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

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;

A List implementation with a ListIterator that
allows concurrent modifications to the underlying list.
 This implementation supports all of the optional List operations. It extends AbstractLinkedList and thus provides the stack/queue/dequeue operations available in LinkedList. 
 The main feature of this class is the ability to modify the list and the iterator at the same time. Both the listIterator() and cursor() methods provides access to a Cursor instance which extends
ListIterator. The cursor allows changes to the list concurrent with changes to the iterator. Note that the iterator() method and sublists do not provide this cursor behaviour.

The Cursor class is provided partly for backwards compatibility
and partly because it allows the cursor to be directly closed. Closing the
cursor is optional because references are held via a WeakReference.
For most purposes, simply modify the iterator and list at will, and then let
the garbage collector to the rest.

Note that this implementation is not synchronized.
Author:
Rodney Waldhoff, Janek Bogucki, Simon Kitching, Stephen Colebourne
See Also:
LinkedList
Since:
Commons Collections 1.0
Version:
$Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $/**
 * A <code>List</code> implementation with a <code>ListIterator</code> that
 * allows concurrent modifications to the underlying list.
 * <p>
 * This implementation supports all of the optional {@link List} operations.
 * It extends <code>AbstractLinkedList</code> and thus provides the
 * stack/queue/dequeue operations available in {@link java.util.LinkedList}.
 * <p>
 * The main feature of this class is the ability to modify the list and the
 * iterator at the same time. Both the {@link #listIterator()} and {@link #cursor()}
 * methods provides access to a <code>Cursor</code> instance which extends
 * <code>ListIterator</code>. The cursor allows changes to the list concurrent
 * with changes to the iterator. Note that the {@link #iterator()} method and
 * sublists do <b>not</b> provide this cursor behaviour.
 * <p>
 * The <code>Cursor</code> class is provided partly for backwards compatibility
 * and partly because it allows the cursor to be directly closed. Closing the
 * cursor is optional because references are held via a <code>WeakReference</code>.
 * For most purposes, simply modify the iterator and list at will, and then let
 * the garbage collector to the rest.
 * <p>
 * <b>Note that this implementation is not synchronized.</b>
 *
 * @see java.util.LinkedList
 * @since Commons Collections 1.0
 * @version $Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
 * 
 * @author Rodney Waldhoff
 * @author Janek Bogucki
 * @author Simon Kitching
 * @author Stephen Colebourne
 */
public class CursorableLinkedList extends AbstractLinkedList implements Serializable {

    
Ensure serialization compatibility /** Ensure serialization compatibility */
    private static final long serialVersionUID = 8836393098519411393L;

    
A list of the cursor currently open on this list /** A list of the cursor currently open on this list */
    protected transient List cursors = new ArrayList();

    //-----------------------------------------------------------------------
    
Constructor that creates.
/**
     * Constructor that creates.
     */
    public CursorableLinkedList() {
        super();
        init(); // must call init() as use super();
    }

    
Constructor that copies the specified collection
Params:
coll –  the collection to copy/**
     * Constructor that copies the specified collection
     * 
     * @param coll  the collection to copy
     */
    public CursorableLinkedList(Collection coll) {
        super(coll);
    }

    
The equivalent of a default constructor called
by any constructor and by readObject.
/**
     * The equivalent of a default constructor called
     * by any constructor and by <code>readObject</code>.
     */
    protected void init() {
        super.init();
        cursors = new ArrayList();
    }

    //-----------------------------------------------------------------------
    
Returns an iterator that does not support concurrent modification.
 If the underlying list is modified while iterating using this iterator a ConcurrentModificationException will occur. The cursor behaviour is available via listIterator(). 
Returns:
a new iterator that does not support concurrent modification/**
     * Returns an iterator that does <b>not</b> support concurrent modification.
     * <p>
     * If the underlying list is modified while iterating using this iterator
     * a ConcurrentModificationException will occur.
     * The cursor behaviour is available via {@link #listIterator()}.
     * 
     * @return a new iterator that does <b>not</b> support concurrent modification
     */
    public Iterator iterator() {
        return super.listIterator(0);
    }

    
Returns a cursor iterator that allows changes to the underlying list in parallel.

