xref: /dports/lang/mono/mono-5.10.1.57/mcs/class/Mono.C5/C5/linkedlists/LinkedList.cs

/*
 Copyright (c) 2003-2006 Niels Kokholm and Peter Sestoft
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
 in the Software without restriction, including without limitation the rights
 to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 copies of the Software, and to permit persons to whom the Software is
 furnished to do so, subject to the following conditions:

 The above copyright notice and this permission notice shall be included in
 all copies or substantial portions of the Software.

 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
*/

#define HASHINDEXnot

using System;
using System.Diagnostics;
using SCG = System.Collections.Generic;

namespace C5
{
  /// <summary>
  /// A list collection class based on a doubly linked list data structure.
  /// </summary>
  [Serializable]
  public class LinkedList<T> : SequencedBase<T>, IList<T>, SCG.IList<T>
#if HASHINDEX
#else
, IStack<T>, IQueue<T>
#endif
  {
    #region Fields
    /// <summary>
    /// IExtensible.Add(T) always does AddLast(T), fIFO determines
    /// if T Remove() does RemoveFirst() or RemoveLast()
    /// </summary>
    bool fIFO = true;

    #region Events

    /// <summary>
    ///
    /// </summary>
    /// <value></value>
    public override EventTypeEnum ListenableEvents { get { return underlying == null ? EventTypeEnum.All : EventTypeEnum.None; } }

    #endregion

    //Invariant:  startsentinel != null && endsentinel != null
    //If size==0: startsentinel.next == endsentinel && endsentinel.prev == startsentinel
    //Else:      startsentinel.next == First && endsentinel.prev == Last)
    /// <summary>
    /// Node to the left of first node
    /// </summary>
    Node startsentinel;
    /// <summary>
    /// Node to the right of last node
    /// </summary>
    Node endsentinel;
    /// <summary>
    /// Offset of this view in underlying list
    /// </summary>
#if HASHINDEX
    int? offset;
#else
    int offset;
#endif

    /// <summary>
    /// underlying list of this view (or null for the underlying list)
    /// </summary>
    LinkedList<T> underlying;

    //Note: all views will have the same views list since all view objects are created by MemberwiseClone()
    WeakViewList<LinkedList<T>> views;
    WeakViewList<LinkedList<T>>.Node myWeakReference;

    /// <summary>
    /// Has this list or view not been invalidated by some operation (by someone calling Dispose())
    /// </summary>
    bool isValid = true;


#if HASHINDEX
    HashDictionary<T, Node> dict;
    /// <summary>
    /// Number of taggroups
    /// </summary>
    int taggroups;
    /// <summary>
    ///
    /// </summary>
    /// <value></value>
    int Taggroups
    {
      get { return underlying == null ? taggroups : underlying.taggroups; }
      set { if (underlying == null) taggroups = value; else underlying.taggroups = value; }
    }
#endif

    #endregion

    #region Util

    bool equals(T i1, T i2) { return itemequalityComparer.Equals(i1, i2); }

    #region Check utilities
    /// <summary>
    /// Check if it is valid to perform updates and increment stamp of
    /// underlying if this is a view.
    /// <para>This method should be called in every public modifying
    /// methods before any modifications are performed.
    /// </para>
    /// </summary>
    /// <exception cref="InvalidOperationException"> if check fails.</exception>
    protected override void updatecheck()
    {
      validitycheck();
      base.updatecheck();
      if (underlying != null)
        underlying.stamp++;
    }

    /// <summary>
    /// Check if we are a view that the underlyinglist has only been updated through us.
    /// <br/>
    /// This method should be called from enumerators etc to guard against
    /// modification of the base collection.
    /// </summary>
    /// <exception cref="InvalidOperationException"> if check fails.</exception>
    void validitycheck()
    {
      if (!isValid)
        throw new ViewDisposedException();
    }

    /// <summary>
    /// Check that the list has not been updated since a particular time.
    /// </summary>
    /// <param name="stamp">The stamp indicating the time.</param>
    /// <exception cref="CollectionModifiedException"> if check fails.</exception>
    protected override void modifycheck(int stamp)
    {
      validitycheck();
      if ((underlying != null ? underlying.stamp : this.stamp) != stamp)
        throw new CollectionModifiedException();
    }
    #endregion

    #region Searching
    bool contains(T item, out Node node)
    {
#if HASHINDEX
      if (dict.Find(item, out node))
        return insideview(node);
#else
      //TODO: search from both ends? Or search from the end selected by FIFO?
      node = startsentinel.next;
      while (node != endsentinel)
      {
        if (equals(item, node.item))
          return true;
        node = node.next;
      }
#endif
      return false;
    }

    /// <summary>
    /// Search forwards from a node for a node with a particular item.
    /// </summary>
    /// <param name="item">The item to look for</param>
    /// <param name="node">On input, the node to start at. If item was found, the node found on output.</param>
    /// <param name="index">If node was found, the value will be the number of links followed higher than
    /// the value on input. If item was not found, the value on output is undefined.</param>
    /// <returns>True if node was found.</returns>
    bool find(T item, ref Node node, ref int index)
    {
      while (node != endsentinel)
      {
        //if (item.Equals(node.item))
        if (itemequalityComparer.Equals(item, node.item))
          return true;

        index++;
        node = node.next;
      }

      return false;
    }

    bool dnif(T item, ref Node node, ref int index)
    {
      while (node != startsentinel)
      {
        //if (item.Equals(node.item))
        if (itemequalityComparer.Equals(item, node.item))
          return true;

        index--;
        node = node.prev;
      }

      return false;
    }

#if HASHINDEX
    bool insideview(Node node)
    {
      if (underlying == null)
        return true;
      return (startsentinel.precedes(node) && node.precedes(endsentinel));
    }
#endif

    #endregion

    #region Indexing
    /// <summary>
    /// Return the node at position pos
    /// </summary>
    /// <param name="pos"></param>
    /// <returns></returns>
    Node get(int pos)
    {
      if (pos < 0 || pos >= size)
        throw new IndexOutOfRangeException();
      else if (pos < size / 2)
      {              // Closer to front
        Node node = startsentinel;

        for (int i = 0; i <= pos; i++)
          node = node.next;

        return node;
      }
      else
      {                            // Closer to end
        Node node = endsentinel;

        for (int i = size; i > pos; i--)
          node = node.prev;

        return node;
      }
    }

    /// <summary>
    /// Find the distance from pos to the set given by positions. Return the
    /// signed distance as return value and as an out parameter, the
    /// array index of the nearest position. This is used for up to length 5 of
    /// positions, and we do not assume it is sorted.
    /// </summary>
    /// <param name="pos"></param>
    /// <param name="positions"></param>
    /// <param name="nearest"></param>
    /// <returns></returns>
    int dist(int pos, out int nearest, int[] positions)
    {
      nearest = -1;
      int bestdist = int.MaxValue;
      int signeddist = bestdist;
      for (int i = 0; i < positions.Length; i++)
      {
        int thisdist = positions[i] - pos;
        if (thisdist >= 0 && thisdist < bestdist) { nearest = i; bestdist = thisdist; signeddist = thisdist; }
        if (thisdist < 0 && -thisdist < bestdist) { nearest = i; bestdist = -thisdist; signeddist = thisdist; }
      }
      return signeddist;
    }

    /// <summary>
    /// Find the node at position pos, given known positions of several nodes.
    /// </summary>
    /// <param name="pos"></param>
    /// <param name="positions"></param>
    /// <param name="nodes"></param>
    /// <returns></returns>
    Node get(int pos, int[] positions, Node[] nodes)
    {
      int nearest;
      int delta = dist(pos, out nearest, positions);
      Node node = nodes[nearest];
      if (delta > 0)
        for (int i = 0; i < delta; i++)
          node = node.prev;
      else
        for (int i = 0; i > delta; i--)
          node = node.next;
      return node;
    }

    /// <summary>
    /// Get nodes at positions p1 and p2, given nodes at several positions.
    /// </summary>
    /// <param name="p1"></param>
    /// <param name="p2"></param>
    /// <param name="n1"></param>
    /// <param name="n2"></param>
    /// <param name="positions"></param>
    /// <param name="nodes"></param>
    void getPair(int p1, int p2, out Node n1, out Node n2, int[] positions, Node[] nodes)
    {
      int nearest1, nearest2;
      int delta1 = dist(p1, out nearest1, positions), d1 = delta1 < 0 ? -delta1 : delta1;
      int delta2 = dist(p2, out nearest2, positions), d2 = delta2 < 0 ? -delta2 : delta2;

      if (d1 < d2)
      {
        n1 = get(p1, positions, nodes);
        n2 = get(p2, new int[] { positions[nearest2], p1 }, new Node[] { nodes[nearest2], n1 });
      }
      else
      {
        n2 = get(p2, positions, nodes);
        n1 = get(p1, new int[] { positions[nearest1], p2 }, new Node[] { nodes[nearest1], n2 });
      }
    }
    #endregion

    #region Insertion
#if HASHINDEX
    void insert(int index, Node succ, T item)
    {
      Node newnode = new Node(item);
      if (dict.FindOrAdd(item, ref newnode))
        throw new DuplicateNotAllowedException("Item already in indexed list");
      insertNode(true, succ, newnode);
    }

    /// <summary>
    /// Insert a Node before another one. Unchecked version.
    /// </summary>
    /// <param name="succ">The successor to be</param>
    /// <param name="newnode">Node to insert</param>
    /// <param name="updateViews">update overlapping view in this call</param>
    void insertNode(bool updateViews, Node succ, Node newnode)
    {
      newnode.next = succ;
      Node pred = newnode.prev = succ.prev;
      succ.prev.next = newnode;
      succ.prev = newnode;
      size++;
      if (underlying != null)
        underlying.size++;
      settag(newnode);
      if (updateViews)
        fixViewsAfterInsert(succ, pred, 1, 0);
    }
#else
    /// <summary>
    ///
    /// </summary>
    /// <param name="index">The index in this view</param>
    /// <param name="succ"></param>
    /// <param name="item"></param>
    /// <returns></returns>
    Node insert(int index, Node succ, T item)
    {
      Node newnode = new Node(item, succ.prev, succ);
      succ.prev.next = newnode;
      succ.prev = newnode;
      size++;
      if (underlying != null)
        underlying.size++;
      fixViewsAfterInsert(succ, newnode.prev, 1, Offset + index);
      return newnode;
    }
#endif
    #endregion

    #region Removal
    T remove(Node node, int index)
    {
      fixViewsBeforeSingleRemove(node, Offset + index);
      node.prev.next = node.next;
      node.next.prev = node.prev;
      size--;
      if (underlying != null)
        underlying.size--;
#if HASHINDEX
      removefromtaggroup(node);
#endif
      return node.item;
    }

#if HASHINDEX
    private bool dictremove(T item, out Node node)
    {
      if (underlying == null)
      {
        if (!dict.Remove(item, out node))
          return false;
      }
      else
      {
        //We cannot avoid calling dict twice - have to intersperse the listorder test!
        if (!contains(item, out node))
          return false;
        dict.Remove(item);
      }
      return true;
    }
#endif
    #endregion

    #region fixView utilities
    /// <summary>
    ///
    /// </summary>
    /// <param name="added">The actual number of inserted nodes</param>
    /// <param name="pred">The predecessor of the inserted nodes</param>
    /// <param name="succ">The successor of the added nodes</param>
    /// <param name="realInsertionIndex"></param>
    void fixViewsAfterInsert(Node succ, Node pred, int added, int realInsertionIndex)
    {
      if (views != null)
        foreach (LinkedList<T> view in views)
        {
          if (view != this)
          {
#if HASHINDEX
            if (pred.precedes(view.startsentinel) || (view.startsentinel == pred && view.size > 0))
              view.offset += added;
            if (view.startsentinel.precedes(pred) && succ.precedes(view.endsentinel))
              view.size += added;
            if (view.startsentinel == pred && view.size > 0)
              view.startsentinel = succ.prev;
            if (view.endsentinel == succ)
              view.endsentinel = pred.next;
#else
            if (view.Offset == realInsertionIndex && view.size > 0)
              view.startsentinel = succ.prev;
            if (view.Offset + view.size == realInsertionIndex)
              view.endsentinel = pred.next;
            if (view.Offset < realInsertionIndex && view.Offset + view.size > realInsertionIndex)
              view.size += added;
            if (view.Offset > realInsertionIndex || (view.Offset == realInsertionIndex && view.size > 0))
              view.offset += added;
#endif
          }
        }
    }

    void fixViewsBeforeSingleRemove(Node node, int realRemovalIndex)
    {
      if (views != null)
        foreach (LinkedList<T> view in views)
        {
          if (view != this)
          {
#if HASHINDEX
            if (view.startsentinel.precedes(node) && node.precedes(view.endsentinel))
              view.size--;
            if (!view.startsentinel.precedes(node))
              view.offset--;
            if (view.startsentinel == node)
              view.startsentinel = node.prev;
            if (view.endsentinel == node)
              view.endsentinel = node.next;
#else
            if (view.offset - 1 == realRemovalIndex)
              view.startsentinel = node.prev;
            if (view.offset + view.size == realRemovalIndex)
              view.endsentinel = node.next;
            if (view.offset <= realRemovalIndex && view.offset + view.size > realRemovalIndex)
              view.size--;
            if (view.offset > realRemovalIndex)
              view.offset--;
#endif
          }
        }
    }

#if HASHINDEX
#else
    void fixViewsBeforeRemove(int start, int count, Node first, Node last)
    {
      int clearend = start + count - 1;
      if (views != null)
        foreach (LinkedList<T> view in views)
        {
          if (view == this)
            continue;
          int viewoffset = view.Offset, viewend = viewoffset + view.size - 1;
          //sentinels
          if (start < viewoffset && viewoffset - 1 <= clearend)
            view.startsentinel = first.prev;
          if (start <= viewend + 1 && viewend < clearend)
            view.endsentinel = last.next;
          //offsets and sizes
          if (start < viewoffset)
          {
            if (clearend < viewoffset)
              view.offset = viewoffset - count;
            else
            {
              view.offset = start;
              view.size = clearend < viewend ? viewend - clearend : 0;
            }
          }
          else if (start <= viewend)
            view.size = clearend <= viewend ? view.size - count : start - viewoffset;
        }
    }
#endif

