// The following class implements a sliding-tile puzzle.
   // The GridStrip control is a custom ToolStrip that arranges
   // its ToolStripButton controls in a grid layout. There is 
   // one empty cell, into which the user can slide an adjacent
   // tile with a drag-and-drop operation. Tiles that are eligible 
   // for moving are highlighted.
   public class GridStrip : ToolStrip
   {
       // The button that is the drag source.
       private ToolStripButton dragButton = null;

       // Settings for the ToolStrip control's TableLayoutPanel.
       // This provides access to the cell position of each
       // ToolStripButton.
       private TableLayoutSettings tableSettings = null;

       // The empty cell. ToolStripButton controls that are
       // adjacent to this button can be moved to this button's
       // cell position.
       private ToolStripButton emptyCellButton = null;

       // The dimensions of each tile. A tile is represented
       // by a ToolStripButton controls.
       private Size tileSize = new Size(128, 128);

       // The number of rows in the GridStrip control.
       private readonly int rows = 5;

       // The number of columns in the GridStrip control.
       private readonly int columns = 5;

       // The one-time initialzation behavior is enforced
       // with this field. For more information, see the 
       // OnPaint method.
       private bool firstTime = false;

       // This is a required by the Windows Forms designer.
       private System.ComponentModel.IContainer components;

       // The default constructor.  
       public GridStrip()
       {
           this.InitializeComponent();

           this.InitializeTableLayoutSettings();
       }

       // This property exposes the empty cell to the 
       // GridStripRenderer class.
       internal ToolStripButton EmptyCell
       {
           get
           {
               return this.emptyCellButton;
           }
       }

       // This utility method initializes the TableLayoutPanel 
       // which contains the ToolStripButton controls.
       private void InitializeTableLayoutSettings()
       {
           // Specify the numbers of rows and columns in the GridStrip control.
           this.tableSettings = base.LayoutSettings as TableLayoutSettings;
           this.tableSettings.ColumnCount = this.rows;
           this.tableSettings.RowCount = this.columns;

           // Create a dummy bitmap with the dimensions of each tile.
           // The GridStrip control sizes itself based on these dimensions.
           Bitmap b = new Bitmap(tileSize.Width, tileSize.Height);

           // Populate the GridStrip control with ToolStripButton controls.
           for (int i = 0; i < this.tableSettings.ColumnCount; i++)
           {
               for (int j = 0; j < this.tableSettings.RowCount; j++)
               {
                   // Create a new ToolStripButton control.
                   ToolStripButton btn = new ToolStripButton();
                   btn.DisplayStyle = ToolStripItemDisplayStyle.Image;
                   btn.Image = b;
                   btn.ImageAlign = ContentAlignment.MiddleCenter;
                   btn.ImageScaling = ToolStripItemImageScaling.None;
                   btn.Margin = Padding.Empty;
                   btn.Padding = Padding.Empty;

                   // Add the new ToolStripButton control to the GridStrip.
                   this.Items.Add(btn);

                   // Set the cell position of the ToolStripButton control.
                   TableLayoutPanelCellPosition cellPos = new TableLayoutPanelCellPosition(i, j);
                   this.tableSettings.SetCellPosition(btn, cellPos);

                   // If this is the ToolStripButton control at cell (0,0),
                   // assign it as the empty cell button.
                   if( i == 0 && j == 0 )
                   {
                       btn.Text = "Empty Cell";
                       btn.Image = b;
                       this.emptyCellButton = btn;
                   }
               }
           }
       }

       // This method defines the Paint event behavior.
       // The GridStripRenderer requires that the GridStrip
       // be fully layed out when it is renders, so this
       // initialization code cannot be placed in the
       // GridStrip constructor. By the time the Paint
       // event is raised, the control layout has been 
       // completed, so the GridStripRenderer can paint
       // correctly. This one-time initialization is
       // implemented with the firstTime field.
       protected override void OnPaint(PaintEventArgs e)
       {
           base.OnPaint(e);

           if (!this.firstTime)
           {
               this.Renderer = new GridStripRenderer();

               // Comment this line to see the unscrambled image.
               this.ScrambleButtons();
               this.firstTime = true;
           }
       }

       // This utility method changes the ToolStripButton control 
       // positions in the TableLayoutPanel. This scrambles the 
       // buttons to initialize the puzzle.
       private void ScrambleButtons()
       {
           int i = 0;
           int lastElement = this.Items.Count - 1;

           while ( (i != lastElement ) &&
                   (lastElement - i > 1) )
           {
               TableLayoutPanelCellPosition pos1 = 
                   this.tableSettings.GetCellPosition(this.Items[i]);

               TableLayoutPanelCellPosition pos2 = 
                   this.tableSettings.GetCellPosition(this.Items[lastElement]);

               this.tableSettings.SetCellPosition(
                   this.Items[i++], 
                   pos2);

               this.tableSettings.SetCellPosition(
                   this.Items[lastElement--], 
                   pos1);
           }
       }

