attributes for the element.
         *
         * @param old the old attribute set
         * @param names the attribute names
         * @return the updated attribute set
         * @see MutableAttributeSet#removeAttributes
         */
        public AttributeSet removeAttributes(AttributeSet old, Enumeration<?> names);

        /**
         * Removes a set of attributes for the element.
         *
         * @param old the old attribute set
         * @param attrs the attributes
         * @return the updated attribute set
         * @see MutableAttributeSet#removeAttributes
         */
        public AttributeSet removeAttributes(AttributeSet old, AttributeSet attrs);

        /**
         * Fetches an empty AttributeSet.
         *
         * @return the attribute set
         */
        public AttributeSet getEmptySet();

        /**
         * Reclaims an attribute set.
         * This is a way for a MutableAttributeSet to mark that it no
         * longer need a particular immutable set.  This is only necessary
         * in 1.1 where there are no weak references.  A 1.1 implementation
         * would call this in its finalize method.
         *
         * @param a the attribute set to reclaim
         */
        public void reclaim(AttributeSet a);
    }

    /**
     * Implements the abstract part of an element.  By default elements
     * support attributes by having a field that represents the immutable
     * part of the current attribute set for the element.  The element itself
     * implements MutableAttributeSet which can be used to modify the set
     * by fetching a new immutable set.  The immutable sets are provided
     * by the AttributeContext associated with the document.
     * <p>
     * <strong>Warning:</strong>
     * Serialized objects of this class will not be compatible with
     * future Swing releases. The current serialization support is
     * appropriate for short term storage or RMI between applications running
     * the same version of Swing.  As of 1.4, support for long term storage
     * of all JavaBeans&trade;
     * has been added to the <code>java.beans</code> package.
     * Please see {@link java.beans.XMLEncoder}.
     */
    public abstract class AbstractElement implements Element, MutableAttributeSet, Serializable, TreeNode {

        /**
         * Creates a new AbstractElement.
         *
         * @param parent the parent element
         * @param a the attributes for the element
         * @since 1.4
         */
        public AbstractElement(Element parent, AttributeSet a) {
            this.parent = parent;
            attributes = getAttributeContext().getEmptySet();
            if (a != null) {
                addAttributes(a);
            }
        }

        private final void indent(PrintWriter out, int n) {
            for (int i = 0; i < n; i++) {
                out.print("  ");
            }
        }

        /**
         * Dumps a debugging representation of the element hierarchy.
         *
         * @param psOut the output stream
         * @param indentAmount the indentation level &gt;= 0
         */
        public void dump(PrintStream psOut, int indentAmount) {
            PrintWriter out;
            try {
                out = new PrintWriter(new OutputStreamWriter(psOut,"JavaEsc"),
                                      true);
            } catch (UnsupportedEncodingException e){
                out = new PrintWriter(psOut,true);
            }
            indent(out, indentAmount);
            if (getName() == null) {
                out.print("<??");
            } else {
                out.print("<" + getName());
            }
            if (getAttributeCount() > 0) {
                out.println("");
                // dump the attributes
                Enumeration names = attributes.getAttributeNames();
                while (names.hasMoreElements()) {
                    Object name = names.nextElement();
                    indent(out, indentAmount + 1);
                    out.println(name + "=" + getAttribute(name));
                }
                indent(out, indentAmount);
            }
            out.println(">");

            if (isLeaf()) {
                indent(out, indentAmount+1);
                out.print("[" + getStartOffset() + "," + getEndOffset() + "]");
                Content c = getContent();
                try {
                    String contentStr = c.getString(getStartOffset(),
                                                    getEndOffset() - getStartOffset())/*.trim()*/;
                    if (contentStr.length() > 40) {
                        contentStr = contentStr.substring(0, 40) + "...";
                    }
                    out.println("["+contentStr+"]");
                } catch (BadLocationException e) {
                }

            } else {
                int n = getElementCount();
                for (int i = 0; i < n; i++) {
                    AbstractElement e = (AbstractElement) getElement(i);
                    e.dump(psOut, indentAmount+1);
                }
            }
        }

        // --- AttributeSet ----------------------------
        // delegated to the immutable field "attributes"

        /**
         * Gets the number of attributes that are defined.
         *
         * @return the number of attributes &gt;= 0
         * @see AttributeSet#getAttributeCount
         */
        public int getAttributeCount() {
            return attributes.getAttributeCount();
        }