    /// <summary>
    ///
    /// </summary>
    /// <param name="otherView"></param>
    /// <returns>The position of View(otherOffset, otherSize) wrt. this view</returns>
    MutualViewPosition viewPosition(LinkedList<T> otherView)
    {
#if HASHINDEX
      Node otherstartsentinel = otherView.startsentinel, otherendsentinel = otherView.endsentinel,
        first = startsentinel.next, last = endsentinel.prev,
        otherfirst = otherstartsentinel.next, otherlast = otherendsentinel.prev;
      if (last.precedes(otherfirst) || otherlast.precedes(first))
        return MutualViewPosition.NonOverlapping;
      if (size == 0 || (otherstartsentinel.precedes(first) && last.precedes(otherendsentinel)))
        return MutualViewPosition.Contains;
      if (otherView.size == 0 || (startsentinel.precedes(otherfirst) && otherlast.precedes(endsentinel)))
        return MutualViewPosition.ContainedIn;
      return MutualViewPosition.Overlapping;
#else
      int end = offset + size, otherOffset = otherView.offset, otherSize = otherView.size, otherEnd = otherOffset + otherSize;
      if (otherOffset >= end || otherEnd <= offset)
        return MutualViewPosition.NonOverlapping;
      if (size == 0 || (otherOffset <= offset && end <= otherEnd))
        return MutualViewPosition.Contains;
      if (otherSize == 0 || (offset <= otherOffset && otherEnd <= end))
        return MutualViewPosition.ContainedIn;
      return MutualViewPosition.Overlapping;
#endif
    }

    void disposeOverlappingViews(bool reverse)
    {
      if (views != null)
      {
        foreach (LinkedList<T> view in views)
        {
          if (view != this)
          {
            switch (viewPosition(view))
            {
              case MutualViewPosition.ContainedIn:
                if (reverse)
                { }
                else
                  view.Dispose();
                break;
              case MutualViewPosition.Overlapping:
                view.Dispose();
                break;
              case MutualViewPosition.Contains:
              case MutualViewPosition.NonOverlapping:
                break;
            }
          }
        }
      }
    }

    #endregion

    #endregion

    #region Constructors

    /// <summary>
    /// Create a linked list with en external item equalityComparer
    /// </summary>
    /// <param name="itemequalityComparer">The external equalityComparer</param>
    public LinkedList(SCG.IEqualityComparer<T> itemequalityComparer)
      : base(itemequalityComparer)
    {
      offset = 0;
      size = stamp = 0;
      startsentinel = new Node(default(T));
      endsentinel = new Node(default(T));
      startsentinel.next = endsentinel;
      endsentinel.prev = startsentinel;
#if HASHINDEX
      //It is important that the sentinels are different:
      startsentinel.taggroup = new TagGroup();
      startsentinel.taggroup.tag = int.MinValue;
      startsentinel.taggroup.count = 0;
      endsentinel.taggroup = new TagGroup();
      endsentinel.taggroup.tag = int.MaxValue;
      endsentinel.taggroup.count = 0;
      dict = new HashDictionary<T, Node>(itemequalityComparer);
#endif
    }

    /// <summary>
    /// Create a linked list with the natural item equalityComparer
    /// </summary>
    public LinkedList() : this(EqualityComparer<T>.Default) { }

    #endregion

    #region Node nested class

    /// <summary>
    /// An individual cell in the linked list
    /// </summary>
    [Serializable]
    class Node
    {
      public Node prev;

      public Node next;

      public T item;

      #region Tag support
#if HASHINDEX
      internal int tag;

      internal TagGroup taggroup;

      internal bool precedes(Node that)
      {
        //Debug.Assert(taggroup != null, "taggroup field null");
        //Debug.Assert(that.taggroup != null, "that.taggroup field null");
        int t1 = taggroup.tag;
        int t2 = that.taggroup.tag;

        return t1 < t2 ? true : t1 > t2 ? false : tag < that.tag;
      }
#endif
      #endregion

      [Tested]
      internal Node(T item) { this.item = item; }

      [Tested]
      internal Node(T item, Node prev, Node next)
      {
        this.item = item; this.prev = prev; this.next = next;
      }

      public override string ToString()
      {
#if HASHINDEX
        return String.Format("Node: (item={0}, tag={1})", item, tag);
#else
        return String.Format("Node(item={0})", item);
#endif
      }
    }

    #endregion

    #region Taggroup nested class and tag maintenance utilities
#if HASHINDEX
    /// <summary>
    /// A group of nodes with the same high tag. Purpose is to be
    /// able to tell the sequence order of two nodes without having to scan through
    /// the list.
    /// </summary>
    [Serializable]
    class TagGroup
    {
      internal int tag, count;

      internal Node first, last;

      /// <summary>
      /// Pretty print a tag group
      /// </summary>
      /// <returns>Formatted tag group</returns>
      public override string ToString()
      { return String.Format("TagGroup(tag={0}, cnt={1}, fst={2}, lst={3})", tag, count, first, last); }
    }

    //Constants for tag maintenance
    const int wordsize = 32;

    const int lobits = 3;

    const int hibits = lobits + 1;

    const int losize = 1 << lobits;

    const int hisize = 1 << hibits;

    const int logwordsize = 5;

    TagGroup gettaggroup(Node pred, Node succ, out int lowbound, out int highbound)
    {
      TagGroup predgroup = pred.taggroup, succgroup = succ.taggroup;

      if (predgroup == succgroup)
      {
        lowbound = pred.tag + 1;
        highbound = succ.tag - 1;
        return predgroup;
      }
      else if (predgroup.first != null)
      {
        lowbound = pred.tag + 1;
        highbound = int.MaxValue;
        return predgroup;
      }
      else if (succgroup.first != null)
      {
        lowbound = int.MinValue;
        highbound = succ.tag - 1;
        return succgroup;
      }
      else
      {
        lowbound = int.MinValue;
        highbound = int.MaxValue;
        return new TagGroup();
      }
    }


    /// <summary>
    /// Put a tag on a node (already inserted in the list). Split taggroups and renumber as
    /// necessary.
    /// </summary>
    /// <param name="node">The node to tag</param>
    void settag(Node node)
    {
      Node pred = node.prev, succ = node.next;
      TagGroup predgroup = pred.taggroup, succgroup = succ.taggroup;

      if (predgroup == succgroup)
      {
        node.taggroup = predgroup;
        predgroup.count++;
        if (pred.tag + 1 == succ.tag)
          splittaggroup(predgroup);
        else
          node.tag = (pred.tag + 1) / 2 + (succ.tag - 1) / 2;
      }
      else if (predgroup.first != null)
      {
        node.taggroup = predgroup;
        predgroup.last = node;
        predgroup.count++;
        if (pred.tag == int.MaxValue)
          splittaggroup(predgroup);
        else
          node.tag = pred.tag / 2 + int.MaxValue / 2 + 1;
      }
      else if (succgroup.first != null)
      {
        node.taggroup = succgroup;
        succgroup.first = node;
        succgroup.count++;
        if (succ.tag == int.MinValue)
          splittaggroup(node.taggroup);
        else
          node.tag = int.MinValue / 2 + (succ.tag - 1) / 2;
      }
      else
      {
        Debug.Assert(Taggroups == 0);

        TagGroup newgroup = new TagGroup();

        Taggroups = 1;
        node.taggroup = newgroup;
        newgroup.first = newgroup.last = node;
        newgroup.count = 1;
        return;
      }
    }


    /// <summary>
    /// Remove a node from its taggroup.
    /// <br/> When this is called, node must already have been removed from the underlying list
    /// </summary>
    /// <param name="node">The node to remove</param>
    void removefromtaggroup(Node node)
    {

      TagGroup taggroup = node.taggroup;

      if (--taggroup.count == 0)
      {
        Taggroups--;
        return;
      }

      if (node == taggroup.first)
        taggroup.first = node.next;

      if (node == taggroup.last)
        taggroup.last = node.prev;

      //node.taggroup = null;
      if (taggroup.count != losize || Taggroups == 1)
        return;

      TagGroup otg;
      // bug20070911:
      Node neighbor;
      if ((neighbor = taggroup.first.prev) != startsentinel
          && (otg = neighbor.taggroup).count <= losize)
        taggroup.first = otg.first;
      else if ((neighbor = taggroup.last.next) != endsentinel
               && (otg = neighbor.taggroup).count <= losize)
        taggroup.last = otg.last;
      else
        return;

      Node n = otg.first;

      for (int i = 0, length = otg.count; i < length; i++)
      {
        n.taggroup = taggroup;
        n = n.next;
      }

      taggroup.count += otg.count;
      Taggroups--;
      n = taggroup.first;

      const int ofs = wordsize - hibits;

      for (int i = 0, count = taggroup.count; i < count; i++)
      {
        n.tag = (i - losize) << ofs; //(i-8)<<28
        n = n.next;
      }
    }


    /// <summary>
    /// Split a tag group to make rom for more tags.
    /// </summary>
    /// <param name="taggroup">The tag group</param>
    void splittaggroup(TagGroup taggroup)
    {
      Node n = taggroup.first;
      int ptgt = taggroup.first.prev.taggroup.tag;
      int ntgt = taggroup.last.next.taggroup.tag;

      Debug.Assert(ptgt + 1 <= ntgt - 1);

      int ofs = wordsize - hibits;
      int newtgs = (taggroup.count - 1) / hisize;
      int tgtdelta = (int)((ntgt + 0.0 - ptgt) / (newtgs + 2)), tgtag = ptgt;

      tgtdelta = tgtdelta == 0 ? 1 : tgtdelta;
      for (int j = 0; j < newtgs; j++)
      {
        TagGroup newtaggroup = new TagGroup();

        newtaggroup.tag = (tgtag = tgtag >= ntgt - tgtdelta ? ntgt : tgtag + tgtdelta);
        newtaggroup.first = n;
        newtaggroup.count = hisize;
        for (int i = 0; i < hisize; i++)
        {
          n.taggroup = newtaggroup;
          n.tag = (i - losize) << ofs; //(i-8)<<28
          n = n.next;
        }

        newtaggroup.last = n.prev;
      }

      int rest = taggroup.count - hisize * newtgs;

      taggroup.first = n;
      taggroup.count = rest;
      taggroup.tag = (tgtag = tgtag >= ntgt - tgtdelta ? ntgt : tgtag + tgtdelta); ofs--;
      for (int i = 0; i < rest; i++)
      {
        n.tag = (i - hisize) << ofs; //(i-16)<<27
        n = n.next;
      }

      taggroup.last = n.prev;
      Taggroups += newtgs;
      if (tgtag == ntgt)
        redistributetaggroups(taggroup);
    }


    private void redistributetaggroups(TagGroup taggroup)
    {
      TagGroup pred = taggroup, succ = taggroup, tmp;
      double limit = 1, bigt = Math.Pow(Taggroups, 1.0 / 30);//?????
      int bits = 1, count = 1, lowmask = 0, himask = 0, target = 0;

      do
      {
        bits++;
        lowmask = (1 << bits) - 1;
        himask = ~lowmask;
        target = taggroup.tag & himask;
        while ((tmp = pred.first.prev.taggroup).first != null && (tmp.tag & himask) == target)
        { count++; pred = tmp; }

        while ((tmp = succ.last.next.taggroup).last != null && (tmp.tag & himask) == target)
        { count++; succ = tmp; }

        limit *= bigt;
      } while (count > limit);

      //redistibute tags
      int lob = pred.first.prev.taggroup.tag, upb = succ.last.next.taggroup.tag;
      int delta = upb / (count + 1) - lob / (count + 1);

      Debug.Assert(delta > 0);
      for (int i = 0; i < count; i++)
      {
        pred.tag = lob + (i + 1) * delta;
        pred = pred.last.next.taggroup;
      }
    }
#endif

    #endregion

    #region Position, PositionComparer and ViewHandler nested types
    class PositionComparer : SCG.IComparer<Position>
    {
      static PositionComparer _default;
      PositionComparer() { }
      public static PositionComparer Default { get { return _default ?? (_default = new PositionComparer()); } }
      public int Compare(Position a, Position b)
      {
#if HASHINDEX
        return a.Endpoint == b.Endpoint ? 0 : a.Endpoint.precedes(b.Endpoint) ? -1 : 1;
#else
        return a.Index.CompareTo(b.Index);
#endif
      }
    }
    /// <summary>
    /// During RemoveAll, we need to cache the original endpoint indices of views
    /// </summary>
    struct Position
    {
      public readonly LinkedList<T> View;
      public bool Left;
#if HASHINDEX
      public readonly Node Endpoint;
#else
      public readonly int Index;
#endif
      public Position(LinkedList<T> view, bool left)
      {
        View = view;
        Left = left;
#if HASHINDEX
        Endpoint = left ? view.startsentinel.next : view.endsentinel.prev;
#else
        Index = left ? view.Offset : view.Offset + view.size - 1;
#endif
      }
#if HASHINDEX
      public Position(Node node, int foo) { this.Endpoint = node; View = null; Left = false; }
#else
      public Position(int index) { this.Index = index; View = null; Left = false; }
#endif
    }

