gui-cs.Terminal.Gui/Terminal.Gui/Views/TableView.cs

using NStack;
using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;

namespace Terminal.Gui {

	/// <summary>
	/// Describes how to render a given column in  a <see cref="TableView"/> including <see cref="Alignment"/> and textual representation of cells (e.g. date formats)
	/// </summary>
	public class ColumnStyle {
		
		/// <summary>
		/// Defines the default alignment for all values rendered in this column.  For custom alignment based on cell contents use <see cref="AlignmentGetter"/>.
		/// </summary>
		public TextAlignment Alignment {get;set;}
	
		/// <summary>
		/// Defines a delegate for returning custom alignment per cell based on cell values.  When specified this will override <see cref="Alignment"/>
		/// </summary>
		public Func<object,TextAlignment> AlignmentGetter;

		/// <summary>
		/// Defines a delegate for returning custom representations of cell values.  If not set then <see cref="object.ToString()"/> is used.  Return values from your delegate may be truncated e.g. based on <see cref="MaxWidth"/>
		/// </summary>
		public Func<object,string> RepresentationGetter;

		/// <summary>
		/// Defines the format for values e.g. "yyyy-MM-dd" for dates
		/// </summary>
		public string Format{get;set;}

		/// <summary>
		/// Set the maximum width of the column in characters.  This value will be ignored if more than the tables <see cref="TableView.MaxCellWidth"/>.  Defaults to <see cref="TableView.DefaultMaxCellWidth"/>
		/// </summary>
		public int MaxWidth {get;set;} = TableView.DefaultMaxCellWidth;

		/// <summary>
		/// Set the minimum width of the column in characters.  This value will be ignored if more than the tables <see cref="TableView.MaxCellWidth"/> or the <see cref="MaxWidth"/>
		/// </summary>
		public int MinWidth {get;set;}

		/// <summary>
		/// Returns the alignment for the cell based on <paramref name="cellValue"/> and <see cref="AlignmentGetter"/>/<see cref="Alignment"/>
		/// </summary>
		/// <param name="cellValue"></param>
		/// <returns></returns>
		public TextAlignment GetAlignment(object cellValue)
		{
			if(AlignmentGetter != null)
				return AlignmentGetter(cellValue);

			return Alignment;
		}

		/// <summary>
		/// Returns the full string to render (which may be truncated if too long) that the current style says best represents the given <paramref name="value"/>
		/// </summary>
		/// <param name="value"></param>
		/// <returns></returns>
		public string GetRepresentation (object value)
		{
			if(!string.IsNullOrWhiteSpace(Format)) {

				if(value is IFormattable f)
					return f.ToString(Format,null);
			}
				

			if(RepresentationGetter != null)
				return RepresentationGetter(value);

			return value?.ToString();
		}
	}
	/// <summary>
	/// Defines rendering options that affect how the table is displayed
	/// </summary>
	public class TableStyle {
		
		/// <summary>
		/// When scrolling down always lock the column headers in place as the first row of the table
		/// </summary>
		public bool AlwaysShowHeaders {get;set;} = false;

		/// <summary>
		/// True to render a solid line above the headers
		/// </summary>
		public bool ShowHorizontalHeaderOverline {get;set;} = true;

		/// <summary>
		/// True to render a solid line under the headers
		/// </summary>
		public bool ShowHorizontalHeaderUnderline {get;set;} = true;

		/// <summary>
		/// True to render a solid line vertical line between cells
		/// </summary>
		public bool ShowVerticalCellLines {get;set;} = true;

		/// <summary>
		/// True to render a solid line vertical line between headers
		/// </summary>
		public bool ShowVerticalHeaderLines {get;set;} = true;

		/// <summary>
		/// Collection of columns for which you want special rendering (e.g. custom column lengths, text alignment etc)
		/// </summary>
		public Dictionary<DataColumn,ColumnStyle> ColumnStyles {get;set; }  = new Dictionary<DataColumn, ColumnStyle>();

		/// <summary>
		/// Returns the entry from <see cref="ColumnStyles"/> for the given <paramref name="col"/> or null if no custom styling is defined for it
		/// </summary>
		/// <param name="col"></param>
		/// <returns></returns>
		public ColumnStyle GetColumnStyleIfAny (DataColumn col)
		{
			return ColumnStyles.TryGetValue(col,out ColumnStyle result) ? result : null;
		}
	}
	