The cursor enables iteration and list changes to occur in any order without
invalidating the iterator (from one thread). When elements are added to the
list, an event is fired to all active cursors enabling them to adjust to the
change in the list.
 When the "current" (i.e., last returned by ListIterator.next or ListIterator.previous) element of the list is removed, the cursor automatically adjusts to the change (invalidating the last returned value such that it cannot be removed). 
Returns:
a new cursor iterator/**
     * Returns a cursor iterator that allows changes to the underlying list in parallel.
     * <p>
     * The cursor enables iteration and list changes to occur in any order without
     * invalidating the iterator (from one thread). When elements are added to the
     * list, an event is fired to all active cursors enabling them to adjust to the
     * change in the list.
     * <p>
     * When the "current" (i.e., last returned by {@link ListIterator#next}
     * or {@link ListIterator#previous}) element of the list is removed,
     * the cursor automatically adjusts to the change (invalidating the
     * last returned value such that it cannot be removed).
     * 
     * @return a new cursor iterator
     */
    public ListIterator listIterator() {
        return cursor(0);
    }

    
Returns a cursor iterator that allows changes to the underlying list in parallel.

The cursor enables iteration and list changes to occur in any order without
invalidating the iterator (from one thread). When elements are added to the
list, an event is fired to all active cursors enabling them to adjust to the
change in the list.
 When the "current" (i.e., last returned by ListIterator.next or ListIterator.previous) element of the list is removed, the cursor automatically adjusts to the change (invalidating the last returned value such that it cannot be removed). 
Params:
fromIndex –  the index to start from
Returns:
a new cursor iterator/**
     * Returns a cursor iterator that allows changes to the underlying list in parallel.
     * <p>
     * The cursor enables iteration and list changes to occur in any order without
     * invalidating the iterator (from one thread). When elements are added to the
     * list, an event is fired to all active cursors enabling them to adjust to the
     * change in the list.
     * <p>
     * When the "current" (i.e., last returned by {@link ListIterator#next}
     * or {@link ListIterator#previous}) element of the list is removed,
     * the cursor automatically adjusts to the change (invalidating the
     * last returned value such that it cannot be removed).
     * 
     * @param fromIndex  the index to start from
     * @return a new cursor iterator
     */
    public ListIterator listIterator(int fromIndex) {
        return cursor(fromIndex);
    }

    
Returns a Cursor for iterating through the elements of this list. 

A Cursor is a ListIterator with an additional
close() method. Calling this method immediately discards the
references to the cursor. If it is not called, then the garbage collector
will still remove the reference as it is held via a WeakReference.

The cursor enables iteration and list changes to occur in any order without
invalidating the iterator (from one thread). When elements are added to the
list, an event is fired to all active cursors enabling them to adjust to the
change in the list.
 When the "current" (i.e., last returned by ListIterator.next or ListIterator.previous) element of the list is removed, the cursor automatically adjusts to the change (invalidating the last returned value such that it cannot be removed). 
 The listIterator() method returns the same as this method, and can be cast to a Cursor if the close method is required.
Returns:
a new cursor iterator/**
     * Returns a {@link Cursor} for iterating through the elements of this list.
     * <p>
     * A <code>Cursor</code> is a <code>ListIterator</code> with an additional
     * <code>close()</code> method. Calling this method immediately discards the
     * references to the cursor. If it is not called, then the garbage collector
     * will still remove the reference as it is held via a <code>WeakReference</code>.
     * <p>
     * The cursor enables iteration and list changes to occur in any order without
     * invalidating the iterator (from one thread). When elements are added to the
     * list, an event is fired to all active cursors enabling them to adjust to the
     * change in the list.
     * <p>
     * When the "current" (i.e., last returned by {@link ListIterator#next}
     * or {@link ListIterator#previous}) element of the list is removed,
     * the cursor automatically adjusts to the change (invalidating the
     * last returned value such that it cannot be removed).
     * <p>
     * The {@link #listIterator()} method returns the same as this method, and can
     * be cast to a <code>Cursor</code> if the <code>close</code> method is required.
     *
     * @return a new cursor iterator
     */
    public CursorableLinkedList.Cursor cursor() {
        return cursor(0);
    }

