System.Linq.ItemPosition.GetInsertionIndex C# (CSharp) Метод

        public int GetInsertionIndex()
        {
            return Index + Math.Min(1, Offset);
        }

            protected override void UpdateOnMove(NotifyCollectionChangedEventArgs e)
            {
                base.UpdateOnMove(e);

                System.Diagnostics.Debug.Assert(e.NewItems.Count == e.OldItems.Count);

                var items =
                    e.NewItems.
                    OfType <T>().
                    Select((item, idx) => new { Item = item, Index = idx }).
                    Where(x => _predicate(x.Item));

                var position = new ItemPosition(-1, 1);

                ItemPosition oldDestinationPosition = position, newDestinationPosition = position;

                for (int idx = 0; idx < _source.Count; idx++)
                {
                    if (idx == e.NewStartingIndex)
                    {
                        newDestinationPosition = position;
                    }
                    if (idx == e.OldStartingIndex)
                    {
                        oldDestinationPosition = position;
                    }

                    T item = default(T);

                    if (idx >= e.OldStartingIndex && idx < e.OldStartingIndex + e.OldItems.Count)
                    {
                        item = (T)e.OldItems[idx - e.OldStartingIndex];
                    }
                    else
                    {
                        int afterChangeIndex = idx;
                        if (afterChangeIndex >= e.OldStartingIndex)
                        {
                            afterChangeIndex = afterChangeIndex - e.OldItems.Count;
                        }
                        if (afterChangeIndex >= e.NewStartingIndex)
                        {
                            afterChangeIndex = afterChangeIndex + e.NewItems.Count;
                        }

                        item = _source[afterChangeIndex];
                    }

                    if (_predicate(item))
                    {
                        position = new ItemPosition(position.Index + 1, 0);
                    }
                    else
                    {
                        position = new ItemPosition(position.Index, position.Offset + 1);
                    }
                }

                foreach (var moved in items)
                {
                    this.Move(oldDestinationPosition.GetInsertionIndex() + moved.Index, newDestinationPosition.GetInsertionIndex() + moved.Index);
                }
            }