	/// <summary>
	/// View for tabular data based on a <see cref="DataTable"/>
	/// </summary>
	public class TableView : View {

		private int columnOffset;
		private int rowOffset;
		private int selectedRow;
		private int selectedColumn;
		private DataTable table;
		private TableStyle style = new TableStyle();

		/// <summary>
		/// The default maximum cell width for <see cref="TableView.MaxCellWidth"/> and <see cref="ColumnStyle.MaxWidth"/>
		/// </summary>
		public const int DefaultMaxCellWidth = 100;

		/// <summary>
		/// The data table to render in the view.  Setting this property automatically updates and redraws the control.
		/// </summary>
		public DataTable Table { get => table; set {table = value; Update(); } }
		
		/// <summary>
		/// Contains options for changing how the table is rendered
		/// </summary>
		public TableStyle Style { get => style; set {style = value; Update(); } }
						
		/// <summary>
		/// True to select the entire row at once.  False to select individual cells.  Defaults to false
		/// </summary>
		public bool FullRowSelect {get;set;}

		/// <summary>
		/// True to allow regions to be selected 
		/// </summary>
		/// <value></value>
		public bool MultiSelect {get;set;} = true;

		/// <summary>
		/// When <see cref="MultiSelect"/> is enabled this property contain all rectangles of selected cells.  Rectangles describe column/rows selected in <see cref="Table"/> (not screen coordinates)
		/// </summary>
		/// <returns></returns>
		public Stack<TableSelection> MultiSelectedRegions {get;} = new Stack<TableSelection>();

		/// <summary>
		/// Horizontal scroll offset.  The index of the first column in <see cref="Table"/> to display when when rendering the view.
		/// </summary>
		/// <remarks>This property allows very wide tables to be rendered with horizontal scrolling</remarks>
		public int ColumnOffset {
			get => columnOffset;

			//try to prevent this being set to an out of bounds column
			set => columnOffset = Table == null ? 0 :Math.Max (0,Math.Min (Table.Columns.Count - 1,  value));
		}

		/// <summary>
		/// Vertical scroll offset.  The index of the first row in <see cref="Table"/> to display in the first non header line of the control when rendering the view.
		/// </summary>
		public int RowOffset {
			get => rowOffset;
			set => rowOffset = Table == null ? 0 : Math.Max (0,Math.Min (Table.Rows.Count - 1, value));
		}

		/// <summary>
		/// The index of <see cref="DataTable.Columns"/> in <see cref="Table"/> that the user has currently selected
		/// </summary>
		public int SelectedColumn {
			get => selectedColumn;

			set {
				var oldValue = selectedColumn;

				//try to prevent this being set to an out of bounds column
				selectedColumn = Table == null ? 0 :  Math.Min (Table.Columns.Count - 1, Math.Max (0, value));

				if(oldValue != selectedColumn)
					OnSelectedCellChanged(new SelectedCellChangedEventArgs(Table,oldValue,SelectedColumn,SelectedRow,SelectedRow));
			} 
		}

		/// <summary>
		/// The index of <see cref="DataTable.Rows"/> in <see cref="Table"/> that the user has currently selected
		/// </summary>
		public int SelectedRow {
			get => selectedRow;
			set {

				var oldValue = selectedRow;

				selectedRow =  Table == null ? 0 : Math.Min (Table.Rows.Count - 1, Math.Max (0, value));

				if(oldValue != selectedRow)
					OnSelectedCellChanged(new SelectedCellChangedEventArgs(Table,SelectedColumn,SelectedColumn,oldValue,selectedRow));
			}
		}

		/// <summary>
		/// The maximum number of characters to render in any given column.  This prevents one long column from pushing out all the others
		/// </summary>
		public int MaxCellWidth { get; set; } = DefaultMaxCellWidth;

		/// <summary>
		/// The text representation that should be rendered for cells with the value <see cref="DBNull.Value"/>
		/// </summary>
		public string NullSymbol { get; set; } = "-";

		/// <summary>
		/// The symbol to add after each cell value and header value to visually seperate values (if not using vertical gridlines)
		/// </summary>
		public char SeparatorSymbol { get; set; } = ' ';

		/// <summary>
		/// This event is raised when the selected cell in the table changes.
		/// </summary>
		public event Action<SelectedCellChangedEventArgs> SelectedCellChanged;

		/// <summary>
		/// Initialzies a <see cref="TableView"/> class using <see cref="LayoutStyle.Computed"/> layout. 
		/// </summary>
		/// <param name="table">The table to display in the control</param>
		public TableView (DataTable table) : this ()
		{
			this.Table = table;
		}