    
Returns a Cursor for iterating through the elements of this list starting from a specified index. 

A Cursor is a ListIterator with an additional
close() method. Calling this method immediately discards the
references to the cursor. If it is not called, then the garbage collector
will still remove the reference as it is held via a WeakReference.

The cursor enables iteration and list changes to occur in any order without
invalidating the iterator (from one thread). When elements are added to the
list, an event is fired to all active cursors enabling them to adjust to the
change in the list.
 When the "current" (i.e., last returned by ListIterator.next or ListIterator.previous) element of the list is removed, the cursor automatically adjusts to the change (invalidating the last returned value such that it cannot be removed). 
 The listIterator(int) method returns the same as this method, and can be cast to a Cursor if the close method is required.
Params:
fromIndex –  the index to start from
Throws:
IndexOutOfBoundsException – if the index is out of range
     (index < 0 || index > size()).
Returns:
a new cursor iterator/**
     * Returns a {@link Cursor} for iterating through the elements of this list
     * starting from a specified index.
     * <p>
     * A <code>Cursor</code> is a <code>ListIterator</code> with an additional
     * <code>close()</code> method. Calling this method immediately discards the
     * references to the cursor. If it is not called, then the garbage collector
     * will still remove the reference as it is held via a <code>WeakReference</code>.
     * <p>
     * The cursor enables iteration and list changes to occur in any order without
     * invalidating the iterator (from one thread). When elements are added to the
     * list, an event is fired to all active cursors enabling them to adjust to the
     * change in the list.
     * <p>
     * When the "current" (i.e., last returned by {@link ListIterator#next}
     * or {@link ListIterator#previous}) element of the list is removed,
     * the cursor automatically adjusts to the change (invalidating the
     * last returned value such that it cannot be removed).
     * <p>
     * The {@link #listIterator(int)} method returns the same as this method, and can
     * be cast to a <code>Cursor</code> if the <code>close</code> method is required.
     *
     * @param fromIndex  the index to start from
     * @return a new cursor iterator
     * @throws IndexOutOfBoundsException if the index is out of range
     *      (index &lt; 0 || index &gt; size()).
     */
    public CursorableLinkedList.Cursor cursor(int fromIndex) {
        Cursor cursor = new Cursor(this, fromIndex);
        registerCursor(cursor);
        return cursor;
    }

    //-----------------------------------------------------------------------
    
Updates the node with a new value.
This implementation sets the value on the node.
Subclasses can override this to record the change.
Params:
node –  node to update
value –  new value of the node/**
     * Updates the node with a new value.
     * This implementation sets the value on the node.
     * Subclasses can override this to record the change.
     * 
     * @param node  node to update
     * @param value  new value of the node
     */
    protected void updateNode(Node node, Object value) {
        super.updateNode(node, value);
        broadcastNodeChanged(node);
    }

    
Inserts a new node into the list.
Params:
nodeToInsert –  new node to insert
insertBeforeNode –  node to insert before
Throws:
NullPointerException – if either node is null/**
     * Inserts a new node into the list.
     *
     * @param nodeToInsert  new node to insert
     * @param insertBeforeNode  node to insert before
     * @throws NullPointerException if either node is null
     */
    protected void addNode(Node nodeToInsert, Node insertBeforeNode) {
        super.addNode(nodeToInsert, insertBeforeNode);
        broadcastNodeInserted(nodeToInsert);
    }
    