    //TODO: merge the two implementations using Position values as arguments
    /// <summary>
    /// Handle the update of (other) views during a multi-remove operation.
    /// </summary>
    struct ViewHandler
    {
      ArrayList<Position> leftEnds;
      ArrayList<Position> rightEnds;
      int leftEndIndex, rightEndIndex, leftEndIndex2, rightEndIndex2;
      internal readonly int viewCount;
      internal ViewHandler(LinkedList<T> list)
      {
        leftEndIndex = rightEndIndex = leftEndIndex2 = rightEndIndex2 = viewCount = 0;
        leftEnds = rightEnds = null;
        if (list.views != null)
          foreach (LinkedList<T> v in list.views)
            if (v != list)
            {
              if (leftEnds == null)
              {
                leftEnds = new ArrayList<Position>();
                rightEnds = new ArrayList<Position>();
              }
              leftEnds.Add(new Position(v, true));
              rightEnds.Add(new Position(v, false));
            }
        if (leftEnds == null)
          return;
        viewCount = leftEnds.Count;
        leftEnds.Sort(PositionComparer.Default);
        rightEnds.Sort(PositionComparer.Default);
      }
#if HASHINDEX
      internal void skipEndpoints(int removed, Node n)
      {
        if (viewCount > 0)
        {
          Position endpoint;
          while (leftEndIndex < viewCount && ((endpoint = leftEnds[leftEndIndex]).Endpoint.prev.precedes(n)))
          {
            LinkedList<T> view = endpoint.View;
            view.offset = view.offset - removed;//TODO: extract offset.Value?
            view.size += removed;
            leftEndIndex++;
          }
          while (rightEndIndex < viewCount && (endpoint = rightEnds[rightEndIndex]).Endpoint.precedes(n))
          {
            LinkedList<T> view = endpoint.View;
            view.size -= removed;
            rightEndIndex++;
          }
        }
        if (viewCount > 0)
        {
          Position endpoint;
          while (leftEndIndex2 < viewCount && (endpoint = leftEnds[leftEndIndex2]).Endpoint.prev.precedes(n))
            leftEndIndex2++;
          while (rightEndIndex2 < viewCount && (endpoint = rightEnds[rightEndIndex2]).Endpoint.next.precedes(n))
            rightEndIndex2++;
        }
      }
      /// <summary>
      /// To be called with n pointing to the right of each node to be removed in a stretch.
      /// And at the endsentinel.
      ///
      /// Update offset of a view whose left endpoint (has not already been handled and) is n or precedes n.
      /// I.e. startsentinel precedes n.
      /// Also update the size as a prelude to handling the right endpoint.
      ///
      /// Update size of a view not already handled and whose right endpoint precedes n.
      /// </summary>
      /// <param name="removed">The number of nodes left of n to be removed</param>
      /// <param name="n"></param>
      internal void updateViewSizesAndCounts(int removed, Node n)
      {
        if (viewCount > 0)
        {
          Position endpoint;
          while (leftEndIndex < viewCount && ((endpoint = leftEnds[leftEndIndex]).Endpoint.prev.precedes(n)))
          {
            LinkedList<T> view = endpoint.View;
            view.offset = view.offset - removed; //TODO: fix use of offset
            view.size += removed;
            leftEndIndex++;
          }
          while (rightEndIndex < viewCount && (endpoint = rightEnds[rightEndIndex]).Endpoint.precedes(n))
          {
            LinkedList<T> view = endpoint.View;
            view.size -= removed;
            rightEndIndex++;
          }
        }
      }
      /// <summary>
      /// To be called with n being the first not-to-be-removed node after a (stretch of) node(s) to be removed.
      ///
      /// It will update the startsentinel of views (that have not been handled before and)
      /// whose startsentinel precedes n, i.e. is to be deleted.
      ///
      /// It will update the endsentinel of views (...) whose endsentinel precedes n, i.e. is to be deleted.
      ///
      /// PROBLEM: DOESNT WORK AS ORIGINALLY ADVERTISED. WE MUST DO THIS BEFORE WE ACTUALLY REMOVE THE NODES. WHEN THE
      /// NODES HAVE BEEN REMOVED, THE precedes METHOD WILL NOT WORK!
      /// </summary>
      /// <param name="n"></param>
      /// <param name="newstart"></param>
      /// <param name="newend"></param>
      internal void updateSentinels(Node n, Node newstart, Node newend)
      {
        if (viewCount > 0)
        {
          Position endpoint;
          while (leftEndIndex2 < viewCount && (endpoint = leftEnds[leftEndIndex2]).Endpoint.prev.precedes(n))
          {
            LinkedList<T> view = endpoint.View;
            view.startsentinel = newstart;
            leftEndIndex2++;
          }
          while (rightEndIndex2 < viewCount && (endpoint = rightEnds[rightEndIndex2]).Endpoint.next.precedes(n))
          {
            LinkedList<T> view = endpoint.View;
            view.endsentinel = newend;
            rightEndIndex2++;
          }
        }
      }
#else
      /// <summary>
      /// This is to be called with realindex pointing to the first node to be removed after a (stretch of) node that was not removed
      /// </summary>
      /// <param name="removed"></param>
      /// <param name="realindex"></param>
      internal void skipEndpoints(int removed, int realindex)
      {
        if (viewCount > 0)
        {
          Position endpoint;
          while (leftEndIndex < viewCount && (endpoint = leftEnds[leftEndIndex]).Index <= realindex)
          {
            LinkedList<T> view = endpoint.View;
            view.offset = view.offset - removed;
            view.size += removed;
            leftEndIndex++;
          }
          while (rightEndIndex < viewCount && (endpoint = rightEnds[rightEndIndex]).Index < realindex)
          {
            LinkedList<T> view = endpoint.View;
            view.size -= removed;
            rightEndIndex++;
          }
        }
        if (viewCount > 0)
        {
          Position endpoint;
          while (leftEndIndex2 < viewCount && (endpoint = leftEnds[leftEndIndex2]).Index <= realindex)
            leftEndIndex2++;
          while (rightEndIndex2 < viewCount && (endpoint = rightEnds[rightEndIndex2]).Index < realindex - 1)
            rightEndIndex2++;
        }
      }
      internal void updateViewSizesAndCounts(int removed, int realindex)
      {
        if (viewCount > 0)
        {
          Position endpoint;
          while (leftEndIndex < viewCount && (endpoint = leftEnds[leftEndIndex]).Index <= realindex)
          {
            LinkedList<T> view = endpoint.View;
            view.offset = view.Offset - removed;
            view.size += removed;
            leftEndIndex++;
          }
          while (rightEndIndex < viewCount && (endpoint = rightEnds[rightEndIndex]).Index < realindex)
          {
            LinkedList<T> view = endpoint.View;
            view.size -= removed;
            rightEndIndex++;
          }
        }
      }
      internal void updateSentinels(int realindex, Node newstart, Node newend)
      {
        if (viewCount > 0)
        {
          Position endpoint;
          while (leftEndIndex2 < viewCount && (endpoint = leftEnds[leftEndIndex2]).Index <= realindex)
          {
            LinkedList<T> view = endpoint.View;
            view.startsentinel = newstart;
            leftEndIndex2++;
          }
          while (rightEndIndex2 < viewCount && (endpoint = rightEnds[rightEndIndex2]).Index < realindex - 1)
          {
            LinkedList<T> view = endpoint.View;
            view.endsentinel = newend;
            rightEndIndex2++;
          }
        }
      }
#endif
    }
    #endregion

    #region Range nested class

    class Range : DirectedCollectionValueBase<T>, IDirectedCollectionValue<T>
    {
      int start, count, rangestamp;
      Node startnode, endnode;

      LinkedList<T> list;

      bool forwards;


      internal Range(LinkedList<T> list, int start, int count, bool forwards)
      {
        this.list = list; this.rangestamp = list.underlying != null ? list.underlying.stamp : list.stamp;
        this.start = start; this.count = count; this.forwards = forwards;
        if (count > 0)
        {
          startnode = list.get(start);
          endnode = list.get(start + count - 1);
        }
      }

      public override bool IsEmpty { get { list.modifycheck(rangestamp); return count == 0; } }

      [Tested]
      public override int Count { [Tested]get { list.modifycheck(rangestamp); return count; } }


      public override Speed CountSpeed { get { list.modifycheck(rangestamp); return Speed.Constant; } }


      public override T Choose()
      {
        list.modifycheck(rangestamp);
        if (count > 0) return startnode.item;
        throw new NoSuchItemException();
      }


      [Tested]
      public override SCG.IEnumerator<T> GetEnumerator()
      {
        int togo = count;

        list.modifycheck(rangestamp);
        if (togo == 0)
          yield break;

        Node cursor = forwards ? startnode : endnode;

        yield return cursor.item;
        while (--togo > 0)
        {
          cursor = forwards ? cursor.next : cursor.prev;
          list.modifycheck(rangestamp);
          yield return cursor.item;
        }
      }


      [Tested]
      public override IDirectedCollectionValue<T> Backwards()
      {
        list.modifycheck(rangestamp);

        Range b = (Range)MemberwiseClone();

        b.forwards = !forwards;
        return b;
      }


      [Tested]
      IDirectedEnumerable<T> IDirectedEnumerable<T>.Backwards() { return Backwards(); }


      [Tested]
      public override EnumerationDirection Direction
      {
        [Tested]
        get
        { return forwards ? EnumerationDirection.Forwards : EnumerationDirection.Backwards; }
      }
    }


    #endregion

    #region IDisposable Members

    /// <summary>
    /// Invalidate this list. If a view, just invalidate the view.
    /// If not a view, invalidate the list and all views on it.
    /// </summary>
    public virtual void Dispose()
    {
      Dispose(false);
    }

    void Dispose(bool disposingUnderlying)
    {
      if (isValid)
      {
        if (underlying != null)
        {
          isValid = false;
          if (!disposingUnderlying && views != null)
            views.Remove(myWeakReference);
          endsentinel = null;
          startsentinel = null;
          underlying = null;
          views = null;
          myWeakReference = null;
        }
        else
        {
          //isValid = false;
          //endsentinel = null;
          //startsentinel = null;
          if (views != null)
            foreach (LinkedList<T> view in views)
              view.Dispose(true);
          //views = null;
          Clear();
        }
      }
    }

    #endregion IDisposable stuff

    #region IList<T> Members

    /// <summary>
    /// </summary>
    /// <exception cref="NoSuchItemException"> if this list is empty.</exception>
    /// <value>The first item in this list.</value>
    [Tested]
    public virtual T First
    {
      [Tested]
      get
      {
        validitycheck();
        if (size == 0)
          throw new NoSuchItemException();
        return startsentinel.next.item;
      }
    }


    /// <summary>
    /// </summary>
    /// <exception cref="NoSuchItemException"> if this list is empty.</exception>
    /// <value>The last item in this list.</value>
    [Tested]
    public virtual T Last
    {
      [Tested]
      get
      {
        validitycheck();
        if (size == 0)
          throw new NoSuchItemException();
        return endsentinel.prev.item;
      }
    }

    /// <summary>
    /// Since <code>Add(T item)</code> always add at the end of the list,
    /// this describes if list has FIFO or LIFO semantics.
    /// </summary>
    /// <value>True if the <code>Remove()</code> operation removes from the
    /// start of the list, false if it removes from the end. THe default for a new linked list is true.</value>
    [Tested]
    public virtual bool FIFO
    {
      [Tested]
      get { validitycheck(); return fIFO; }
      [Tested]
      set { updatecheck(); fIFO = value; }
    }

    /// <summary>
    ///
    /// </summary>
    public virtual bool IsFixedSize
    {
      get { validitycheck(); return false; }
    }

    /// <summary>
    /// On this list, this indexer is read/write.
    /// <exception cref="IndexOutOfRangeException"/> if i is negative or
    /// &gt;= the size of the collection.
    /// </summary>
    /// <value>The i'th item of this list.</value>
    /// <param name="index">The index of the item to fetch or store.</param>
    [Tested]
    public virtual T this[int index]
    {
      [Tested]
      get { validitycheck(); return get(index).item; }
      [Tested]
      set
      {
        updatecheck();
        Node n = get(index);
        //
        T item = n.item;
#if HASHINDEX

        if (itemequalityComparer.Equals(value, item))
        {
          n.item = value;
          dict.Update(value, n);
        }
        else if (!dict.FindOrAdd(value, ref n))
        {
          dict.Remove(item);
          n.item = value;
        }
        else
          throw new ArgumentException("Item already in indexed list");
#else
        n.item = value;
#endif
        (underlying ?? this).raiseForSetThis(index, value, item);
      }
    }

    /// <summary>
    ///
    /// </summary>
    /// <value></value>
    public virtual Speed IndexingSpeed { get { return Speed.Linear; } }

    /// <summary>
    /// Insert an item at a specific index location in this list.
    /// <exception cref="IndexOutOfRangeException"/> if i is negative or
    /// &gt; the size of the collection.</summary>
    /// <param name="i">The index at which to insert.</param>
    /// <param name="item">The item to insert.</param>
    [Tested]
    public virtual void Insert(int i, T item)
    {
      updatecheck();
      insert(i, i == size ? endsentinel : get(i), item);
      if (ActiveEvents != EventTypeEnum.None)
        (underlying ?? this).raiseForInsert(i + Offset, item);
    }

    /// <summary>
    /// Insert an item at the end of a compatible view, used as a pointer.
    /// <para>The <code>pointer</code> must be a view on the same list as
    /// <code>this</code> and the endpoitn of <code>pointer</code> must be
    /// a valid insertion point of <code>this</code></para>
    /// </summary>
    /// <exception cref="IncompatibleViewException">If <code>pointer</code>
    /// is not a view on the same list as <code>this</code></exception>
    /// <exception cref="IndexOutOfRangeException"><b>??????</b> if the endpoint of
    ///  <code>pointer</code> is not inside <code>this</code></exception>
    /// <exception cref="DuplicateNotAllowedException"> if the list has
    /// <code>AllowsDuplicates==false</code> and the item is
    /// already in the list.</exception>
    /// <param name="pointer"></param>
    /// <param name="item"></param>
    public void Insert(IList<T> pointer, T item)
    {
      updatecheck();
      if ((pointer == null) || ((pointer.Underlying ?? pointer) != (underlying ?? this)))
        throw new IncompatibleViewException();
#warning INEFFICIENT
      //TODO: make this efficient (the whole point of the method:
      //Do NOT use Insert, but insert the node at pointer.endsentinel, checking
      //via the ordering that this is a valid insertion point
      Insert(pointer.Offset + pointer.Count - Offset, item);
    }