		/// <summary>
		/// Initialzies a <see cref="TableView"/> class using <see cref="LayoutStyle.Computed"/> layout. Set the <see cref="Table"/> property to begin editing
		/// </summary>
		public TableView () : base ()
		{
			CanFocus = true;
		}

		///<inheritdoc/>
		public override void Redraw (Rect bounds)
		{
			Move (0, 0);
			var frame = Frame;

			// What columns to render at what X offset in viewport
			var columnsToRender = CalculateViewport(bounds).ToArray();

			Driver.SetAttribute (ColorScheme.Normal);
			
			//invalidate current row (prevents scrolling around leaving old characters in the frame
			Driver.AddStr (new string (' ', bounds.Width));

			int line = 0;

			if(ShouldRenderHeaders()){
				// Render something like:
				/*
					┌────────────────────┬──────────┬───────────┬──────────────┬─────────┐
					│ArithmeticComparator│chi       │Healthboard│Interpretation│Labnumber│
					└────────────────────┴──────────┴───────────┴──────────────┴─────────┘
				*/
				if(Style.ShowHorizontalHeaderOverline){
					RenderHeaderOverline(line,bounds.Width,columnsToRender);
					line++;
				}

				RenderHeaderMidline(line,columnsToRender);
				line++;

				if(Style.ShowHorizontalHeaderUnderline){
					RenderHeaderUnderline(line,bounds.Width,columnsToRender);
					line++;
				}
			}
					
			int headerLinesConsumed = line;

			//render the cells
			for (; line < frame.Height; line++) {

				ClearLine(line,bounds.Width);

				//work out what Row to render
				var rowToRender = RowOffset + (line - headerLinesConsumed);

				//if we have run off the end of the table
				if ( Table == null || rowToRender >= Table.Rows.Count || rowToRender < 0)
					continue;

				RenderRow(line,rowToRender,columnsToRender);
			}
		}

		/// <summary>
		/// Clears a line of the console by filling it with spaces
		/// </summary>
		/// <param name="row"></param>
		/// <param name="width"></param>
		private void ClearLine(int row, int width)
		{            
			Move (0, row);
			Driver.SetAttribute (ColorScheme.Normal);
			Driver.AddStr (new string (' ', width));
		}

		/// <summary>
		/// Returns the amount of vertical space required to display the header
		/// </summary>
		/// <returns></returns>
		private int GetHeaderHeight()
		{
			int heightRequired = 1;
			
			if(Style.ShowHorizontalHeaderOverline)
				heightRequired++;

			if(Style.ShowHorizontalHeaderUnderline)
				heightRequired++;
			
			return heightRequired;
		}

		private void RenderHeaderOverline(int row,int availableWidth, ColumnToRender[] columnsToRender)
		{
			// Renders a line above table headers (when visible) like:
			// ┌────────────────────┬──────────┬───────────┬──────────────┬─────────┐

			for(int c = 0;c< availableWidth;c++) {

				var rune = Driver.HLine;

				if (Style.ShowVerticalHeaderLines){
							
					if(c == 0){
						rune = Driver.ULCorner;
					}	
					// if the next column is the start of a header
					else if(columnsToRender.Any(r=>r.X == c+1)){
						rune = Driver.TopTee;
					}
					else if(c == availableWidth -1){
						rune = Driver.URCorner;
					}
				}

				AddRuneAt(Driver,c,row,rune);
			}
		}

		private void RenderHeaderMidline(int row, ColumnToRender[] columnsToRender)
		{
			// Renders something like:
			// │ArithmeticComparator│chi       │Healthboard│Interpretation│Labnumber│
						
			ClearLine(row,Bounds.Width);

			//render start of line
			if(style.ShowVerticalHeaderLines)
				AddRune(0,row,Driver.VLine);

			for(int i =0 ; i<columnsToRender.Length;i++) {
				
				var current =  columnsToRender[i];
				var availableWidthForCell = GetCellWidth(columnsToRender,i);

				var colStyle = Style.GetColumnStyleIfAny(current.Column);
				var colName = current.Column.ColumnName;

				RenderSeparator(current.X-1,row,true);
									
				Move (current.X, row);
				
				Driver.AddStr(TruncateOrPad(colName,colName,availableWidthForCell ,colStyle));

			}

			//render end of line
			if(style.ShowVerticalHeaderLines)
				AddRune(Bounds.Width-1,row,Driver.VLine);
		}