    
Removes the specified node from the list.
Params:
node –  the node to remove
Throws:
NullPointerException – if node is null/**
     * Removes the specified node from the list.
     *
     * @param node  the node to remove
     * @throws NullPointerException if <code>node</code> is null
     */
    protected void removeNode(Node node) {
        super.removeNode(node);
        broadcastNodeRemoved(node);
    }

    
Removes all nodes by iteration.
/**
     * Removes all nodes by iteration.
     */
    protected void removeAllNodes() {
        if (size() > 0) {
            // superclass implementation would break all the iterators
            Iterator it = iterator();
            while (it.hasNext()) {
                it.next();
                it.remove();
            }
        }
    }

    //-----------------------------------------------------------------------
    
Registers a cursor to be notified of changes to this list.
Params:
cursor –  the cursor to register/**
     * Registers a cursor to be notified of changes to this list.
     * 
     * @param cursor  the cursor to register
     */
    protected void registerCursor(Cursor cursor) {
        // We take this opportunity to clean the cursors list
        // of WeakReference objects to garbage-collected cursors.
        for (Iterator it = cursors.iterator(); it.hasNext();) {
            WeakReference ref = (WeakReference) it.next();
            if (ref.get() == null) {
                it.remove();
            }
        }
        cursors.add(new WeakReference(cursor));
    }

    
Deregisters a cursor from the list to be notified of changes.
Params:
cursor –  the cursor to deregister/**
     * Deregisters a cursor from the list to be notified of changes.
     * 
     * @param cursor  the cursor to deregister
     */
    protected void unregisterCursor(Cursor cursor) {
        for (Iterator it = cursors.iterator(); it.hasNext();) {
            WeakReference ref = (WeakReference) it.next();
            Cursor cur = (Cursor) ref.get();
            if (cur == null) {
                // some other unrelated cursor object has been 
                // garbage-collected; let's take the opportunity to
                // clean up the cursors list anyway..
                it.remove();

            } else if (cur == cursor) {
                ref.clear();
                it.remove();
                break;
            }
        }
    }

    //-----------------------------------------------------------------------
    
Informs all of my registered cursors that the specified
element was changed.
Params:
node –  the node that was changed/**
     * Informs all of my registered cursors that the specified
     * element was changed.
     * 
     * @param node  the node that was changed
     */
    protected void broadcastNodeChanged(Node node) {
        Iterator it = cursors.iterator();
        while (it.hasNext()) {
            WeakReference ref = (WeakReference) it.next();
            Cursor cursor = (Cursor) ref.get();
            if (cursor == null) {
                it.remove(); // clean up list
            } else {
                cursor.nodeChanged(node);
            }
        }
    }

    
Informs all of my registered cursors that the specified
element was just removed from my list.
Params:
node –  the node that was changed/**
     * Informs all of my registered cursors that the specified
     * element was just removed from my list.
     * 
     * @param node  the node that was changed
     */
    protected void broadcastNodeRemoved(Node node) {
        Iterator it = cursors.iterator();
        while (it.hasNext()) {
            WeakReference ref = (WeakReference) it.next();
            Cursor cursor = (Cursor) ref.get();
            if (cursor == null) {
                it.remove(); // clean up list
            } else {
                cursor.nodeRemoved(node);
            }
        }
    }

    
Informs all of my registered cursors that the specified
element was just added to my list.
Params:
node –  the node that was changed/**
     * Informs all of my registered cursors that the specified
     * element was just added to my list.
     * 
     * @param node  the node that was changed
     */
    protected void broadcastNodeInserted(Node node) {
        Iterator it = cursors.iterator();
        while (it.hasNext()) {
            WeakReference ref = (WeakReference) it.next();
            Cursor cursor = (Cursor) ref.get();
            if (cursor == null) {
                it.remove(); // clean up list
            } else {
                cursor.nodeInserted(node);
            }
        }
    }