        /**
         * Checks whether a given attribute is defined.
         *
         * @param attrName the non-null attribute name
         * @return true if the attribute is defined
         * @see AttributeSet#isDefined
         */
        public boolean isDefined(Object attrName) {
            return attributes.isDefined(attrName);
        }

        /**
         * Checks whether two attribute sets are equal.
         *
         * @param attr the attribute set to check against
         * @return true if the same
         * @see AttributeSet#isEqual
         */
        public boolean isEqual(AttributeSet attr) {
            return attributes.isEqual(attr);
        }

        /**
         * Copies a set of attributes.
         *
         * @return the copy
         * @see AttributeSet#copyAttributes
         */
        public AttributeSet copyAttributes() {
            return attributes.copyAttributes();
        }

        /**
         * Gets the value of an attribute.
         *
         * @param attrName the non-null attribute name
         * @return the attribute value
         * @see AttributeSet#getAttribute
         */
        public Object getAttribute(Object attrName) {
            Object value = attributes.getAttribute(attrName);
            if (value == null) {
                // The delegate nor it's resolvers had a match,
                // so we'll try to resolve through the parent
                // element.
                AttributeSet a = (parent != null) ? parent.getAttributes() : null;
                if (a != null) {
                    value = a.getAttribute(attrName);
                }
            }
            return value;
        }

        /**
         * Gets the names of all attributes.
         *
         * @return the attribute names as an enumeration
         * @see AttributeSet#getAttributeNames
         */
        public Enumeration<?> getAttributeNames() {
            return attributes.getAttributeNames();
        }

        /**
         * Checks whether a given attribute name/value is defined.
         *
         * @param name the non-null attribute name
         * @param value the attribute value
         * @return true if the name/value is defined
         * @see AttributeSet#containsAttribute
         */
        public boolean containsAttribute(Object name, Object value) {
            return attributes.containsAttribute(name, value);
        }


        /**
         * Checks whether the element contains all the attributes.
         *
         * @param attrs the attributes to check
         * @return true if the element contains all the attributes
         * @see AttributeSet#containsAttributes
         */
        public boolean containsAttributes(AttributeSet attrs) {
            return attributes.containsAttributes(attrs);
        }

        /**
         * Gets the resolving parent.
         * If not overridden, the resolving parent defaults to
         * the parent element.
         *
         * @return the attributes from the parent, <code>null</code> if none
         * @see AttributeSet#getResolveParent
         */
        public AttributeSet getResolveParent() {
            AttributeSet a = attributes.getResolveParent();
            if ((a == null) && (parent != null)) {
                a = parent.getAttributes();
            }
            return a;
        }

        // --- MutableAttributeSet ----------------------------------
        // should fetch a new immutable record for the field
        // "attributes".

        /**
         * Adds an attribute to the element.
         *
         * @param name the non-null attribute name
         * @param value the attribute value
         * @see MutableAttributeSet#addAttribute
         */
        public void addAttribute(Object name, Object value) {
            checkForIllegalCast();
            AttributeContext context = getAttributeContext();
            attributes = context.addAttribute(attributes, name, value);
        }

        /**
         * Adds a set of attributes to the element.
         *
         * @param attr the attributes to add
         * @see MutableAttributeSet#addAttribute
         */
        public void addAttributes(AttributeSet attr) {
            checkForIllegalCast();
            AttributeContext context = getAttributeContext();
            attributes = context.addAttributes(attributes, attr);
        }

        /**
         * Removes an attribute from the set.
         *
         * @param name the non-null attribute name
         * @see MutableAttributeSet#removeAttribute
         */
        public void removeAttribute(Object name) {
            checkForIllegalCast();
            AttributeContext context = getAttributeContext();
            attributes = context.removeAttribute(attributes, name);
        }

        /**
         * Removes a set of attributes for the element.
         *
         * @param names the attribute names
         * @see MutableAttributeSet#removeAttributes
         */
        public void removeAttributes(Enumeration<?> names) {
            checkForIllegalCast();
            AttributeContext context = getAttributeContext();
            attributes = context.removeAttributes(attributes, names);
        }

        /**
         * Removes a set of attributes for the element.
         *
         * @param attrs the attributes
         * @see MutableAttributeSet#removeAttributes
         */
        public void removeAttributes(AttributeSet attrs) {
            checkForIllegalCast();
            AttributeContext context = getAttributeContext();
            if (attrs == this) {
                attributes = context.getEmptySet();
            } else {
                attributes = context.removeAttributes(attributes, attrs);
            }
        }