		/// <summary>
		/// Calculates how much space is available to render index <paramref name="i"/> of the <paramref name="columnsToRender"/> given the remaining horizontal space
		/// </summary>
		/// <param name="columnsToRender"></param>
		/// <param name="i"></param>
		private int GetCellWidth (ColumnToRender [] columnsToRender, int i)
		{
			var current =  columnsToRender[i];
			var next = i+1 < columnsToRender.Length ? columnsToRender[i+1] : null;

			if(next == null) {
				// cell can fill to end of the line
				return Bounds.Width - current.X;
			}
			else {
				// cell can fill up to next cell start				
				return next.X - current.X;
			}

		}

		private void RenderHeaderUnderline(int row,int availableWidth, ColumnToRender[] columnsToRender)
		{
			// Renders a line below the table headers (when visible) like:
			// ├──────────┼───────────┼───────────────────┼──────────┼────────┼─────────────┤
								
			for(int c = 0;c< availableWidth;c++) {

				var rune = Driver.HLine;

				if (Style.ShowVerticalHeaderLines){
					if(c == 0){
						rune = Style.ShowVerticalCellLines ? Driver.LeftTee : Driver.LLCorner;
					}	
					// if the next column is the start of a header
					else if(columnsToRender.Any(r=>r.X == c+1)){
					
						/*TODO: is ┼ symbol in Driver?*/ 
						rune = Style.ShowVerticalCellLines ? '┼' :Driver.BottomTee;
					}
					else if(c == availableWidth -1){
						rune = Style.ShowVerticalCellLines ? Driver.RightTee : Driver.LRCorner;
					}
				}

				AddRuneAt(Driver,c,row,rune);
			}
			
		}
		private void RenderRow(int row, int rowToRender, ColumnToRender[] columnsToRender)
		{
			//render start of line
			if(style.ShowVerticalCellLines)
				AddRune(0,row,Driver.VLine);

			// Render cells for each visible header for the current row
			for(int i=0;i< columnsToRender.Length ;i++) {

				var current = columnsToRender[i];
				var availableWidthForCell = GetCellWidth(columnsToRender,i);

				var colStyle = Style.GetColumnStyleIfAny(current.Column);

				// move to start of cell (in line with header positions)
				Move (current.X, row);

				// Set color scheme based on whether the current cell is the selected one
				bool isSelectedCell = IsSelected(current.Column.Ordinal,rowToRender);

				Driver.SetAttribute (isSelectedCell ? ColorScheme.HotFocus : ColorScheme.Normal);

				var val = Table.Rows [rowToRender][current.Column];

				// Render the (possibly truncated) cell value
				var representation = GetRepresentation(val,colStyle);
				
				Driver.AddStr (TruncateOrPad(val,representation,availableWidthForCell,colStyle));
				
				// If not in full row select mode always, reset color scheme to normal and render the vertical line (or space) at the end of the cell
				if(!FullRowSelect)
					Driver.SetAttribute (ColorScheme.Normal);

				RenderSeparator(current.X-1,row,false);
			}

			//render end of line
			if(style.ShowVerticalCellLines)
				AddRune(Bounds.Width-1,row,Driver.VLine);
		}
		
		private void RenderSeparator(int col, int row,bool isHeader)
		{
			if(col<0)
				return;
				
			var renderLines = isHeader ? style.ShowVerticalHeaderLines : style.ShowVerticalCellLines;

			Rune symbol =  renderLines ? Driver.VLine : SeparatorSymbol;
			AddRune(col,row,symbol);
		}

		void AddRuneAt (ConsoleDriver d,int col, int row, Rune ch)
		{
			Move (col, row);
			d.AddRune (ch);
		}

		/// <summary>
		/// Truncates or pads <paramref name="representation"/> so that it occupies a exactly <paramref name="availableHorizontalSpace"/> using the alignment specified in <paramref name="colStyle"/> (or left if no style is defined)
		/// </summary>
		/// <param name="originalCellValue">The object in this cell of the <see cref="Table"/></param>
		/// <param name="representation">The string representation of <paramref name="originalCellValue"/></param>
		/// <param name="availableHorizontalSpace"></param>
		/// <param name="colStyle">Optional style indicating custom alignment for the cell</param>
		/// <returns></returns>
		private string TruncateOrPad (object originalCellValue,string representation, int availableHorizontalSpace, ColumnStyle colStyle)
		{
			if (string.IsNullOrEmpty (representation))
				return representation;