    //-----------------------------------------------------------------------
    
Serializes the data held in this object to the stream specified.
/**
     * Serializes the data held in this object to the stream specified.
     */
    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        doWriteObject(out);
    }

    
Deserializes the data held in this object to the stream specified.
/**
     * Deserializes the data held in this object to the stream specified.
     */
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        doReadObject(in);
    }

    //-----------------------------------------------------------------------
    
Creates a list iterator for the sublist.
Params:
subList –  the sublist to get an iterator for
fromIndex –  the index to start from, relative to the sublist/**
     * Creates a list iterator for the sublist.
     * 
     * @param subList  the sublist to get an iterator for
     * @param fromIndex  the index to start from, relative to the sublist
     */
    protected ListIterator createSubListListIterator(LinkedSubList subList, int fromIndex) {
        SubCursor cursor = new SubCursor(subList, fromIndex);
        registerCursor(cursor);
        return cursor;
    }

    //-----------------------------------------------------------------------
    
An extended ListIterator that allows concurrent changes to
the underlying list.
/**
     * An extended <code>ListIterator</code> that allows concurrent changes to
     * the underlying list.
     */
    public static class Cursor extends AbstractLinkedList.LinkedListIterator {
        
Is the cursor valid (not closed) /** Is the cursor valid (not closed) */
        boolean valid = true;
        
Is the next index valid /** Is the next index valid */
        boolean nextIndexValid = true;
        
Flag to indicate if the current element was removed by another object. /** Flag to indicate if the current element was removed by another object. */
        boolean currentRemovedByAnother = false;
        
        
Constructs a new cursor.
Params:
index –  the index to start from/**
         * Constructs a new cursor.
         * 
         * @param index  the index to start from
         */
        protected Cursor(CursorableLinkedList parent, int index) {
            super(parent, index);
            valid = true;
        }

        
Removes the item last returned by this iterator.

There may have been subsequent alterations to the list
since you obtained this item, however you can still remove it.
You can even remove it if the item is no longer in the main list.
However, you can't call this method on the same iterator more
than once without calling next() or previous().
Throws:
IllegalStateException – if there is no item to remove/**
         * Removes the item last returned by this iterator.
         * <p>
         * There may have been subsequent alterations to the list
         * since you obtained this item, however you can still remove it.
         * You can even remove it if the item is no longer in the main list.
         * However, you can't call this method on the same iterator more
         * than once without calling next() or previous().
         *
         * @throws IllegalStateException if there is no item to remove
         */
        public void remove() {
            // overridden, as the nodeRemoved() method updates the iterator
            // state in the parent.removeNode() call below
            if (current == null && currentRemovedByAnother) {
                // quietly ignore, as the last returned node was removed
                // by the list or some other iterator
                // by ignoring it, we keep this iterator independent from
                // other changes as much as possible
            } else {
                checkModCount();
                parent.removeNode(getLastNodeReturned());
            }
            currentRemovedByAnother = false;
        }

        
Adds an object to the list.
The object added here will be the new 'previous' in the iterator.
Params:
obj –  the object to add/**
         * Adds an object to the list.
         * The object added here will be the new 'previous' in the iterator.
         * 
         * @param obj  the object to add
         */
        public void add(Object obj) {
            // overridden, as the nodeInserted() method updates the iterator state
            super.add(obj);
            // matches the (next.previous == node) clause in nodeInserted()
            // thus next gets changed - reset it again here
            next = next.next;
        }
        
        // set is not overridden, as it works ok
        // note that we want it to throw an exception if the element being
        // set has been removed from the real list (compare this with the
        // remove method where we silently ignore this case)