        /**
         * Sets the resolving parent.
         *
         * @param parent the parent, null if none
         * @see MutableAttributeSet#setResolveParent
         */
        public void setResolveParent(AttributeSet parent) {
            checkForIllegalCast();
            AttributeContext context = getAttributeContext();
            if (parent != null) {
                attributes =
                    context.addAttribute(attributes, StyleConstants.ResolveAttribute,
                                         parent);
            } else {
                attributes =
                    context.removeAttribute(attributes, StyleConstants.ResolveAttribute);
            }
        }

        private final void checkForIllegalCast() {
            Thread t = getCurrentWriter();
            if ((t == null) || (t != Thread.currentThread())) {
                throw new StateInvariantError("Illegal cast to MutableAttributeSet");
            }
        }

        // --- Element methods -------------------------------------

        /**
         * Retrieves the underlying model.
         *
         * @return the model
         */
        public Document getDocument() {
            return AbstractDocument.this;
        }

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

        /**
         * Gets the attributes for the element.
         *
         * @return the attribute set
         */
        public AttributeSet getAttributes() {
            return this;
        }

        /**
         * Gets the name of the element.
         *
         * @return the name, null if none
         */
        public String getName() {
            if (attributes.isDefined(ElementNameAttribute)) {
                return (String) attributes.getAttribute(ElementNameAttribute);
            }
            return null;
        }

        /**
         * Gets the starting offset in the model for the element.
         *
         * @return the offset &gt;= 0
         */
        public abstract int getStartOffset();

        /**
         * Gets the ending offset in the model for the element.
         *
         * @return the offset &gt;= 0
         */
        public abstract int getEndOffset();

        /**
         * Gets a child element.
         *
         * @param index the child index, &gt;= 0 &amp;&amp; &lt; getElementCount()
         * @return the child element
         */
        public abstract Element getElement(int index);

        /**
         * Gets the number of children for the element.
         *
         * @return the number of children &gt;= 0
         */
        public abstract int getElementCount();

        /**
         * Gets the child element index closest to the given model offset.
         *
         * @param offset the offset &gt;= 0
         * @return the element index &gt;= 0
         */
        public abstract int getElementIndex(int offset);

        /**
         * Checks whether the element is a leaf.
         *
         * @return true if a leaf
         */
        public abstract boolean isLeaf();

        // --- TreeNode methods -------------------------------------

        /**
         * Returns the child <code>TreeNode</code> at index
         * <code>childIndex</code>.
         */
        public TreeNode getChildAt(int childIndex) {
            return (TreeNode)getElement(childIndex);
        }

        /**
         * Returns the number of children <code>TreeNode</code>'s
         * receiver contains.
         * @return the number of children <code>TreeNodews</code>'s
         * receiver contains
         */
        public int getChildCount() {
            return getElementCount();
        }

        /**
         * Returns the parent <code>TreeNode</code> of the receiver.
         * @return the parent <code>TreeNode</code> of the receiver
         */
        public TreeNode getParent() {
            return (TreeNode)getParentElement();
        }

        /**
         * Returns the index of <code>node</code> in the receivers children.
         * If the receiver does not contain <code>node</code>, -1 will be
         * returned.
         * @param node the location of interest
         * @return the index of <code>node</code> in the receiver's
         * children, or -1 if absent
         */
        public int getIndex(TreeNode node) {
            for(int counter = getChildCount() - 1; counter >= 0; counter--)
                if(getChildAt(counter) == node)
                    return counter;
            return -1;
        }

        /**
         * Returns true if the receiver allows children.
         * @return true if the receiver allows children, otherwise false
         */
        public abstract boolean getAllowsChildren();


        /**
         * Returns the children of the receiver as an
         * <code>Enumeration</code>.
         * @return the children of the receiver as an <code>Enumeration</code>
         */
        public abstract Enumeration children();


        // --- serialization ---------------------------------------------

        private void writeObject(ObjectOutputStream s) throws IOException {
            s.defaultWriteObject();
            StyleContext.writeAttributeSet(s, attributes);
        }

        private void readObject(ObjectInputStream s)
            throws ClassNotFoundException, IOException
        {
            s.defaultReadObject();
            MutableAttributeSet attr = new SimpleAttributeSet();
            StyleContext.readAttributeSet(s, attr);
            AttributeContext context = getAttributeContext();
            attributes = context.addAttributes(SimpleAttributeSet.EMPTY, attr);
        }