			// if value is not wide enough
			if(representation.Sum(c=>Rune.ColumnWidth(c)) < availableHorizontalSpace) {
				
				// pad it out with spaces to the given alignment
				int toPad = availableHorizontalSpace - (representation.Sum(c=>Rune.ColumnWidth(c)) +1 /*leave 1 space for cell boundary*/);

				switch(colStyle?.GetAlignment(originalCellValue) ?? TextAlignment.Left) {

					case TextAlignment.Left : 
						return representation + new string(' ',toPad);
					case TextAlignment.Right : 
						return new string(' ',toPad) + representation;
					
					// TODO: With single line cells, centered and justified are the same right?
					case TextAlignment.Centered : 
					case TextAlignment.Justified : 
						return 
							new string(' ',(int)Math.Floor(toPad/2.0)) + // round down
							representation +
							 new string(' ',(int)Math.Ceiling(toPad/2.0)) ; // round up
				}
			}

			// value is too wide
			return new string(representation.TakeWhile(c=>(availableHorizontalSpace-= Rune.ColumnWidth(c))>0).ToArray());
		}

		/// <inheritdoc/>
		public override bool ProcessKey (KeyEvent keyEvent)
		{
			switch (keyEvent.Key) {
			case Key.CursorLeft:
			case Key.ShiftMask | Key.CursorLeft:
				ChangeSelectionByOffset(-1,0,keyEvent.Key.HasFlag(Key.ShiftMask));
				Update ();
				break;
			case Key.CursorRight:
			case Key.ShiftMask | Key.CursorRight:
				ChangeSelectionByOffset(1,0,keyEvent.Key.HasFlag(Key.ShiftMask));
				Update ();
				break;
			case Key.CursorDown:
			case Key.ShiftMask | Key.CursorDown:
				ChangeSelectionByOffset(0,1,keyEvent.Key.HasFlag(Key.ShiftMask));
				Update ();
				break;
			case Key.CursorUp:
			case Key.ShiftMask | Key.CursorUp:
				ChangeSelectionByOffset(0,-1,keyEvent.Key.HasFlag(Key.ShiftMask));
				Update ();
				break;
			case Key.PageUp:
				SelectedRow -= Frame.Height;
				Update ();
				break;
			case Key.PageDown:
				SelectedRow += Frame.Height;
				Update ();
				break;
			case Key.Home | Key.CtrlMask:
				SelectedRow = 0;
				SelectedColumn = 0;
				Update ();
				break;
			case Key.Home:
				SelectedColumn = 0;
				Update ();
				break;
			case Key.End | Key.CtrlMask:
				//jump to end of table
				SelectedRow =  Table == null ? 0 : Table.Rows.Count - 1;
				SelectedColumn =  Table == null ? 0 : Table.Columns.Count - 1;
				Update ();
				break;
			case Key.End:
				//jump to end of row
				SelectedColumn =  Table == null ? 0 : Table.Columns.Count - 1;
				Update ();
				break;
			default:
				// Not a keystroke we care about
				return false;
			}
			PositionCursor ();
			return true;
		}

		private void ChangeSelectionByOffset (int offsetX, int offsetY, bool extendExistingSelection)
		{
			if(!MultiSelect || !extendExistingSelection)
				MultiSelectedRegions.Clear();

			if(extendExistingSelection)
			{
				// If we are extending current selection but there isn't one
				if(MultiSelectedRegions.Count == 0)
				{
					// Create a new region between the old active cell and the new offset
					var rect = CreateTableSelection(SelectedColumn,SelectedRow,SelectedColumn+offsetX,SelectedRow+offsetY);
					MultiSelectedRegions.Push(rect);
				}
				else
				{
					// Extend the current head selection to include the new offset
					var head = MultiSelectedRegions.Pop();
					var newRect = CreateTableSelection(head.Origin.X,head.Origin.Y,SelectedColumn + offsetX,SelectedRow + offsetY);
					MultiSelectedRegions.Push(newRect);
				}
			}

			SelectedColumn += offsetX;
			SelectedRow += offsetY;
			
		}


		/// <summary>
		/// Returns a new rectangle between the two points with positive width/height regardless of relative positioning of the points.  pt1 is always considered the <see cref="TableSelection.Origin"/> point
		/// </summary>
		/// <param name="pt1X">Origin point for the selection in X</param>
		/// <param name="pt1Y">Origin point for the selection in Y</param>
		/// <param name="pt2X">End point for the selection in X</param>
		/// <param name="pt2Y">End point for the selection in Y</param>
		/// <returns></returns>
		private TableSelection CreateTableSelection (int pt1X, int pt1Y, int pt2X, int pt2Y)
		{
			var top = Math.Min(pt1Y,pt2Y);
			var bot = Math.Max(pt1Y,pt2Y);

			var left = Math.Min(pt1X,pt2X);
			var right = Math.Max(pt1X,pt2X);