    /// <summary>
    /// Insert into this list all items from an enumerable collection starting
    /// at a particular index.
    /// <exception cref="IndexOutOfRangeException"/> if i is negative or
    /// &gt; the size of the collection.
    /// </summary>
    /// <param name="i">Index to start inserting at</param>
    /// <param name="items">Items to insert</param>
    /// <typeparam name="U"></typeparam>
    [Tested]
    public virtual void InsertAll<U>(int i, SCG.IEnumerable<U> items) where U : T
    {
      insertAll(i, items, true);
    }

    void insertAll<U>(int i, SCG.IEnumerable<U> items, bool insertion) where U : T
    {
      updatecheck();
      Node succ, node, pred;
      int count = 0;
      succ = i == size ? endsentinel : get(i);
      pred = node = succ.prev;
#if HASHINDEX
      TagGroup taggroup = null;
      int taglimit = 0, thetag = 0;
      taggroup = gettaggroup(node, succ, out thetag, out taglimit);
      try
      {
        foreach (T item in items)
        {
          Node tmp = new Node(item, node, null);
          if (!dict.FindOrAdd(item, ref tmp))
          {
            tmp.tag = thetag < taglimit ? ++thetag : thetag;
            tmp.taggroup = taggroup;
            node.next = tmp;
            count++;
            node = tmp;
          }
          else
            throw new DuplicateNotAllowedException("Item already in indexed list");
        }
      }
      finally
      {
        if (count != 0)
        {
          taggroup.count += count;
          if (taggroup != pred.taggroup)
            taggroup.first = pred.next;
          if (taggroup != succ.taggroup)
            taggroup.last = node;
          succ.prev = node;
          node.next = succ;
          if (node.tag == node.prev.tag)
            splittaggroup(taggroup);
          size += count;
          if (underlying != null)
            underlying.size += count;
          fixViewsAfterInsert(succ, pred, count, 0);
          raiseForInsertAll(pred, i, count, insertion);
        }
      }
#else
      foreach (T item in items)
      {
        Node tmp = new Node(item, node, null);
        node.next = tmp;
        count++;
        node = tmp;
      }
      if (count == 0)
        return;
      succ.prev = node;
      node.next = succ;
      size += count;
      if (underlying != null)
        underlying.size += count;
      if (count > 0)
      {
        fixViewsAfterInsert(succ, pred, count, offset + i);
        raiseForInsertAll(pred, i, count, insertion);
      }
#endif
    }

    private void raiseForInsertAll(Node node, int i, int added, bool insertion)
    {
      if (ActiveEvents != 0)
      {
        int index = Offset + i;
        if ((ActiveEvents & (EventTypeEnum.Added | EventTypeEnum.Inserted)) != 0)
          for (int j = index; j < index + added; j++)
          {
#warning must we check stamps here?
            node = node.next;
            T item = node.item;
            if (insertion) raiseItemInserted(item, j);
            raiseItemsAdded(item, 1);
          }
        raiseCollectionChanged();
      }
    }

    /// <summary>
    /// Insert an item at the front of this list.
    /// </summary>
    /// <param name="item">The item to insert.</param>
    [Tested]
    public virtual void InsertFirst(T item)
    {
      updatecheck();
      insert(0, startsentinel.next, item);
      if (ActiveEvents != EventTypeEnum.None)
        (underlying ?? this).raiseForInsert(0 + Offset, item);
    }

    /// <summary>
    /// Insert an item at the back of this list.
    /// </summary>
    /// <param name="item">The item to insert.</param>
    [Tested]
    public virtual void InsertLast(T item)
    {
      updatecheck();
      insert(size, endsentinel, item);
      if (ActiveEvents != EventTypeEnum.None)
        (underlying ?? this).raiseForInsert(size - 1 + Offset, item);
    }

    /// <summary>
    /// Create a new list consisting of the results of mapping all items of this
    /// list.
    /// </summary>
    /// <param name="mapper">The delegate defining the map.</param>
    /// <returns>The new list.</returns>
    [Tested]
    public IList<V> Map<V>(Fun<T, V> mapper)
    {
      validitycheck();

      LinkedList<V> retval = new LinkedList<V>();
      return map<V>(mapper, retval);
    }

    /// <summary>
    /// Create a new list consisting of the results of mapping all items of this
    /// list. The new list will use a specified equalityComparer for the item type.
    /// </summary>
    /// <typeparam name="V">The type of items of the new list</typeparam>
    /// <param name="mapper">The delegate defining the map.</param>
    /// <param name="equalityComparer">The equalityComparer to use for the new list</param>
    /// <returns>The new list.</returns>
    public IList<V> Map<V>(Fun<T, V> mapper, SCG.IEqualityComparer<V> equalityComparer)
    {
      validitycheck();

      LinkedList<V> retval = new LinkedList<V>(equalityComparer);
      return map<V>(mapper, retval);
    }

    private IList<V> map<V>(Fun<T, V> mapper, LinkedList<V> retval)
    {
      if (size == 0)
        return retval;
      int stamp = this.stamp;
      Node cursor = startsentinel.next;
      LinkedList<V>.Node mcursor = retval.startsentinel;

#if HASHINDEX
      double tagdelta = int.MaxValue / (size + 1.0);
      int count = 1;
      LinkedList<V>.TagGroup taggroup = null;
      taggroup = new LinkedList<V>.TagGroup();
      retval.taggroups = 1;
      taggroup.count = size;
#endif
      while (cursor != endsentinel)
      {
        V v = mapper(cursor.item);
        modifycheck(stamp);
        mcursor.next = new LinkedList<V>.Node(v, mcursor, null);
        cursor = cursor.next;
        mcursor = mcursor.next;
#if HASHINDEX
        retval.dict.Add(v, mcursor);
        mcursor.taggroup = taggroup;
        mcursor.tag = (int)(tagdelta * count++);
#endif
      }

#if HASHINDEX
      taggroup.first = retval.startsentinel.next;
      taggroup.last = mcursor;
#endif
      retval.endsentinel.prev = mcursor;
      mcursor.next = retval.endsentinel;
      retval.size = size;
      return retval;
    }

    /// <summary>
    /// Remove one item from the list: from the front if <code>FIFO</code>
    /// is true, else from the back.
    /// <exception cref="NoSuchItemException"/> if this list is empty.
    /// </summary>
    /// <returns>The removed item.</returns>
    [Tested]
    public virtual T Remove()
    {
      updatecheck();
      if (size == 0)
        throw new NoSuchItemException("List is empty");
      T item = fIFO ? remove(startsentinel.next, 0) : remove(endsentinel.prev, size - 1);
#if HASHINDEX
      dict.Remove(item);
#endif
      (underlying ?? this).raiseForRemove(item);
      return item;
    }

    /// <summary>
    /// Remove one item from the front of the list.
    /// <exception cref="NoSuchItemException"/> if this list is empty.
    /// </summary>
    /// <returns>The removed item.</returns>
    [Tested]
    public virtual T RemoveFirst()
    {
      updatecheck();
      if (size == 0)
        throw new NoSuchItemException("List is empty");

      T item = remove(startsentinel.next, 0);
#if HASHINDEX
      dict.Remove(item);
#endif
      if (ActiveEvents != EventTypeEnum.None)
        (underlying ?? this).raiseForRemoveAt(Offset, item);
      return item;
    }

    /// <summary>
    /// Remove one item from the back of the list.
    /// <exception cref="NoSuchItemException"/> if this list is empty.
    /// </summary>
    /// <returns>The removed item.</returns>
    [Tested]
    public virtual T RemoveLast()
    {
      updatecheck();
      if (size == 0)
        throw new NoSuchItemException("List is empty");

      T item = remove(endsentinel.prev, size - 1);
#if HASHINDEX
      dict.Remove(item);
#endif
      if (ActiveEvents != EventTypeEnum.None)
        (underlying ?? this).raiseForRemoveAt(size + Offset, item);
      return item;
    }

    /// <summary>
    /// Create a list view on this list.
    /// </summary>
    /// <exception cref="ArgumentOutOfRangeException"> if the start or count is negative</exception>
    /// <exception cref="ArgumentException"> if the range does not fit within list.</exception>
    /// <param name="start">The index in this list of the start of the view.</param>
    /// <param name="count">The size of the view.</param>
    /// <returns>The new list view.</returns>
    [Tested]
    public virtual IList<T> View(int start, int count)
    {
      checkRange(start, count);
      validitycheck();
      if (views == null)
        views = new WeakViewList<LinkedList<T>>();
      LinkedList<T> retval = (LinkedList<T>)MemberwiseClone();
      retval.underlying = underlying != null ? underlying : this;
      retval.offset = offset + start;
      retval.size = count;
      getPair(start - 1, start + count, out retval.startsentinel, out retval.endsentinel,
          new int[] { -1, size }, new Node[] { startsentinel, endsentinel });
      //retval.startsentinel = start == 0 ? startsentinel : get(start - 1);
      //retval.endsentinel = start + count == size ? endsentinel : get(start + count);

      //TODO: for the purpose of Dispose, we need to retain a ref to the node
      retval.myWeakReference = views.Add(retval);
      return retval;
    }

    /// <summary>
    /// Create a list view on this list containing the (first) occurrence of a particular item.
    /// </summary>
    /// <exception cref="ArgumentException"> if the item is not in this list.</exception>
    /// <param name="item">The item to find.</param>
    /// <returns>The new list view.</returns>
    public virtual IList<T> ViewOf(T item)
    {
#if HASHINDEX
      Node n;
      validitycheck();
      if (!contains(item, out n))
        return null;
      LinkedList<T> retval = (LinkedList<T>)MemberwiseClone();
      retval.underlying = underlying != null ? underlying : this;
      retval.offset = null;
      retval.startsentinel = n.prev;
      retval.endsentinel = n.next;
      retval.size = 1;
      return retval;
#else
      int index = 0;
      Node n = startsentinel.next;
      if (!find(item, ref n, ref index))
        return null;
      //TODO: optimize with getpair!
      return View(index, 1);
#endif
    }

    /// <summary>
    /// Create a list view on this list containing the last occurrence of a particular item.
    /// <exception cref="ArgumentException"/> if the item is not in this list.
    /// </summary>
    /// <param name="item">The item to find.</param>
    /// <returns>The new list view.</returns>
    public virtual IList<T> LastViewOf(T item)
    {
#if HASHINDEX
      return ViewOf(item);
#else
      int index = size - 1;
      Node n = endsentinel.prev;
      if (!dnif(item, ref n, ref index))
        return null;
      return View(index, 1);
#endif
    }

    /// <summary>
    /// Null if this list is not a view.
    /// </summary>
    /// <value>Underlying list for view.</value>
    [Tested]
    public virtual IList<T> Underlying { [Tested]get { validitycheck(); return underlying; } }

    /// <summary>
    ///
    /// </summary>
    /// <value></value>
    public virtual bool IsValid { get { return isValid; } }

    /// <summary>
    /// </summary>
    /// <value>Offset for this list view or 0 for a underlying list.</value>
    [Tested]
    public virtual int Offset
    {
      [Tested]
      get
      {
        validitycheck();
#if HASHINDEX
        if (offset == null && underlying != null)
        {
          //TODO: search from both ends simultaneously!
          Node n = underlying.startsentinel;
          int i = 0;
          while (n != startsentinel) { n = n.next; i++; }
          offset = i;
        }
#endif
        return (int)offset;
      }
    }

    /// <summary>
    /// Slide this list view along the underlying list.
    /// </summary>
    /// <exception cref="NotAViewException"> if this list is not a view.</exception>
    /// <exception cref="ArgumentOutOfRangeException"> if the operation
    /// would bring either end of the view outside the underlying list.</exception>
    /// <param name="offset">The signed amount to slide: positive to slide
    /// towards the end.</param>
    [Tested]
    public IList<T> Slide(int offset)
    {
      if (!TrySlide(offset, size))
        throw new ArgumentOutOfRangeException();
      return this;
    }

    //TODO: more test cases
    /// <summary>
    /// Slide this list view along the underlying list, perhaps changing its size.
    /// </summary>
    /// <exception cref="NotAViewException"> if this list is not a view.</exception>
    /// <exception cref="ArgumentOutOfRangeException"> if the operation
    /// would bring either end of the view outside the underlying list.</exception>
    /// <param name="offset">The signed amount to slide: positive to slide
    /// towards the end.</param>
    /// <param name="size">The new size of the view.</param>
    public IList<T> Slide(int offset, int size)
    {
      if (!TrySlide(offset, size))
        throw new ArgumentOutOfRangeException();
      return this;
    }

    /// <summary>
    ///
    /// </summary>
    /// <param name="offset"></param>
    /// <returns></returns>
    public virtual bool TrySlide(int offset) { return TrySlide(offset, size); }

    /// <summary>
    ///
    /// </summary>
    /// <param name="offset"></param>
    /// <param name="size"></param>
    /// <returns></returns>
    public virtual bool TrySlide(int offset, int size)
    {
      updatecheck();
      if (underlying == null)
        throw new NotAViewException("List not a view");

#pragma warning disable 472
      if (this.offset == null) //Note: only possible with HASHINDEX
#pragma warning restore 472
      {
#pragma warning disable 162
        try
        {
          getPair(offset - 1, offset + size, out startsentinel, out endsentinel,
              new int[] { -1, this.size }, new Node[] { startsentinel, endsentinel });
          //TODO: maybe-update offset field
        }
        catch (NullReferenceException)
        {
          return false;
        }
#pragma warning restore 162
      }
      else
      {
        if (offset + this.offset < 0 || offset + this.offset + size > underlying.size)
          return false;
        int oldoffset = (int)(this.offset);
        getPair(offset - 1, offset + size, out startsentinel, out endsentinel,
            new int[] { -oldoffset - 1, -1, this.size, underlying.size - oldoffset },
            new Node[] { underlying.startsentinel, startsentinel, endsentinel, underlying.endsentinel });
      }
      this.size = size;
      this.offset += offset;
      return true;
    }