        // ---- variables -----------------------------------------------------

        private Element parent;
        private transient AttributeSet attributes;

    }

    /**
     * Implements a composite element that contains other elements.
     * <p>
     * <strong>Warning:</strong>
     * Serialized objects of this class will not be compatible with
     * future Swing releases. The current serialization support is
     * appropriate for short term storage or RMI between applications running
     * the same version of Swing.  As of 1.4, support for long term storage
     * of all JavaBeans&trade;
     * has been added to the <code>java.beans</code> package.
     * Please see {@link java.beans.XMLEncoder}.
     */
    public class BranchElement extends AbstractElement {

        /**
         * Constructs a composite element that initially contains
         * no children.
         *
         * @param parent  The parent element
         * @param a the attributes for the element
         * @since 1.4
         */
        public BranchElement(Element parent, AttributeSet a) {
            super(parent, a);
            children = new AbstractElement[1];
            nchildren = 0;
            lastIndex = -1;
        }

        /**
         * Gets the child element that contains
         * the given model position.
         *
         * @param pos the position &gt;= 0
         * @return the element, null if none
         */
        public Element positionToElement(int pos) {
            int index = getElementIndex(pos);
            Element child = children[index];
            int p0 = child.getStartOffset();
            int p1 = child.getEndOffset();
            if ((pos >= p0) && (pos < p1)) {
                return child;
            }
            return null;
        }

        /**
         * Replaces content with a new set of elements.
         *
         * @param offset the starting offset &gt;= 0
         * @param length the length to replace &gt;= 0
         * @param elems the new elements
         */
        public void replace(int offset, int length, Element[] elems) {
            int delta = elems.length - length;
            int src = offset + length;
            int nmove = nchildren - src;
            int dest = src + delta;
            if ((nchildren + delta) >= children.length) {
                // need to grow the array
                int newLength = Math.max(2*children.length, nchildren + delta);
                AbstractElement[] newChildren = new AbstractElement[newLength];
                System.arraycopy(children, 0, newChildren, 0, offset);
                System.arraycopy(elems, 0, newChildren, offset, elems.length);
                System.arraycopy(children, src, newChildren, dest, nmove);
                children = newChildren;
            } else {
                // patch the existing array
                System.arraycopy(children, src, children, dest, nmove);
                System.arraycopy(elems, 0, children, offset, elems.length);
            }
            nchildren = nchildren + delta;
        }

        /**
         * Converts the element to a string.
         *
         * @return the string
         */
        public String toString() {
            return "BranchElement(" + getName() + ") " + getStartOffset() + "," +
                getEndOffset() + "\n";
        }

        // --- Element methods -----------------------------------

        /**
         * Gets the element name.
         *
         * @return the element name
         */
        public String getName() {
            String nm = super.getName();
            if (nm == null) {
                nm = ParagraphElementName;
            }
            return nm;
        }

        /**
         * Gets the starting offset in the model for the element.
         *
         * @return the offset &gt;= 0
         */
        public int getStartOffset() {
            return children[0].getStartOffset();
        }

        /**
         * Gets the ending offset in the model for the element.
         * @throws NullPointerException if this element has no children
         *
         * @return the offset &gt;= 0
         */
        public int getEndOffset() {
            Element child =
                (nchildren > 0) ? children[nchildren - 1] : children[0];
            return child.getEndOffset();
        }

        /**
         * Gets a child element.
         *
         * @param index the child index, &gt;= 0 &amp;&amp; &lt; getElementCount()
         * @return the child element, null if none
         */
        public Element getElement(int index) {
            if (index < nchildren) {
                return children[index];
            }
            return null;
        }

        /**
         * Gets the number of children for the element.
         *
         * @return the number of children &gt;= 0
         */
        public int getElementCount()  {
            return nchildren;
        }

        /**
         * Gets the child element index closest to the given model offset.
         *
         * @param offset the offset &gt;= 0
         * @return the element index &gt;= 0
         */
        public int getElementIndex(int offset) {
            int index;
            int lower = 0;
            int upper = nchildren - 1;
            int mid = 0;
            int p0 = getStartOffset();
            int p1;

            if (nchildren == 0) {
                return 0;
            }
            if (offset >= getEndOffset()) {
                return nchildren - 1;
            }