       // This method defines the MouseDown event behavior. 
       // If the user has clicked on a valid drag source, 
       // the drag operation starts.
       protected override void OnMouseDown(MouseEventArgs mea)
       {
           base.OnMouseDown(mea);

           ToolStripButton btn = this.GetItemAt(mea.Location) as ToolStripButton;

           if (btn != null)
           {
               if (this.IsValidDragSource(btn))
               {
                   this.dragButton = btn;
               }
           }
       }

       // This method defines the MouseMove event behavior. 
       protected override void OnMouseMove(MouseEventArgs mea)
       {
           base.OnMouseMove(mea);

           // Is a drag operation pending?
           if (this.dragButton != null)
           {
               // A drag operation is pending. Call DoDragDrop to 
               // determine the disposition of the operation.
               DragDropEffects dropEffect = this.DoDragDrop(
                   new DataObject(this.dragButton), 
                   DragDropEffects.Move);
           }
       }

       // This method defines the DragOver event behavior. 
       protected override void OnDragOver(DragEventArgs dea)
       {
           base.OnDragOver(dea);

           // Get the ToolStripButton control 
           // at the given mouse position.
           Point p = new Point(dea.X, dea.Y);
           ToolStripButton item = this.GetItemAt(
               this.PointToClient(p)) as ToolStripButton;

           // If the ToolStripButton control is the empty cell,
           // indicate that the move operation is valid.
           if( item == this.emptyCellButton )
           {
               // Set the drag operation to indicate a valid move.
               dea.Effect = DragDropEffects.Move;
           }
       }

       // This method defines the DragDrop event behavior. 
       protected override void OnDragDrop(DragEventArgs dea)
       {
           base.OnDragDrop(dea);

           // Did a valid move operation occur?
           if (dea.Effect == DragDropEffects.Move)
           {
               // The move operation is valid. Adjust the state
               // of the GridStrip control's TableLayoutPanel,
               // by swapping the positions of the source button
               // and the empty cell button.

               // Get the cell of the control to move.
               TableLayoutPanelCellPosition sourcePos = 
                   tableSettings.GetCellPosition(this.dragButton);

               // Get the cell of the emptyCellButton.
               TableLayoutPanelCellPosition dropPos = 
                   tableSettings.GetCellPosition(this.emptyCellButton);

               // Move the control to the empty cell.
               tableSettings.SetCellPosition(this.dragButton, dropPos);

               // Set the position of the empty cell to 
               // that of the previously occupied cell.
               tableSettings.SetCellPosition(this.emptyCellButton, sourcePos);

               // Reset the drag operation.
               this.dragButton = null;
           }
       }

       // This method defines the DragLeave event behavior. 
       // If the mouse leaves the client area of the GridStrip
       // control, the drag operation is canceled.
       protected override void OnDragLeave(EventArgs e)
       {
           base.OnDragLeave(e);

           // Reset the drag operation.
           this.dragButton = null;
       }

       // This method defines the ueryContinueDrag event behavior. 
       // If the mouse leaves the client area of the GridStrip
       // control, the drag operation is canceled.
       protected override void OnQueryContinueDrag(QueryContinueDragEventArgs qcdevent)
       {
           base.OnQueryContinueDrag(qcdevent);

           // Get the current mouse position, in screen coordinates.
           Point mousePos = this.PointToClient(Control.MousePosition);

           // If the mouse position is outside the GridStrip control's
           // client area, cancel the drag operation. Be sure to
           // transform the mouse's screen coordinates to client coordinates. 
           if (!this.ClientRectangle.Contains(mousePos))
           {
               qcdevent.Action = DragAction.Cancel;
           }
       }

       // This utility method determines if a button
       // is positioned relative to the empty cell 
       // such that it can be dragged into the empty cell.
       private bool IsValidDragSource(ToolStripButton b)
       {
           TableLayoutPanelCellPosition sourcePos = 
               tableSettings.GetCellPosition(b);

           TableLayoutPanelCellPosition emptyPos = 
               tableSettings.GetCellPosition(this.emptyCellButton);

           return (IsValidDragSource(sourcePos, emptyPos));
       }

       // This utility method determines if a cell position
       // is adjacent to the empty cell.
       internal static bool IsValidDragSource(
           TableLayoutPanelCellPosition sourcePos, 
           TableLayoutPanelCellPosition emptyPos)
       {
           bool returnValue = false;

           // A cell is considered to be a valid drag source if it
           // is adjacent to the empty cell. Cells that are positioned
           // on a diagonal are not valid.
           if (((sourcePos.Column == emptyPos.Column - 1) && (sourcePos.Row == emptyPos.Row)) ||
               ((sourcePos.Column == emptyPos.Column + 1) && (sourcePos.Row == emptyPos.Row)) ||
               ((sourcePos.Column == emptyPos.Column) && (sourcePos.Row == emptyPos.Row - 1)) ||
               ((sourcePos.Column == emptyPos.Column) && (sourcePos.Row == emptyPos.Row + 1)))
           {
               returnValue = true;
           }

           return returnValue;
       }