    //TODO: improve the complexity of the implementation
    /// <summary>
    ///
    /// <para>Returns null if <code>otherView</code> is strictly to the left of this view</para>
    /// </summary>
    /// <param name="otherView"></param>
    /// <exception cref="IncompatibleViewException">If otherView does not have the same underlying list as this</exception>
    /// <returns></returns>
    public virtual IList<T> Span(IList<T> otherView)
    {
      if ((otherView == null) || ((otherView.Underlying ?? otherView) != (underlying ?? this)))
        throw new IncompatibleViewException();
      if (otherView.Offset + otherView.Count - Offset < 0)
        return null;
      return (underlying ?? this).View(Offset, otherView.Offset + otherView.Count - Offset);
    }


    //Question: should we swap items or move nodes around?
    //The first seems much more efficient unless the items are value types
    //with a large memory footprint.
    //(Swapping will do count*3/2 T assignments, linking around will do
    // 4*count ref assignments; note that ref assignments are more expensive
    //than copying non-ref bits)
    /// <summary>
    /// Reverse the list so the items are in the opposite sequence order.
    /// </summary>
    [Tested]
    public virtual void Reverse()
    {
      updatecheck();
      if (size == 0)
        return;

      Position[] positions = null;
      int poslow = 0, poshigh = 0;
      if (views != null)
      {
        CircularQueue<Position> _positions = null;
        foreach (LinkedList<T> view in views)
        {
          if (view != this)
          {
            switch (viewPosition(view))
            {
              case MutualViewPosition.ContainedIn:
                (_positions ?? (_positions = new CircularQueue<Position>())).Enqueue(new Position(view, true));
                _positions.Enqueue(new Position(view, false));
                break;
              case MutualViewPosition.Overlapping:
                view.Dispose();
                break;
              case MutualViewPosition.Contains:
              case MutualViewPosition.NonOverlapping:
                break;
            }
          }
        }
        if (_positions != null)
        {
          positions = _positions.ToArray();
          Sorting.IntroSort<Position>(positions, 0, positions.Length, PositionComparer.Default);
          poshigh = positions.Length - 1;
        }
      }

      Node a = get(0), b = get(size - 1);
      for (int i = 0; i < size / 2; i++)
      {
        T swap;
        swap = a.item; a.item = b.item; b.item = swap;
#if HASHINDEX
        dict[a.item] = a; dict[b.item] = b;
#endif
        if (positions != null)
          mirrorViewSentinelsForReverse(positions, ref poslow, ref poshigh, a, b, i);
        a = a.next; b = b.prev;
      }
      if (positions != null && size % 2 != 0)
        mirrorViewSentinelsForReverse(positions, ref poslow, ref poshigh, a, b, size / 2);
      (underlying ?? this).raiseCollectionChanged();
    }

    private void mirrorViewSentinelsForReverse(Position[] positions, ref int poslow, ref int poshigh, Node a, Node b, int i)
    {
#if HASHINDEX
      int? aindex = offset + i, bindex = offset + size - 1 - i;
#else
      int aindex = offset + i, bindex = offset + size - 1 - i;
#endif
      Position pos;
#if HASHINDEX
      while (poslow <= poshigh && (pos = positions[poslow]).Endpoint == a)
#else
      while (poslow <= poshigh && (pos = positions[poslow]).Index == aindex)
#endif
      {
        //TODO: Note: in the case og hashed linked list, if this.offset == null, but pos.View.offset!=null
        //we may at this point compute this.offset and non-null values of aindex and bindex
        if (pos.Left)
          pos.View.endsentinel = b.next;
        else
        {
          pos.View.startsentinel = b.prev;
          pos.View.offset = bindex;
        }
        poslow++;
      }
#if HASHINDEX
      while (poslow < poshigh && (pos = positions[poshigh]).Endpoint == b)
#else
      while (poslow < poshigh && (pos = positions[poshigh]).Index == bindex)
#endif
      {
        if (pos.Left)
          pos.View.endsentinel = a.next;
        else
        {
          pos.View.startsentinel = a.prev;
          pos.View.offset = aindex;
        }
        poshigh--;
      }
    }

    /// <summary>
    /// Check if this list is sorted according to the default sorting order
    /// for the item type T, as defined by the <see cref="T:C5.Comparer`1"/> class
    /// </summary>
    /// <exception cref="NotComparableException">if T is not comparable</exception>
    /// <returns>True if the list is sorted, else false.</returns>
    public bool IsSorted() { return IsSorted(Comparer<T>.Default); }

    /// <summary>
    /// Check if this list is sorted according to a specific sorting order.
    /// </summary>
    /// <param name="c">The comparer defining the sorting order.</param>
    /// <returns>True if the list is sorted, else false.</returns>
    [Tested]
    public virtual bool IsSorted(SCG.IComparer<T> c)
    {
      validitycheck();
      if (size <= 1)
        return true;

      Node node = startsentinel.next;
      T prevItem = node.item;

      node = node.next;
      while (node != endsentinel)
      {
        if (c.Compare(prevItem, node.item) > 0)
          return false;
        else
        {
          prevItem = node.item;
          node = node.next;
        }
      }

      return true;
    }

    /// <summary>
    /// Sort the items of the list according to the default sorting order
    /// for the item type T, as defined by the Comparer[T] class.
    /// (<see cref="T:C5.Comparer`1"/>).
    /// The sorting is stable.
    /// </summary>
    /// <exception cref="InvalidOperationException">if T is not comparable</exception>
    public virtual void Sort() { Sort(Comparer<T>.Default); }

    // Sort the linked list using mergesort
    /// <summary>
    /// Sort the items of the list according to a specific sorting order.
    /// The sorting is stable.
    /// </summary>
    /// <param name="c">The comparer defining the sorting order.</param>
    [Tested]
    public virtual void Sort(SCG.IComparer<T> c)
    {
      updatecheck();
      if (size == 0)
        return;
      disposeOverlappingViews(false);
#if HASHINDEX
      if (underlying != null)
      {
        Node cursor = startsentinel.next;
        while (cursor != endsentinel)
        {
          cursor.taggroup.count--;
          cursor = cursor.next;
        }
      }
#endif
      // Build a linked list of non-empty runs.
      // The prev field in first node of a run points to next run's first node
      Node runTail = startsentinel.next;
      Node prevNode = startsentinel.next;

      endsentinel.prev.next = null;
      while (prevNode != null)
      {
        Node node = prevNode.next;

        while (node != null && c.Compare(prevNode.item, node.item) <= 0)
        {
          prevNode = node;
          node = prevNode.next;
        }

        // Completed a run; prevNode is the last node of that run
        prevNode.next = null;	// Finish the run
        runTail.prev = node;	// Link it into the chain of runs
        runTail = node;
        if (c.Compare(endsentinel.prev.item, prevNode.item) <= 0)
          endsentinel.prev = prevNode;	// Update last pointer to point to largest

        prevNode = node;		// Start a new run
      }

      // Repeatedly merge runs two and two, until only one run remains
      while (startsentinel.next.prev != null)
      {
        Node run = startsentinel.next;
        Node newRunTail = null;

        while (run != null && run.prev != null)
        { // At least two runs, merge
          Node nextRun = run.prev.prev;
          Node newrun = mergeRuns(run, run.prev, c);

          if (newRunTail != null)
            newRunTail.prev = newrun;
          else
            startsentinel.next = newrun;

          newRunTail = newrun;
          run = nextRun;
        }

        if (run != null) // Add the last run, if any
          newRunTail.prev = run;
      }

      endsentinel.prev.next = endsentinel;
      startsentinel.next.prev = startsentinel;

      //assert invariant();
      //assert isSorted();
#if HASHINDEX
      {
        Node cursor = startsentinel.next, end = endsentinel;
        int tag, taglimit;
        TagGroup t = gettaggroup(startsentinel, endsentinel, out tag, out taglimit);
        int tagdelta = taglimit / (size + 1) - tag / (size + 1);
        tagdelta = tagdelta == 0 ? 1 : tagdelta;
        if (underlying == null)
          taggroups = 1;
        while (cursor != end)
        {
          tag = tag + tagdelta > taglimit ? taglimit : tag + tagdelta;
          cursor.tag = tag;
          t.count++;
          cursor.taggroup = t;
          cursor = cursor.next;
        }
        if (t != startsentinel.taggroup)
          t.first = startsentinel.next;
        if (t != endsentinel.taggroup)
          t.last = endsentinel.prev;
        if (tag == taglimit)
          splittaggroup(t);
      }
#endif
      (underlying ?? this).raiseCollectionChanged();
    }

    private static Node mergeRuns(Node run1, Node run2, SCG.IComparer<T> c)
    {
      //assert run1 != null && run2 != null;
      Node prev;
      bool prev1;	// is prev from run1?

      if (c.Compare(run1.item, run2.item) <= 0)
      {
        prev = run1;
        prev1 = true;
        run1 = run1.next;
      }
      else
      {
        prev = run2;
        prev1 = false;
        run2 = run2.next;
      }

      Node start = prev;

      //assert start != null;
      start.prev = null;
      while (run1 != null && run2 != null)
      {
        if (prev1)
        {
          //assert prev.next == run1;
          //Comparable run2item = (Comparable)run2.item;
          while (run1 != null && c.Compare(run2.item, run1.item) >= 0)
          {
            prev = run1;
            run1 = prev.next;
          }

          if (run1 != null)
          { // prev.item <= run2.item < run1.item; insert run2
            prev.next = run2;
            run2.prev = prev;
            prev = run2;
            run2 = prev.next;
            prev1 = false;
          }
        }
        else
        {
          //assert prev.next == run2;
          //Comparable run1item = (Comparable)run1.item;
          while (run2 != null && c.Compare(run1.item, run2.item) > 0)
          {
            prev = run2;
            run2 = prev.next;
          }

          if (run2 != null)
          { // prev.item < run1.item <= run2.item; insert run1
            prev.next = run1;
            run1.prev = prev;
            prev = run1;
            run1 = prev.next;
            prev1 = true;
          }
        }
      }

      //assert !(run1 != null && prev1) && !(run2 != null && !prev1);
      if (run1 != null)
      { // last run2 < all of run1; attach run1 at end
        prev.next = run1;
        run1.prev = prev;
      }
      else if (run2 != null)
      { // last run1
        prev.next = run2;
        run2.prev = prev;
      }

      return start;
    }

    /// <summary>
    /// Randomly shuffle the items of this list.
    /// <para>Will invalidate overlapping views???</para>
    /// </summary>
    public virtual void Shuffle() { Shuffle(new C5Random()); }


    /// <summary>
    /// Shuffle the items of this list according to a specific random source.
    /// <para>Will invalidate overlapping views???</para>
    /// </summary>
    /// <param name="rnd">The random source.</param>
    public virtual void Shuffle(Random rnd)
    {
      updatecheck();
      if (size == 0)
        return;
      disposeOverlappingViews(false);
      ArrayList<T> a = new ArrayList<T>();
      a.AddAll(this);
      a.Shuffle(rnd);
      Node cursor = startsentinel.next;
      int j = 0;
      while (cursor != endsentinel)
      {
        cursor.item = a[j++];
#if HASHINDEX
        dict[cursor.item] = cursor;
#endif
        cursor = cursor.next;
      }
      (underlying ?? this).raiseCollectionChanged();
    }

    #endregion

    #region IIndexed<T> Members

    /// <summary>
    /// <exception cref="IndexOutOfRangeException"/>.
    /// </summary>
    /// <value>The directed collection of items in a specific index interval.</value>
    /// <param name="start">The low index of the interval (inclusive).</param>
    /// <param name="count">The size of the range.</param>
    [Tested]
    public IDirectedCollectionValue<T> this[int start, int count]
    {
      [Tested]
      get
      {
        validitycheck();
        checkRange(start, count);
        return new Range(this, start, count, true);
      }
    }

    /// <summary>
    /// Searches for an item in the list going forwrds from the start.
    /// </summary>
    /// <param name="item">Item to search for.</param>
    /// <returns>Index of item from start.</returns>
    [Tested]
    public virtual int IndexOf(T item)
    {
      validitycheck();
      Node node;
#if HASHINDEX
      if (!dict.Find(item, out node) || !insideview(node))
        return ~size;
#endif
      node = startsentinel.next;
      int index = 0;
      if (find(item, ref node, ref index))
        return index;
      else
        return ~size;
    }

    /// <summary>
    /// Searches for an item in the list going backwords from the end.
    /// </summary>
    /// <param name="item">Item to search for.</param>
    /// <returns>Index of of item from the end.</returns>
    [Tested]
    public virtual int LastIndexOf(T item)
    {
#if HASHINDEX
      return IndexOf(item);
#else
      validitycheck();

      Node node = endsentinel.prev;
      int index = size - 1;

      if (dnif(item, ref node, ref index))
        return index;
      else
        return ~size;
#endif
    }

    /// <summary>
    /// Remove the item at a specific position of the list.
    /// <exception cref="IndexOutOfRangeException"/> if i is negative or
    /// &gt;= the size of the collection.
    /// </summary>
    /// <param name="i">The index of the item to remove.</param>
    /// <returns>The removed item.</returns>
    [Tested]
    public virtual T RemoveAt(int i)
    {
      updatecheck();
      T retval = remove(get(i), i);
#if HASHINDEX
      dict.Remove(retval);
#endif
      if (ActiveEvents != EventTypeEnum.None)
        (underlying ?? this).raiseForRemoveAt(Offset + i, retval);
      return retval;
    }

    /// <summary>
    /// Remove all items in an index interval.
    /// <exception cref="IndexOutOfRangeException"/>???.
    /// </summary>
    /// <param name="start">The index of the first item to remove.</param>
    /// <param name="count">The number of items to remove.</param>
    [Tested]
    public virtual void RemoveInterval(int start, int count)
    {
#if HASHINDEX
      updatecheck();
      checkRange(start, count);
      if (count == 0)
        return;

      View(start, count).Clear();
#else
      //Note: this is really almost equaivalent to Clear on a view
      updatecheck();
      checkRange(start, count);
      if (count == 0)
        return;