            // see if the last index can be used.
            if ((lastIndex >= lower) && (lastIndex <= upper)) {
                Element lastHit = children[lastIndex];
                p0 = lastHit.getStartOffset();
                p1 = lastHit.getEndOffset();
                if ((offset >= p0) && (offset < p1)) {
                    return lastIndex;
                }

                // last index wasn't a hit, but it does give useful info about
                // where a hit (if any) would be.
                if (offset < p0) {
                    upper = lastIndex;
                } else  {
                    lower = lastIndex;
                }
            }

            while (lower <= upper) {
                mid = lower + ((upper - lower) / 2);
                Element elem = children[mid];
                p0 = elem.getStartOffset();
                p1 = elem.getEndOffset();
                if ((offset >= p0) && (offset < p1)) {
                    // found the location
                    index = mid;
                    lastIndex = index;
                    return index;
                } else if (offset < p0) {
                    upper = mid - 1;
                } else {
                    lower = mid + 1;
                }
            }

            // didn't find it, but we indicate the index of where it would belong
            if (offset < p0) {
                index = mid;
            } else {
                index = mid + 1;
            }
            lastIndex = index;
            return index;
        }

        /**
         * Checks whether the element is a leaf.
         *
         * @return true if a leaf
         */
        public boolean isLeaf() {
            return false;
        }


        // ------ TreeNode ----------------------------------------------

        /**
         * Returns true if the receiver allows children.
         * @return true if the receiver allows children, otherwise false
         */
        public boolean getAllowsChildren() {
            return true;
        }


        /**
         * Returns the children of the receiver as an
         * <code>Enumeration</code>.
         * @return the children of the receiver
         */
        public Enumeration children() {
            if(nchildren == 0)
                return null;

            Vector<AbstractElement> tempVector = new Vector<AbstractElement>(nchildren);

            for(int counter = 0; counter < nchildren; counter++)
                tempVector.addElement(children[counter]);
            return tempVector.elements();
        }

        // ------ members ----------------------------------------------

        private AbstractElement[] children;
        private int nchildren;
        private int lastIndex;
    }

    /**
     * Implements an element that directly represents content of
     * some kind.
     * <p>
     * <strong>Warning:</strong>
     * Serialized objects of this class will not be compatible with
     * future Swing releases. The current serialization support is
     * appropriate for short term storage or RMI between applications running
     * the same version of Swing.  As of 1.4, support for long term storage
     * of all JavaBeans&trade;
     * has been added to the <code>java.beans</code> package.
     * Please see {@link java.beans.XMLEncoder}.
     *
     * @see     Element
     */
    public class LeafElement extends AbstractElement {

        /**
         * Constructs an element that represents content within the
         * document (has no children).
         *
         * @param parent  The parent element
         * @param a       The element attributes
         * @param offs0   The start offset &gt;= 0
         * @param offs1   The end offset &gt;= offs0
         * @since 1.4
         */
        public LeafElement(Element parent, AttributeSet a, int offs0, int offs1) {
            super(parent, a);
            try {
                p0 = createPosition(offs0);
                p1 = createPosition(offs1);
            } catch (BadLocationException e) {
                p0 = null;
                p1 = null;
                throw new StateInvariantError("Can't create Position references");
            }
        }

        /**
         * Converts the element to a string.
         *
         * @return the string
         */
        public String toString() {
            return "LeafElement(" + getName() + ") " + p0 + "," + p1 + "\n";
        }

        // --- Element methods ---------------------------------------------

        /**
         * Gets the starting offset in the model for the element.
         *
         * @return the offset &gt;= 0
         */
        public int getStartOffset() {
            return p0.getOffset();
        }

        /**
         * Gets the ending offset in the model for the element.
         *
         * @return the offset &gt;= 0
         */
        public int getEndOffset() {
            return p1.getOffset();
        }

        /**
         * Gets the element name.
         *
         * @return the name
         */
        public String getName() {
            String nm = super.getName();
            if (nm == null) {
                nm = ContentElementName;
            }
            return nm;
        }

        /**
         * Gets the child element index closest to the given model offset.
         *
         * @param pos the offset &gt;= 0
         * @return the element index &gt;= 0
         */
        public int getElementIndex(int pos) {
            return -1;
        }

        /**
         * Gets a child element.
         *
         * @param index the child index, &gt;= 0 &amp;&amp; &lt; getElementCount()
         * @return the child element
         */
        public Element getElement(int index) {
            return null;
        }