        
Gets the index of the next element to be returned.
Returns:
the next index/**
         * Gets the index of the next element to be returned.
         * 
         * @return the next index
         */
        public int nextIndex() {
            if (nextIndexValid == false) {
                if (next == parent.header) {
                    nextIndex = parent.size();
                } else {
                    int pos = 0;
                    Node temp = parent.header.next;
                    while (temp != next) {
                        pos++;
                        temp = temp.next;
                    }
                    nextIndex = pos;
                }
                nextIndexValid = true;
            }
            return nextIndex;
        }

        
Handle event from the list when a node has changed.
Params:
node –  the node that changed/**
         * Handle event from the list when a node has changed.
         * 
         * @param node  the node that changed
         */
        protected void nodeChanged(Node node) {
            // do nothing
        }

        
Handle event from the list when a node has been removed.
Params:
node –  the node that was removed/**
         * Handle event from the list when a node has been removed.
         * 
         * @param node  the node that was removed
         */
        protected void nodeRemoved(Node node) {
            if (node == next && node == current) {
                // state where next() followed by previous()
                next = node.next;
                current = null;
                currentRemovedByAnother = true;
            } else if (node == next) {
                // state where next() not followed by previous()
                // and we are matching next node
                next = node.next;
                currentRemovedByAnother = false;
            } else if (node == current) {
                // state where next() not followed by previous()
                // and we are matching current (last returned) node
                current = null;
                currentRemovedByAnother = true;
                nextIndex--;
            } else {
                nextIndexValid = false;
                currentRemovedByAnother = false;
            }
        }

        
Handle event from the list when a node has been added.
Params:
node –  the node that was added/**
         * Handle event from the list when a node has been added.
         * 
         * @param node  the node that was added
         */
        protected void nodeInserted(Node node) {
            if (node.previous == current) {
                next = node;
            } else if (next.previous == node) {
                next = node;
            } else {
                nextIndexValid = false;
            }
        }

        
Override superclass modCount check, and replace it with our valid flag.
/**
         * Override superclass modCount check, and replace it with our valid flag.
         */
        protected void checkModCount() {
            if (!valid) {
                throw new ConcurrentModificationException("Cursor closed");
            }
        }

        
Mark this cursor as no longer being needed. Any resources
associated with this cursor are immediately released.
In previous versions of this class, it was mandatory to close
all cursor objects to avoid memory leaks. It is no longer
necessary to call this close method; an instance of this class
can now be treated exactly like a normal iterator.
/**
         * Mark this cursor as no longer being needed. Any resources
         * associated with this cursor are immediately released.
         * In previous versions of this class, it was mandatory to close
         * all cursor objects to avoid memory leaks. It is <i>no longer</i>
         * necessary to call this close method; an instance of this class
         * can now be treated exactly like a normal iterator.
         */
        public void close() {
            if (valid) {
                ((CursorableLinkedList) parent).unregisterCursor(this);
                valid = false;
            }
        }
    }

    //-----------------------------------------------------------------------
    
A cursor for the sublist based on LinkedSubListIterator.
Since:
Commons Collections 3.2/**
     * A cursor for the sublist based on LinkedSubListIterator.
     *
     * @since Commons Collections 3.2
     */
    protected static class SubCursor extends Cursor {

        
The parent list /** The parent list */
        protected final LinkedSubList sub;

        
Constructs a new cursor.
Params:
index –  the index to start from/**
         * Constructs a new cursor.
         * 
         * @param index  the index to start from
         */
        protected SubCursor(LinkedSubList sub, int index) {
            super((CursorableLinkedList) sub.parent, index + sub.offset);
            this.sub = sub;
        }

        public boolean hasNext() {
            return (nextIndex() < sub.size);
        }

        public boolean hasPrevious() {
            return (previousIndex() >= 0);
        }

        public int nextIndex() {
            return (super.nextIndex() - sub.offset);
        }

        public void add(Object obj) {
            super.add(obj);
            sub.expectedModCount = parent.modCount;
            sub.size++;
        }

        public void remove() {
            super.remove();
            sub.expectedModCount = parent.modCount;
            sub.size--;
        }
    }

}