      //for small count: optimize
      //use an optimal get(int i, int j, ref Node ni, ref Node nj)?
      Node a = get(start), b = get(start + count - 1);
      fixViewsBeforeRemove(start, count, a, b);
      a.prev.next = b.next;
      b.next.prev = a.prev;
      if (underlying != null)
        underlying.size -= count;

      size -= count;
      if (ActiveEvents != EventTypeEnum.None)
        (underlying ?? this).raiseForRemoveInterval(start + Offset, count);
#endif
    }

    void raiseForRemoveInterval(int start, int count)
    {
      if (ActiveEvents != 0)
      {
        raiseCollectionCleared(size == 0, count, start);
        raiseCollectionChanged();
      }
    }
    #endregion

    #region ISequenced<T> Members

    /// <summary>
    ///
    /// </summary>
    /// <returns></returns>
    [Tested]
    public override int GetSequencedHashCode() { validitycheck(); return base.GetSequencedHashCode(); }

    /// <summary>
    ///
    /// </summary>
    /// <param name="that"></param>
    /// <returns></returns>
    [Tested]
    public override bool SequencedEquals(ISequenced<T> that) { validitycheck(); return base.SequencedEquals(that); }

    #endregion

    #region IDirectedCollection<T> Members

    /// <summary>
    /// Create a collection containing the same items as this collection, but
    /// whose enumerator will enumerate the items backwards. The new collection
    /// will become invalid if the original is modified. Method typicaly used as in
    /// <code>foreach (T x in coll.Backwards()) {...}</code>
    /// </summary>
    /// <returns>The backwards collection.</returns>
    [Tested]
    public override IDirectedCollectionValue<T> Backwards()
    { return this[0, size].Backwards(); }

    #endregion

    #region IDirectedEnumerable<T> Members

    [Tested]
    IDirectedEnumerable<T> IDirectedEnumerable<T>.Backwards() { return Backwards(); }

    #endregion

    #region IEditableCollection<T> Members

    /// <summary>
    /// The value is symbolic indicating the type of asymptotic complexity
    /// in terms of the size of this collection (worst-case or amortized as
    /// relevant).
    /// </summary>
    /// <value>Speed.Linear</value>
    [Tested]
    public virtual Speed ContainsSpeed
    {
      [Tested]
      get
      {
#if HASHINDEX
        return Speed.Constant;
#else
        return Speed.Linear;
#endif
      }
    }

    /// <summary>
    /// Performs a check for view validity before calling base.GetUnsequencedHashCode()
    /// </summary>
    /// <returns></returns>
    [Tested]
    public override int GetUnsequencedHashCode()
    { validitycheck(); return base.GetUnsequencedHashCode(); }

    /// <summary>
    ///
    /// </summary>
    /// <param name="that"></param>
    /// <returns></returns>
    [Tested]
    public override bool UnsequencedEquals(ICollection<T> that)
    { validitycheck(); return base.UnsequencedEquals(that); }

    /// <summary>
    /// Check if this collection contains (an item equivalent to according to the
    /// itemequalityComparer) a particular value.
    /// </summary>
    /// <param name="item">The value to check for.</param>
    /// <returns>True if the items is in this collection.</returns>
    [Tested]
    public virtual bool Contains(T item)
    {
      validitycheck();
      Node node;
      return contains(item, out node);
    }

    /// <summary>
    /// Check if this collection contains an item equivalent according to the
    /// itemequalityComparer to a particular value. If so, return in the ref argument (a
    /// binary copy of) the actual value found.
    /// </summary>
    /// <param name="item">The value to look for.</param>
    /// <returns>True if the items is in this collection.</returns>
    [Tested]
    public virtual bool Find(ref T item)
    {
      validitycheck();
      Node node;
      if (contains(item, out node)) { item = node.item; return true; }
      return false;
    }

    /// <summary>
    /// Check if this collection contains an item equivalent according to the
    /// itemequalityComparer to a particular value. If so, update the item in the collection
    /// to with a binary copy of the supplied value. Will update a single item.
    /// </summary>
    /// <param name="item">Value to update.</param>
    /// <returns>True if the item was found and hence updated.</returns>
    [Tested]
    public virtual bool Update(T item) { T olditem; return Update(item, out olditem); }

    /// <summary>
    ///
    /// </summary>
    /// <param name="item"></param>
    /// <param name="olditem"></param>
    /// <returns></returns>
    public virtual bool Update(T item, out T olditem)
    {
      updatecheck();
      Node node;

      if (contains(item, out node))
      {
        olditem = node.item;
        node.item = item;
#if HASHINDEX
        //Avoid clinging onto a reference to olditem via dict!
        dict.Update(item, node);
#endif
        (underlying ?? this).raiseForUpdate(item, olditem);
        return true;
      }

      olditem = default(T);
      return false;
    }

    /// <summary>
    /// Check if this collection contains an item equivalent according to the
    /// itemequalityComparer to a particular value. If so, return in the ref argument (a
    /// binary copy of) the actual value found. Else, add the item to the collection.
    /// </summary>
    /// <param name="item">The value to look for.</param>
    /// <returns>True if the item was found (hence not added).</returns>
    [Tested]
    public virtual bool FindOrAdd(ref T item)
    {
      updatecheck();
#if HASHINDEX
      //This is an extended myinsert:
      Node node = new Node(item);
      if (!dict.FindOrAdd(item, ref node))
      {
        insertNode(true, endsentinel, node);
        (underlying ?? this).raiseForAdd(item);
        return false;
      }
      if (!insideview(node))
        throw new ArgumentException("Item alredy in indexed list but outside view");
      item = node.item;
      return true;
#else
      if (Find(ref item))
        return true;

      Add(item);
      return false;
#endif
    }

    /// <summary>
    /// Check if this collection contains an item equivalent according to the
    /// itemequalityComparer to a particular value. If so, update the item in the collection
    /// to with a binary copy of the supplied value; else add the value to the collection.
    /// </summary>
    /// <param name="item">Value to add or update.</param>
    /// <returns>True if the item was found and updated (hence not added).</returns>
    [Tested]
    public virtual bool UpdateOrAdd(T item) { T olditem; return UpdateOrAdd(item, out olditem); }

    /// <summary>
    ///
    /// </summary>
    /// <param name="item"></param>
    /// <param name="olditem"></param>
    /// <returns></returns>
    public virtual bool UpdateOrAdd(T item, out T olditem)
    {
      updatecheck();
#if HASHINDEX
      Node node = new Node(item);
      //NOTE: it is hard to do this without double access to the dictionary
      //in the update case
      if (dict.FindOrAdd(item, ref node))
      {
        if (!insideview(node))
          throw new ArgumentException("Item in indexed list but outside view");
        olditem = node.item;
        //Avoid clinging onto a reference to olditem via dict!
        dict.Update(item, node);
        node.item = item;
        (underlying ?? this).raiseForUpdate(item, olditem);
        return true;
      }
      insertNode(true, endsentinel, node);
      (underlying ?? this).raiseForAdd(item);
#else
      if (Update(item, out olditem))
        return true;
      Add(item);
#endif
      olditem = default(T);
      return false;
    }

    /// <summary>
    /// Remove a particular item from this collection. Since the collection has bag
    /// semantics only one copy equivalent to the supplied item is removed.
    /// </summary>
    /// <param name="item">The value to remove.</param>
    /// <returns>True if the item was found (and removed).</returns>
    [Tested]
    public virtual bool Remove(T item)
    {
      updatecheck();
      int i = 0;
      Node node;
#if HASHINDEX
      if (!dictremove(item, out node))
#else
      node = fIFO ? startsentinel.next : endsentinel.prev;
      if (!(fIFO ? find(item, ref node, ref i) : dnif(item, ref node, ref i)))
#endif
        return false;
      T removeditem = remove(node, i);
      (underlying ?? this).raiseForRemove(removeditem);
      return true;
    }

    /// <summary>
    /// Remove a particular item from this collection if found (only one copy).
    /// If an item was removed, report a binary copy of the actual item removed in
    /// the argument.
    /// </summary>
    /// <param name="item">The value to remove on input.</param>
    /// <param name="removeditem">The value removed.</param>
    /// <returns>True if the item was found (and removed).</returns>
    [Tested]
    public virtual bool Remove(T item, out T removeditem)
    {
      updatecheck();
      int i = 0;
      Node node;
#if HASHINDEX
      if (!dictremove(item, out node))
#else
      node = fIFO ? startsentinel.next : endsentinel.prev;
      if (!(fIFO ? find(item, ref node, ref i) : dnif(item, ref node, ref i)))
#endif
      {
        removeditem = default(T);
        return false;
      }
      removeditem = node.item;
      remove(node, i);
      (underlying ?? this).raiseForRemove(removeditem);
      return true;
    }

    /// <summary>
    /// Remove all items in another collection from this one, taking multiplicities into account.
    /// <para>Always removes from the front of the list.
    /// </para>
    /// <para>The asymptotic running time complexity of this method is <code>O(n+m+v*log(v))</code>,
    /// where <code>n</code> is the size of this list, <code>m</code> is the size of the
    /// <code>items</code> collection and <code>v</code> is the number of views.
    /// The method will temporarily allocate memory of size <code>O(m+v)</code>.
    /// </para>
    /// </summary>
    /// <typeparam name="U"></typeparam>
    /// <param name="items">The items to remove.</param>
    [Tested]
    public virtual void RemoveAll<U>(SCG.IEnumerable<U> items) where U : T
    {
      updatecheck();
      if (size == 0)
        return;
      RaiseForRemoveAllHandler raiseHandler = new RaiseForRemoveAllHandler(underlying ?? this);
      bool mustFire = raiseHandler.MustFire;
#if HASHINDEX
      Node node;
      foreach (T item in items)
        if (dictremove(item, out node))
        {
          if (mustFire)
            raiseHandler.Remove(node.item);
          remove(node, 118);
        }
#else
      HashBag<T> toremove = new HashBag<T>(itemequalityComparer);
      toremove.AddAll(items);
      ViewHandler viewHandler = new ViewHandler(this);
      int index = 0, removed = 0, myoffset = Offset;
      Node node = startsentinel.next;
      while (node != endsentinel)
      {
        //pass by a stretch of nodes
        while (node != endsentinel && !toremove.Contains(node.item))
        {
          node = node.next;
          index++;
        }
        viewHandler.skipEndpoints(removed, myoffset + index);
        //Remove a stretch of nodes
        Node localend = node.prev; //Latest node not to be removed
        while (node != endsentinel && toremove.Remove(node.item))
        {
          if (mustFire)
            raiseHandler.Remove(node.item);
          removed++;
          node = node.next;
          index++;
          viewHandler.updateViewSizesAndCounts(removed, myoffset + index);
        }
        viewHandler.updateSentinels(myoffset + index, localend, node);
        localend.next = node;
        node.prev = localend;
      }
      index = underlying != null ? underlying.size + 1 - myoffset : size + 1 - myoffset;
      viewHandler.updateViewSizesAndCounts(removed, myoffset + index);
      size -= removed;
      if (underlying != null)
        underlying.size -= removed;
#endif
      raiseHandler.Raise();
    }

    /// <summary>
    ///
    /// </summary>
    /// <param name="predicate"></param>
    void RemoveAll(Fun<T, bool> predicate)
    {
      updatecheck();
      if (size == 0)
        return;
      RaiseForRemoveAllHandler raiseHandler = new RaiseForRemoveAllHandler(underlying ?? this);
      bool mustFire = raiseHandler.MustFire;
#if HASHINDEX
      {
        Node n = startsentinel.next;

        while (n != endsentinel)
        {
          bool removeIt = predicate(n.item);
          updatecheck();
          if (removeIt)
          {
            dict.Remove(n.item);
            remove(n, 119);
            if (mustFire)
              raiseHandler.Remove(n.item);
          }

          n = n.next;
        }
      }
#else
      ViewHandler viewHandler = new ViewHandler(this);
      int index = 0, removed = 0, myoffset = Offset;
      Node node = startsentinel.next;
      while (node != endsentinel)
      {
        //pass by a stretch of nodes
        while (node != endsentinel && !predicate(node.item))
        {
          updatecheck();
          node = node.next;
          index++;
        }
        updatecheck();
        viewHandler.skipEndpoints(removed, myoffset + index);
        //Remove a stretch of nodes
        Node localend = node.prev; //Latest node not to be removed
        while (node != endsentinel && predicate(node.item))
        {
          updatecheck();
          if (mustFire)
            raiseHandler.Remove(node.item);
          removed++;
          node = node.next;
          index++;
          viewHandler.updateViewSizesAndCounts(removed, myoffset + index);
        }
        updatecheck();
        viewHandler.updateSentinels(myoffset + index, localend, node);
        localend.next = node;
        node.prev = localend;
      }
      index = underlying != null ? underlying.size + 1 - myoffset : size + 1 - myoffset;
      viewHandler.updateViewSizesAndCounts(removed, myoffset + index);
      size -= removed;
      if (underlying != null)
        underlying.size -= removed;
#endif
      raiseHandler.Raise();
    }

    /// <summary>
    /// Remove all items from this collection.
    /// </summary>
    [Tested]
    public virtual void Clear()
    {
      updatecheck();
      if (size == 0)
        return;
      int oldsize = size;
#if HASHINDEX
      if (underlying == null)
        dict.Clear();
      else
        foreach (T item in this)
          dict.Remove(item);
#endif
      clear();
      (underlying ?? this).raiseForRemoveInterval(Offset, oldsize);
    }

    void clear()
    {
      if (size == 0)
        return;
#if HASHINDEX
      //TODO: mix with tag maintenance to only run through list once?
      ViewHandler viewHandler = new ViewHandler(this);
      if (viewHandler.viewCount > 0)
      {
        int removed = 0;
        Node n = startsentinel.next;
        viewHandler.skipEndpoints(0, n);
        while (n != endsentinel)
        {
          removed++;
          n = n.next;
          viewHandler.updateViewSizesAndCounts(removed, n);
        }
        viewHandler.updateSentinels(endsentinel, startsentinel, endsentinel);
        if (underlying != null)
          viewHandler.updateViewSizesAndCounts(removed, underlying.endsentinel);
      }
#else
      fixViewsBeforeRemove(Offset, size, startsentinel.next, endsentinel.prev);
#endif
#if HASHINDEX
      if (underlying != null)
      {
        Node n = startsentinel.next;

        while (n != endsentinel)
        {
          n.next.prev = startsentinel;
          startsentinel.next = n.next;
          removefromtaggroup(n);
          n = n.next;
        }
      }
      else
        taggroups = 0;
#endif
      endsentinel.prev = startsentinel;
      startsentinel.next = endsentinel;
      if (underlying != null)
        underlying.size -= size;
      size = 0;
    }