        /**
         * Returns the number of child elements.
         *
         * @return the number of children &gt;= 0
         */
        public int getElementCount()  {
            return 0;
        }

        /**
         * Checks whether the element is a leaf.
         *
         * @return true if a leaf
         */
        public boolean isLeaf() {
            return true;
        }

        // ------ TreeNode ----------------------------------------------

        /**
         * Returns true if the receiver allows children.
         * @return true if the receiver allows children, otherwise false
         */
        public boolean getAllowsChildren() {
            return false;
        }


        /**
         * Returns the children of the receiver as an
         * <code>Enumeration</code>.
         * @return the children of the receiver
         */
        public Enumeration children() {
            return null;
        }

        // --- serialization ---------------------------------------------

        private void writeObject(ObjectOutputStream s) throws IOException {
            s.defaultWriteObject();
            s.writeInt(p0.getOffset());
            s.writeInt(p1.getOffset());
        }

        private void readObject(ObjectInputStream s)
            throws ClassNotFoundException, IOException
        {
            s.defaultReadObject();

            // set the range with positions that track change
            int off0 = s.readInt();
            int off1 = s.readInt();
            try {
                p0 = createPosition(off0);
                p1 = createPosition(off1);
            } catch (BadLocationException e) {
                p0 = null;
                p1 = null;
                throw new IOException("Can't restore Position references");
            }
        }

        // ---- members -----------------------------------------------------

        private transient Position p0;
        private transient Position p1;
    }

    /**
     * Represents the root element of the bidirectional element structure.
     * The root element is the only element in the bidi element structure
     * which contains children.
     */
    class BidiRootElement extends BranchElement {

        BidiRootElement() {
            super( null, null );
        }

        /**
         * Gets the name of the element.
         * @return the name
         */
        public String getName() {
            return "bidi root";
        }
    }

    /**
     * Represents an element of the bidirectional element structure.
     */
    class BidiElement extends LeafElement {

        /**
         * Creates a new BidiElement.
         */
        BidiElement(Element parent, int start, int end, int level) {
            super(parent, new SimpleAttributeSet(), start, end);
            addAttribute(StyleConstants.BidiLevel, Integer.valueOf(level));
            //System.out.println("BidiElement: start = " + start
            //                   + " end = " + end + " level = " + level );
        }

        /**
         * Gets the name of the element.
         * @return the name
         */
        public String getName() {
            return BidiElementName;
        }

        int getLevel() {
            Integer o = (Integer) getAttribute(StyleConstants.BidiLevel);
            if (o != null) {
                return o.intValue();
            }
            return 0;  // Level 0 is base level (non-embedded) left-to-right
        }

        boolean isLeftToRight() {
            return ((getLevel() % 2) == 0);
        }
    }

    /**
     * Stores document changes as the document is being
     * modified.  Can subsequently be used for change notification
     * when done with the document modification transaction.
     * This is used by the AbstractDocument class and its extensions
     * for broadcasting change information to the document listeners.
     */
    public class DefaultDocumentEvent extends CompoundEdit implements DocumentEvent {

        /**
         * Constructs a change record.
         *
         * @param offs the offset into the document of the change &gt;= 0
         * @param len  the length of the change &gt;= 0
         * @param type the type of event (DocumentEvent.EventType)
         * @since 1.4
         */
        public DefaultDocumentEvent(int offs, int len, DocumentEvent.EventType type) {
            super();
            offset = offs;
            length = len;
            this.type = type;
        }

        /**
         * Returns a string description of the change event.
         *
         * @return a string
         */
        public String toString() {
            return edits.toString();
        }

        // --- CompoundEdit methods --------------------------

        /**
         * Adds a document edit.  If the number of edits crosses
         * a threshold, this switches on a hashtable lookup for
         * ElementChange implementations since access of these
         * needs to be relatively quick.
         *
         * @param anEdit a document edit record
         * @return true if the edit was added
         */
        public boolean addEdit(UndoableEdit anEdit) {
            // if the number of changes gets too great, start using
            // a hashtable for to locate the change for a given element.
            if ((changeLookup == null) && (edits.size() > 10)) {
                changeLookup = new Hashtable<Element, ElementChange>();
                int n = edits.size();
                for (int i = 0; i < n; i++) {
                    Object o = edits.elementAt(i);
                    if (o instanceof DocumentEvent.ElementChange) {
                        DocumentEvent.ElementChange ec = (DocumentEvent.ElementChange) o;
                        changeLookup.put(ec.getElement(), ec);
                    }
                }
            }

            // if we have a hashtable... add the entry if it's
            // an ElementChange.
            if ((changeLookup != null) && (anEdit instanceof DocumentEvent.ElementChange)) {
                DocumentEvent.ElementChange ec = (DocumentEvent.ElementChange) anEdit;
                changeLookup.put(ec.getElement(), ec);
            }
            return super.addEdit(anEdit);
        }

        /**
         * Redoes a change.
         *
         * @exception CannotRedoException if the change cannot be redone
         */
        public void redo() throws CannotRedoException {
            writeLock();
            try {
                // change the state
                super.redo();
                // fire a DocumentEvent to notify the view(s)
                UndoRedoDocumentEvent ev = new UndoRedoDocumentEvent(this, false);
                if (type == DocumentEvent.EventType.INSERT) {
                    fireInsertUpdate(ev);
                } else if (type == DocumentEvent.EventType.REMOVE) {
                    fireRemoveUpdate(ev);
                } else {
                    fireChangedUpdate(ev);
                }
            } finally {
                writeUnlock();
            }
        }

        /**
         * Undoes a change.
         *
         * @exception CannotUndoException if the change cannot be undone
         */
        public void undo() throws CannotUndoException {
            writeLock();
            try {
                // change the state
                super.undo();
                // fire a DocumentEvent to notify the view(s)
                UndoRedoDocumentEvent ev = new UndoRedoDocumentEvent(this, true);
                if (type == DocumentEvent.EventType.REMOVE) {
                    fireInsertUpdate(ev);
                } else if (type == DocumentEvent.EventType.INSERT) {
                    fireRemoveUpdate(ev);
                } else {
                    fireChangedUpdate(ev);
                }
            } finally {
                writeUnlock();
            }
        }

        /**
         * DefaultDocument events are significant.  If you wish to aggregate
         * DefaultDocumentEvents to present them as a single edit to the user
         * place them into a CompoundEdit.
         *
         * @return whether the event is significant for edit undo purposes
         */
        public boolean isSignificant() {
            return true;
        }


        /**
         * Provides a localized, human readable description of this edit
         * suitable for use in, say, a change log.
         *
         * @return the description
         */
        public String getPresentationName() {
            DocumentEvent.EventType type = getType();
            if(type == DocumentEvent.EventType.INSERT)
                return UIManager.getString("AbstractDocument.additionText");
            if(type == DocumentEvent.EventType.REMOVE)
                return UIManager.getString("AbstractDocument.deletionText");
            return UIManager.getString("AbstractDocument.styleChangeText");
        }

        /**
         * Provides a localized, human readable description of the undoable
         * form of this edit, e.g. for use as an Undo menu item. Typically
         * derived from getDescription();
         *
         * @return the description
         */
        public String getUndoPresentationName() {
            return UIManager.getString("AbstractDocument.undoText") + " " +
                getPresentationName();
        }

        /**
         * Provides a localized, human readable description of the redoable
         * form of this edit, e.g. for use as a Redo menu item. Typically
         * derived from getPresentationName();
         *
         * @return the description
         */
        public String getRedoPresentationName() {
            return UIManager.getString("AbstractDocument.redoText") + " " +
                getPresentationName();
        }

        // --- DocumentEvent methods --------------------------

        /**
         * Returns the type of event.
         *
         * @return the event type as a DocumentEvent.EventType
         * @see DocumentEvent#getType
         */
        public DocumentEvent.EventType getType() {
            return type;
        }

        /**
         * Returns the offset within the document of the start of the change.
         *
         * @return the offset &gt;= 0
         * @see DocumentEvent#getOffset
         */
        public int getOffset() {
            return offset;
        }

        /**
         * Returns the length of the change.
         *
         * @return the length &gt;= 0
         * @see DocumentEvent#getLength
         */
        public int getLength() {
            return length;
        }

        /**
         * Gets the document that sourced the change event.
         *
         * @return the document
         * @see DocumentEvent#getDocument
         */
        public Document getDocument() {
            return AbstractDocument.this;
        }

        /**
         * Gets the changes for an element.
         *
         * @param elem the element
         * @return the changes
         */
        public DocumentEvent.ElementChange getChange(Element elem) {
            if (changeLookup != null) {
                return changeLookup.get(elem);
            }
            int n = edits.size();
            for (int i = 0; i < n; i++) {
                Object o = edits.elementAt(i);
                if (o instanceof DocumentEvent.ElementChange) {
                    DocumentEvent.ElementChange c = (DocumentEvent.ElementChange) o;
                    if (elem.equals(c.getElement())) {
                        return c;
                    }
                }
            }
            return null;
        }