    /// <summary>
    /// Remove all items not in some other collection from this one, taking multiplicities into account.
    /// <para>The asymptotic running time complexity of this method is <code>O(n+m+v*log(v))</code>,
    /// where <code>n</code> is the size of this collection, <code>m</code> is the size of the
    /// <code>items</code> collection and <code>v</code> is the number of views.
    /// The method will temporarily allocate memory of size <code>O(m+v)</code>. The stated complexitiy
    /// holds under the assumption that the itemequalityComparer of this list is well-behaved.
    /// </para>
    /// </summary>
    /// <typeparam name="U"></typeparam>
    /// <param name="items">The items to retain.</param>
    [Tested]
    public virtual void RetainAll<U>(SCG.IEnumerable<U> items) where U : T
    {
      updatecheck();
      if (size == 0)
        return;
      RaiseForRemoveAllHandler raiseHandler = new RaiseForRemoveAllHandler(underlying ?? this);
      bool mustFire = raiseHandler.MustFire;
#if HASHINDEX
      /*if (underlying == null)
      {
        HashDictionary<T, Node> newdict = new HashDictionary<T, Node>(itemequalityComparer);
        foreach (T item in items)
        {
          Node node;

          if (dict.Remove(item, out node))
            newdict.Add(item, node);
        }
        foreach (KeyValuePair<T, Node> pair in dict)
        {
          Node n = pair.Value;
          fixViewsBeforeSingleRemove(n, 117);
          Node p = n.prev, s = n.next; s.prev = p; p.next = s;
          removefromtaggroup(n);
        }
        dict = newdict;
        size = dict.Count;
        //For a small number of items to retain it might be faster to
        //iterate through the list and splice out the chunks not needed
      }
      else*/
      {
        HashSet<T> toremove = new HashSet<T>(itemequalityComparer);

        foreach (T item in this)
          toremove.Add(item);

        foreach (T item in items)
          toremove.Remove(item);

        Node n = startsentinel.next;

        while (n != endsentinel && toremove.Count > 0)
        {
          if (toremove.Contains(n.item))
          {
            dict.Remove(n.item);
            remove(n, 119);
            if (mustFire)
              raiseHandler.Remove(n.item);
          }

          n = n.next;
        }
      }
#else
      HashBag<T> toretain = new HashBag<T>(itemequalityComparer);
      toretain.AddAll(items);
      ViewHandler viewHandler = new ViewHandler(this);
      int index = 0, removed = 0, myoffset = Offset;
      Node node = startsentinel.next;
      while (node != endsentinel)
      {
        //Skip a stretch of nodes
        while (node != endsentinel && toretain.Remove(node.item))
        {
          node = node.next;
          index++;
        }
        viewHandler.skipEndpoints(removed, myoffset + index);
        //Remove a stretch of nodes
        Node localend = node.prev; //Latest node not to be removed
        while (node != endsentinel && !toretain.Contains(node.item))
        {
          if (mustFire)
            raiseHandler.Remove(node.item);
          removed++;
          node = node.next;
          index++;
          viewHandler.updateViewSizesAndCounts(removed, myoffset + index);
        }
        viewHandler.updateSentinels(myoffset + index, localend, node);
        localend.next = node;
        node.prev = localend;
      }
      index = underlying != null ? underlying.size + 1 - myoffset : size + 1 - myoffset;
      viewHandler.updateViewSizesAndCounts(removed, myoffset + index);
      size -= removed;
      if (underlying != null)
        underlying.size -= removed;
#endif
      raiseHandler.Raise();
    }

    /// <summary>
    ///
    /// </summary>
    /// <param name="predicate"></param>
    void RetainAll(Fun<T, bool> predicate)
    {
      updatecheck();
      if (size == 0)
        return;
      RaiseForRemoveAllHandler raiseHandler = new RaiseForRemoveAllHandler(underlying ?? this);
      bool mustFire = raiseHandler.MustFire;
#if HASHINDEX
      {
        Node n = startsentinel.next;

        while (n != endsentinel)
        {
          bool removeIt = !predicate(n.item);
          updatecheck();
          if (removeIt)
          {
            dict.Remove(n.item);
            remove(n, 119);
            if (mustFire)
              raiseHandler.Remove(n.item);
          }

          n = n.next;
        }
      }
#else
      ViewHandler viewHandler = new ViewHandler(this);
      int index = 0, removed = 0, myoffset = Offset;
      Node node = startsentinel.next;
      while (node != endsentinel)
      {
        //Skip a stretch of nodes
        while (node != endsentinel && predicate(node.item))
        {
          updatecheck();
          node = node.next;
          index++;
        }
        updatecheck();
        viewHandler.skipEndpoints(removed, myoffset + index);
        //Remove a stretch of nodes
        Node localend = node.prev; //Latest node not to be removed
        while (node != endsentinel && !predicate(node.item))
        {
          updatecheck();
          if (mustFire)
            raiseHandler.Remove(node.item);
          removed++;
          node = node.next;
          index++;
          viewHandler.updateViewSizesAndCounts(removed, myoffset + index);
        }
        updatecheck();
        viewHandler.updateSentinels(myoffset + index, localend, node);
        localend.next = node;
        node.prev = localend;
      }
      index = underlying != null ? underlying.size + 1 - myoffset : size + 1 - myoffset;
      viewHandler.updateViewSizesAndCounts(removed, myoffset + index);
      size -= removed;
      if (underlying != null)
        underlying.size -= removed;
#endif
      raiseHandler.Raise();
    }

    /// <summary>
    /// Check if this collection contains all the values in another collection
    /// with respect to multiplicities.
    /// </summary>
    /// <param name="items">The </param>
    /// <typeparam name="U"></typeparam>
    /// <returns>True if all values in <code>items</code>is in this collection.</returns>
    [Tested]
    public virtual bool ContainsAll<U>(SCG.IEnumerable<U> items) where U : T
    {
      validitycheck();
#if HASHINDEX
      Node node;
      foreach (T item in items)
        if (!contains(item, out node))
          return false;
      return true;
#else
      HashBag<T> tocheck = new HashBag<T>(itemequalityComparer);
      tocheck.AddAll(items);
      if (tocheck.Count > size)
        return false;
      Node node = startsentinel.next;
      while (node != endsentinel)
      {
        tocheck.Remove(node.item);
        node = node.next;
      }
      return tocheck.IsEmpty;
#endif
    }


    /// <summary>
    /// Create a new list consisting of the items of this list satisfying a
    /// certain predicate.
    /// </summary>
    /// <param name="filter">The filter delegate defining the predicate.</param>
    /// <returns>The new list.</returns>
    [Tested]
    public IList<T> FindAll(Fun<T, bool> filter)
    {
      validitycheck();
      int stamp = this.stamp;
      LinkedList<T> retval = new LinkedList<T>();
      Node cursor = startsentinel.next;
      Node mcursor = retval.startsentinel;
#if HASHINDEX
      double tagdelta = int.MaxValue / (size + 1.0);
      int count = 1;
      TagGroup taggroup = new TagGroup();
      retval.taggroups = 1;
#endif
      while (cursor != endsentinel)
      {
        bool found = filter(cursor.item);
        modifycheck(stamp);
        if (found)
        {
          mcursor.next = new Node(cursor.item, mcursor, null);
          mcursor = mcursor.next;
          retval.size++;
#if HASHINDEX
          retval.dict.Add(cursor.item, mcursor);
          mcursor.taggroup = taggroup;
          mcursor.tag = (int)(tagdelta * count++);
#endif
        }
        cursor = cursor.next;
      }
#if HASHINDEX
      if (retval.size > 0)
      {
        taggroup.count = retval.size;
        taggroup.first = retval.startsentinel.next;
        taggroup.last = mcursor;
      }
#endif
      retval.endsentinel.prev = mcursor;
      mcursor.next = retval.endsentinel;
      return retval;
    }


    /// <summary>
    /// Count the number of items of the collection equal to a particular value.
    /// Returns 0 if and only if the value is not in the collection.
    /// </summary>
    /// <param name="item">The value to count.</param>
    /// <returns>The number of copies found.</returns>
    [Tested]
    public virtual int ContainsCount(T item)
    {
#if HASHINDEX
      return Contains(item) ? 1 : 0;
#else
      validitycheck();
      int retval = 0;
      Node node = startsentinel.next;
      while (node != endsentinel)
      {
        if (itemequalityComparer.Equals(node.item, item))
          retval++;
        node = node.next;
      }
      return retval;
#endif
    }

    /// <summary>
    ///
    /// </summary>
    /// <returns></returns>
    public virtual ICollectionValue<T> UniqueItems()
    {
#if HASHINDEX
      return this;
#else
      HashBag<T> hashbag = new HashBag<T>(itemequalityComparer);
      hashbag.AddAll(this);
      return hashbag.UniqueItems();
#endif
    }

    /// <summary>
    ///
    /// </summary>
    /// <returns></returns>
    public virtual ICollectionValue<KeyValuePair<T, int>> ItemMultiplicities()
    {
#if HASHINDEX
      return new MultiplicityOne<T>(this);
#else
      HashBag<T> hashbag = new HashBag<T>(itemequalityComparer);
      hashbag.AddAll(this);
      return hashbag.ItemMultiplicities();
#endif
    }

    /// <summary>
    /// Remove all items equivalent to a given value.
    /// <para>The asymptotic complexity of this method is <code>O(n+v*log(v))</code>,
    /// where <code>n</code> is the size of the collection and <code>v</code>
    /// is the number of views.
    /// </para>
    /// </summary>
    /// <param name="item">The value to remove.</param>
    [Tested]
    public virtual void RemoveAllCopies(T item)
    {
#if HASHINDEX
      Remove(item);
#else
      updatecheck();
      if (size == 0)
        return;
      RaiseForRemoveAllHandler raiseHandler = new RaiseForRemoveAllHandler(underlying ?? this);
      bool mustFire = raiseHandler.MustFire;
      ViewHandler viewHandler = new ViewHandler(this);
      int index = 0, removed = 0, myoffset = Offset;
      //
      Node node = startsentinel.next;
      while (node != endsentinel)
      {
        //pass by a stretch of nodes
        while (node != endsentinel && !itemequalityComparer.Equals(node.item, item))
        {
          node = node.next;
          index++;
        }
        viewHandler.skipEndpoints(removed, myoffset + index);
        //Remove a stretch of nodes
        Node localend = node.prev; //Latest node not to be removed
        while (node != endsentinel && itemequalityComparer.Equals(node.item, item))
        {
          if (mustFire)
            raiseHandler.Remove(node.item);
          removed++;
          node = node.next;
          index++;
          viewHandler.updateViewSizesAndCounts(removed, myoffset + index);
        }
        viewHandler.updateSentinels(myoffset + index, localend, node);
        localend.next = node;
        node.prev = localend;
      }
      index = underlying != null ? underlying.size + 1 - myoffset : size + 1 - myoffset;
      viewHandler.updateViewSizesAndCounts(removed, myoffset + index);
      size -= removed;
      if (underlying != null)
        underlying.size -= removed;
      raiseHandler.Raise();
#endif
    }

    #endregion

    #region ICollectionValue<T> Members

    /// <summary>
    ///
    /// </summary>
    /// <value>The number of items in this collection</value>
    [Tested]
    public override int Count { [Tested]get { validitycheck(); return size; } }

    /// <summary>
    /// Choose some item of this collection.
    /// </summary>
    /// <exception cref="NoSuchItemException">if collection is empty.</exception>
    /// <returns></returns>
    [Tested]
    public override T Choose() { return First; }

    /// <summary>
    /// Create an enumerable, enumerating the items of this collection that satisfies
    /// a certain condition.
    /// </summary>
    /// <param name="filter">The T->bool filter delegate defining the condition</param>
    /// <returns>The filtered enumerable</returns>
    public override SCG.IEnumerable<T> Filter(Fun<T, bool> filter) { validitycheck(); return base.Filter(filter); }

    #endregion

    #region IEnumerable<T> Members
    /// <summary>
    /// Create an enumerator for the collection
    /// </summary>
    /// <returns>The enumerator</returns>
    [Tested]
    public override SCG.IEnumerator<T> GetEnumerator()
    {
      validitycheck();
      Node cursor = startsentinel.next;
      int enumeratorstamp = underlying != null ? underlying.stamp : this.stamp;

      while (cursor != endsentinel)
      {
        modifycheck(enumeratorstamp);
        yield return cursor.item;
        cursor = cursor.next;
      }
    }

    #endregion

    #region IExtensible<T> Members
    /// <summary>
    /// Add an item to this collection if possible.
    /// </summary>
    /// <param name="item">The item to add.</param>
    /// <returns>True.</returns>
    [Tested]
    public virtual bool Add(T item)
    {
      updatecheck();
#if HASHINDEX
      Node node = new Node(item);
      if (!dict.FindOrAdd(item, ref node))
      {
        insertNode(true, endsentinel, node);
        (underlying ?? this).raiseForAdd(item);
        return true;
      }
      return false;
#else
      insert(size, endsentinel, item);
      (underlying ?? this).raiseForAdd(item);
      return true;
#endif
    }

    /// <summary>
    ///
    /// </summary>
    /// <value>True since this collection has bag semantics.</value>
    [Tested]
    public virtual bool AllowsDuplicates
    {
      [Tested]
      get
      {
#if HASHINDEX
        return false;
#else
        return true;
#endif
      }
    }

    /// <summary>
    /// By convention this is true for any collection with set semantics.
    /// </summary>
    /// <value>True if only one representative of a group of equal items
    /// is kept in the collection together with the total count.</value>
    public virtual bool DuplicatesByCounting
    {
      get
      {
#if HASHINDEX
        return true;
#else
        return false;
#endif
      }
    }