        // --- member variables ------------------------------------

        private int offset;
        private int length;
        private Hashtable<Element, ElementChange> changeLookup;
        private DocumentEvent.EventType type;

    }

    /**
     * This event used when firing document changes while Undo/Redo
     * operations. It just wraps DefaultDocumentEvent and delegates
     * all calls to it except getType() which depends on operation
     * (Undo or Redo).
     */
    class UndoRedoDocumentEvent implements DocumentEvent {
        private DefaultDocumentEvent src = null;
        private EventType type = null;

        public UndoRedoDocumentEvent(DefaultDocumentEvent src, boolean isUndo) {
            this.src = src;
            if(isUndo) {
                if(src.getType().equals(EventType.INSERT)) {
                    type = EventType.REMOVE;
                } else if(src.getType().equals(EventType.REMOVE)) {
                    type = EventType.INSERT;
                } else {
                    type = src.getType();
                }
            } else {
                type = src.getType();
            }
        }

        public DefaultDocumentEvent getSource() {
            return src;
        }

        // DocumentEvent methods delegated to DefaultDocumentEvent source
        // except getType() which depends on operation (Undo or Redo).
        public int getOffset() {
            return src.getOffset();
        }

        public int getLength() {
            return src.getLength();
        }

        public Document getDocument() {
            return src.getDocument();
        }

        public DocumentEvent.EventType getType() {
            return type;
        }

        public DocumentEvent.ElementChange getChange(Element elem) {
            return src.getChange(elem);
        }
    }

    /**
     * An implementation of ElementChange that can be added to the document
     * event.
     */
    public static class ElementEdit extends AbstractUndoableEdit implements DocumentEvent.ElementChange {

        /**
         * Constructs an edit record.  This does not modify the element
         * so it can safely be used to <em>catch up</em> a view to the
         * current model state for views that just attached to a model.
         *
         * @param e the element
         * @param index the index into the model &gt;= 0
         * @param removed a set of elements that were removed
         * @param added a set of elements that were added
         */
        public ElementEdit(Element e, int index, Element[] removed, Element[] added) {
            super();
            this.e = e;
            this.index = index;
            this.removed = removed;
            this.added = added;
        }

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

        /**
         * Returns the index into the list of elements.
         *
         * @return the index &gt;= 0
         */
        public int getIndex() {
            return index;
        }

        /**
         * Gets a list of children that were removed.
         *
         * @return the list
         */
        public Element[] getChildrenRemoved() {
            return removed;
        }

        /**
         * Gets a list of children that were added.
         *
         * @return the list
         */
        public Element[] getChildrenAdded() {
            return added;
        }

        /**
         * Redoes a change.
         *
         * @exception CannotRedoException if the change cannot be redone
         */
        public void redo() throws CannotRedoException {
            super.redo();

            // Since this event will be reused, switch around added/removed.
            Element[] tmp = removed;
            removed = added;
            added = tmp;

            // PENDING(prinz) need MutableElement interface, canRedo() should check
            ((AbstractDocument.BranchElement)e).replace(index, removed.length, added);
        }

        /**
         * Undoes a change.
         *
         * @exception CannotUndoException if the change cannot be undone
         */
        public void undo() throws CannotUndoException {
            super.undo();
            // PENDING(prinz) need MutableElement interface, canUndo() should check
            ((AbstractDocument.BranchElement)e).replace(index, added.length, removed);

            // Since this event will be reused, switch around added/removed.
            Element[] tmp = removed;
            removed = added;
            added = tmp;
        }

        private Element e;
        private int index;
        private Element[] removed;
        private Element[] added;
    }


    private class DefaultFilterBypass extends DocumentFilter.FilterBypass {
        public Document getDocument() {
            return AbstractDocument.this;
        }

        public void remove(int offset, int length) throws
            BadLocationException {
            handleRemove(offset, length);
        }

        public void insertString(int offset, String string,
                                 AttributeSet attr) throws
                                        BadLocationException {
            handleInsertString(offset, string, attr);
        }

        public void replace(int offset, int length, String text,
                            AttributeSet attrs) throws BadLocationException {
            handleRemove(offset, length);
            handleInsertString(offset, text, attrs);
        }
    }
}