    /// <summary>
    /// Add the elements from another collection with a more specialized item type
    /// to this collection.
    /// </summary>
    /// <typeparam name="U">The type of items to add</typeparam>
    /// <param name="items">The items to add</param>
    [Tested]
    public virtual void AddAll<U>(SCG.IEnumerable<U> items) where U : T
    {
#if HASHINDEX
      updatecheck();
      int added = 0;
      Node pred = endsentinel.prev;
      foreach (U item in items)
      {
        Node node = new Node(item);
        if (!dict.FindOrAdd(item, ref node))
        {
          insertNode(false, endsentinel, node);
          added++;
        }
      }
      if (added > 0)
      {
        fixViewsAfterInsert(endsentinel, pred, added, 0);
        raiseForInsertAll(pred, size - added, added, false);
      }
#else
      insertAll(size, items, false);
#endif
    }

    #endregion

#if HASHINDEX
#else
    #region IStack<T> Members

    /// <summary>
    /// Push an item to the top of the stack.
    /// </summary>
    /// <param name="item">The item</param>
    [Tested]
    public void Push(T item)
    {
      InsertLast(item);
    }

    /// <summary>
    /// Pop the item at the top of the stack from the stack.
    /// </summary>
    /// <returns>The popped item.</returns>
    [Tested]
    public T Pop()
    {
      return RemoveLast();
    }

    #endregion

    #region IQueue<T> Members

    /// <summary>
    /// Enqueue an item at the back of the queue.
    /// </summary>
    /// <param name="item">The item</param>
    [Tested]
    public virtual void Enqueue(T item)
    {
      InsertLast(item);
    }

    /// <summary>
    /// Dequeue an item from the front of the queue.
    /// </summary>
    /// <returns>The item</returns>
    [Tested]
    public virtual T Dequeue()
    {
      return RemoveFirst();
    }
    #endregion
#endif

    #region Diagnostic

    private bool checkViews()
    {
      if (underlying != null)
        throw new InternalException(System.Reflection.MethodInfo.GetCurrentMethod() + " called on a view");
      if (views == null)
        return true;
      bool retval = true;

      Node[] nodes = new Node[size + 2];
      int i = 0;
      Node n = startsentinel;
      while (n != null)
      {
        nodes[i++] = n;
        n = n.next;
      }
      //Console.WriteLine("###");
      foreach (LinkedList<T> view in views)
      {
        if (!view.isValid)
        {
          Console.WriteLine("Invalid view(hash {0}, offset {1}, size {2})",
            view.GetHashCode(), view.offset, view.size);
          retval = false;
          continue;
        }
        if (view.Offset > size || view.Offset < 0)
        {
          Console.WriteLine("Bad view(hash {0}, offset {1}, size {2}), Offset > underlying.size ({2})",
            view.GetHashCode(), view.offset, view.size, size);
          retval = false;
        }
        else if (view.startsentinel != nodes[view.Offset])
        {
          Console.WriteLine("Bad view(hash {0}, offset {1}, size {2}), startsentinel {3} should be {4}",
            view.GetHashCode(), view.offset, view.size,
            view.startsentinel + " " + view.startsentinel.GetHashCode(),
            nodes[view.Offset] + " " + nodes[view.Offset].GetHashCode());
          retval = false;
        }
        if (view.Offset + view.size > size || view.Offset + view.size < 0)
        {
          Console.WriteLine("Bad view(hash {0}, offset {1}, size {2}), end index > underlying.size ({3})",
            view.GetHashCode(), view.offset, view.size, size);
          retval = false;
        }
        else if (view.endsentinel != nodes[view.Offset + view.size + 1])
        {
          Console.WriteLine("Bad view(hash {0}, offset {1}, size {2}), endsentinel {3} should be {4}",
            view.GetHashCode(), view.offset, view.size,
            view.endsentinel + " " + view.endsentinel.GetHashCode(),
            nodes[view.Offset + view.size + 1] + " " + nodes[view.Offset + view.size + 1].GetHashCode());
          retval = false;
        }
        if (view.views != views)
        {
          Console.WriteLine("Bad view(hash {0}, offset {1}, size {2}), wrong views list {3} <> {4}",
            view.GetHashCode(), view.offset, view.size, view.views.GetHashCode(), views.GetHashCode());
          retval = false;
        }
        if (view.underlying != this)
        {
          Console.WriteLine("Bad view(hash {0}, offset {1}, size {2}), wrong underlying {3} <> this {4}",
            view.GetHashCode(), view.offset, view.size, view.underlying.GetHashCode(), GetHashCode());
          retval = false;
        }
        if (view.stamp != stamp)
        {
          //Console.WriteLine("Bad view(hash {0}, offset {1}, size {2}), wrong stamp view:{2} underlying: {3}", view.GetHashCode(),view.offset, view.size, view.stamp, stamp);
          //retval = false;
        }
      }
      return retval;
    }

    string zeitem(Node node)
    {
      return node == null ? "(null node)" : node.item.ToString();
    }

    /// <summary>
    /// Check the sanity of this list
    /// </summary>
    /// <returns>true if sane</returns>
    [Tested]
    public virtual bool Check()
    {
      bool retval = true;

      /*if (underlying != null && underlying.stamp != stamp)
      {
        Console.WriteLine("underlying != null && underlying.stamp({0}) != stamp({1})", underlying.stamp, stamp);
        retval = false;
      }*/

      if (underlying != null)
      {
        //TODO: check that this view is included in viewsEndpoints tree
        return underlying.Check();
      }

      if (startsentinel == null)
      {
        Console.WriteLine("startsentinel == null");
        retval = false;
      }

      if (endsentinel == null)
      {
        Console.WriteLine("endsentinel == null");
        retval = false;
      }

      if (size == 0)
      {
        if (startsentinel != null && startsentinel.next != endsentinel)
        {
          Console.WriteLine("size == 0 but startsentinel.next != endsentinel");
          retval = false;
        }

        if (endsentinel != null && endsentinel.prev != startsentinel)
        {
          Console.WriteLine("size == 0 but endsentinel.prev != startsentinel");
          retval = false;
        }
      }

      if (startsentinel == null)
      {
        Console.WriteLine("NULL startsentinel");
        return retval;
      }

      int count = 0;
      Node node = startsentinel.next, prev = startsentinel;
#if HASHINDEX
      int taggroupsize = 0, oldtaggroupsize = losize + 1, seentaggroups = 0;
      TagGroup oldtg = null;

      if (underlying == null)
      {
        TagGroup tg = startsentinel.taggroup;

        if (tg.count != 0 || tg.first != null || tg.last != null || tg.tag != int.MinValue)
        {
          Console.WriteLine("Bad startsentinel tag group: {0}", tg);
          retval = false;
        }

        tg = endsentinel.taggroup;
        if (tg.count != 0 || tg.first != null || tg.last != null || tg.tag != int.MaxValue)
        {
          Console.WriteLine("Bad endsentinel tag group: {0}", tg);
          retval = false;
        }
      }
#endif
      while (node != endsentinel)
      {
        count++;
        if (node.prev != prev)
        {
          Console.WriteLine("Bad backpointer at node {0}", count);
          retval = false;
        }
#if HASHINDEX
        if (underlying == null)
        {
          if (!node.prev.precedes(node))
          {
            Console.WriteLine("node.prev.tag ({0}, {1}) >= node.tag ({2}, {3}) at index={4} item={5} ", node.prev.taggroup.tag, node.prev.tag, node.taggroup.tag, node.tag, count, node.item);
            retval = false;
          }

          if (node.taggroup != oldtg)
          {

            if (node.taggroup.first != node)
            {
              string ntfi = zeitem(node.taggroup.first);
              Console.WriteLine("Bad first pointer in taggroup: node.taggroup.first.item ({0}), node.item ({1}) at index={2} item={3}", ntfi, node.item, count, node.item);
              retval = false;
            }

            if (oldtg != null)
            {
              if (oldtg.count != taggroupsize)
              {
                Console.WriteLine("Bad taggroupsize: oldtg.count ({0}) != taggroupsize ({1}) at index={2} item={3}", oldtg.count, taggroupsize, count, node.item);
                retval = false;
              }

              if (oldtaggroupsize <= losize && taggroupsize <= losize)
              {
                Console.WriteLine("Two small taggroups in a row: oldtaggroupsize ({0}), taggroupsize ({1}) at index={2} item={3}", oldtaggroupsize, taggroupsize, count, node.item);
                retval = false;
              }

              if (node.taggroup.tag <= oldtg.tag)
              {
                Console.WriteLine("Taggroup tags not strictly increasing: oldtaggrouptag ({0}), taggrouptag ({1}) at index={2} item={3}", oldtg.tag, node.taggroup.tag, count, node.item);
                retval = false;
              }

              if (oldtg.last != node.prev)
              {
                Console.WriteLine("Bad last pointer in taggroup: oldtg.last.item ({0}), node.prev.item ({1}) at index={2} item={3}", oldtg.last.item, node.prev.item, count, node.item);
                retval = false;
              }

              oldtaggroupsize = taggroupsize;
            }

            seentaggroups++;
            oldtg = node.taggroup;
            taggroupsize = 1;
          }
          else
          {
            taggroupsize++;
          }
        }

#endif
        prev = node;
        node = node.next;
        if (node == null)
        {
          Console.WriteLine("Null next pointer at node {0}", count);
          return false;
        }
      }

#if HASHINDEX
      if (underlying == null && size == 0 && taggroups != 0)
      {
        Console.WriteLine("Bad taggroups for empty list: size={0}   taggroups={1}", size, taggroups);
        retval = false;
      }
      if (underlying == null && size > 0)
      {
        oldtg = node.prev.taggroup;
        if (oldtg != null)
        {
          if (oldtg.count != taggroupsize)
          {
            Console.WriteLine("Bad taggroupsize: oldtg.count ({0}) != taggroupsize ({1}) at index={2} item={3}", oldtg.count, taggroupsize, count, node.item);
            retval = false;
          }

          if (oldtaggroupsize <= losize && taggroupsize <= losize)
          {
            Console.WriteLine("Two small taggroups in a row: oldtaggroupsize ({0}), taggroupsize ({1}) at index={2} item={3}", oldtaggroupsize, taggroupsize, count, node.item);
            retval = false;
          }

              if (node.taggroup.tag <= oldtg.tag)
              {
                Console.WriteLine("Taggroup tags not strictly increasing: oldtaggrouptag ({0}), taggrouptag ({1}) at index={2} item={3}", oldtg.tag, node.taggroup.tag, count, node.item);
                retval = false;
              }

              if (oldtg.last != node.prev)
              {
                Console.WriteLine("Bad last pointer in taggroup: oldtg.last.item ({0}), node.prev.item ({1}) at index={2} item={3}", zeitem(oldtg.last), zeitem(node.prev), count, node.item);
                retval = false;
              }
        }

        if (seentaggroups != taggroups)
        {
          Console.WriteLine("seentaggroups ({0}) != taggroups ({1}) (at size {2})", seentaggroups, taggroups, size);
          retval = false;
        }
      }
#endif
      if (count != size)
      {
        Console.WriteLine("size={0} but enumeration gives {1} nodes ", size, count);
        retval = false;
      }

      retval = checkViews() && retval;

#if HASHINDEX
      if (!retval)
        return false;
      if (underlying == null)
      {
        if (size != dict.Count)
        {
          Console.WriteLine("list.size ({0}) != dict.Count ({1})", size, dict.Count);
          retval = false;
        }
        Node n = startsentinel.next, n2;
        while (n != endsentinel)
        {
          if (!dict.Find(n.item, out n2))
          {
            Console.WriteLine("Item in list but not dict: {0}", n.item);
            retval = false;
          }
          else if (n != n2)
          {
            Console.WriteLine("Wrong node in dict for item: {0}", n.item);
            retval = false;
          }
          n = n.next;
        }
      }
#endif
      return retval;
    }
    #endregion

    #region ICloneable Members

    /// <summary>
    /// Make a shallow copy of this LinkedList.
    /// </summary>
    /// <returns></returns>
    public virtual object Clone()
    {
      LinkedList<T> clone = new LinkedList<T>(itemequalityComparer);
      clone.AddAll(this);
      return clone;
    }

    #endregion

    #region System.Collections.Generic.IList<T> Members

    void System.Collections.Generic.IList<T>.RemoveAt(int index)
    {
      RemoveAt(index);
    }

    void System.Collections.Generic.ICollection<T>.Add(T item)
    {
      Add(item);
    }

    #endregion

    #region System.Collections.ICollection Members

    bool System.Collections.ICollection.IsSynchronized
    {
      get { return false; }
    }

    [Obsolete]
    Object System.Collections.ICollection.SyncRoot
    {
      // Presumably safe to use the startsentinel (of type Node, always != null) as SyncRoot
      // since the class Node is private.
      get { return underlying != null ? ((System.Collections.ICollection)underlying).SyncRoot : startsentinel; }
    }

    void System.Collections.ICollection.CopyTo(Array arr, int index)
    {
      if (index < 0 || index + Count > arr.Length)
        throw new ArgumentOutOfRangeException();

      foreach (T item in this)
        arr.SetValue(item, index++);
    }

    #endregion

    #region System.Collections.IList Members

    Object System.Collections.IList.this[int index]
    {
      get { return this[index]; }
      set { this[index] = (T)value; }
    }

    int System.Collections.IList.Add(Object o)
    {
      bool added = Add((T)o);
      // What position to report if item not added? SC.IList.Add doesn't say
      return added ? Count-1 : -1;
    }

    bool System.Collections.IList.Contains(Object o)
    {
      return Contains((T)o);
    }

    int System.Collections.IList.IndexOf(Object o)
    {
      return Math.Max(-1, IndexOf((T)o));
    }

    void System.Collections.IList.Insert(int index, Object o)
    {
      Insert(index, (T)o);
    }

    void System.Collections.IList.Remove(Object o)
    {
      Remove((T)o);
    }

    void System.Collections.IList.RemoveAt(int index)
    {
      RemoveAt(index);
    }

    #endregion
  }
}