			// Rect class is inclusive of Top Left but exclusive of Bottom Right so extend by 1
			return new TableSelection(new Point(pt1X,pt1Y),new Rect(left,top,right-left + 1,bot-top + 1));
		}

		/// <summary>
		/// Returns true if the given cell is selected either because it is the active cell or part of a multi cell selection (e.g. <see cref="FullRowSelect"/>)
		/// </summary>
		/// <param name="col"></param>
		/// <param name="row"></param>
		/// <returns></returns>
		public bool IsSelected(int col, int row)
		{
			// Cell is also selected if 
			if(MultiSelect && MultiSelectedRegions.Any(r=>r.Rect.Contains(col,row)))
				return true;

			return row == SelectedRow && 
					(col == SelectedColumn || FullRowSelect);
		}


		

		/// <summary>
		/// Positions the cursor in the area of the screen in which the start of the active cell is rendered.  Calls base implementation if active cell is not visible due to scrolling or table is loaded etc
		/// </summary>
		public override void PositionCursor()
		{
			if(Table == null) {
				base.PositionCursor();
				return;
			}
				
			var screenPoint = CellToScreen(SelectedColumn,SelectedRow);
			
			if(screenPoint  != null)
				Move(screenPoint.Value.X,screenPoint.Value.Y);
		}

		///<inheritdoc/>
		public override bool MouseEvent (MouseEvent me)
		{
			if (!me.Flags.HasFlag (MouseFlags.Button1Clicked) && !me.Flags.HasFlag (MouseFlags.Button1DoubleClicked) &&
				me.Flags != MouseFlags.WheeledDown && me.Flags != MouseFlags.WheeledUp &&
				me.Flags != MouseFlags.WheeledLeft && me.Flags != MouseFlags.WheeledRight)
				return false;

			if (!HasFocus && CanFocus) {
				SetFocus ();
			}

			if (Table == null) {
				return false;
			}

			// Scroll wheel flags
			switch(me.Flags)
			{
				case MouseFlags.WheeledDown: 
					RowOffset++;
					EnsureValidScrollOffsets();
					SetNeedsDisplay();
					return true;

				case MouseFlags.WheeledUp:
					RowOffset--;
					EnsureValidScrollOffsets();
					SetNeedsDisplay();
					return true;

				case MouseFlags.WheeledRight:
					ColumnOffset++;
					EnsureValidScrollOffsets();
					SetNeedsDisplay();
					return true;

				case  MouseFlags.WheeledLeft:
					ColumnOffset--;
					EnsureValidScrollOffsets();
					SetNeedsDisplay();
					return true;
			}

			if(me.Flags == MouseFlags.Button1Clicked) {
				
				var hit = ScreenToCell(me.OfX,me.OfY);

				if(hit != null) {
					
					SelectedRow = hit.Value.Y;
					SelectedColumn = hit.Value.X;
					Update();
				}
			}

			return false;
		}

		/// <summary>
		/// Returns the column and row of <see cref="Table"/> that corresponds to a given point on the screen (relative to the control client area).  Returns null if the point is in the header, no table is loaded or outside the control bounds
		/// </summary>
		/// <param name="clientX">X offset from the top left of the control</param>
		/// <param name="clientY">Y offset from the top left of the control</param>
		/// <returns></returns>
		public Point? ScreenToCell (int clientX, int clientY)
		{
			if(Table == null)
				return null;

			var viewPort = CalculateViewport(Bounds);
				
			var headerHeight = ShouldRenderHeaders()? GetHeaderHeight():0;

			var col = viewPort.LastOrDefault(c=>c.X <= clientX);
				
			// Click is on the header section of rendered UI
			if(clientY < headerHeight)
				return null;

			var rowIdx = RowOffset - headerHeight + clientY;

			if(col != null && rowIdx >= 0) {
				
				return new Point(col.Column.Ordinal,rowIdx);
			}

			return null;
		}
		
		/// <summary>
		/// Returns the screen position (relative to the control client area) that the given cell is rendered or null if it is outside the current scroll area or no table is loaded
		/// </summary>
		/// <param name="tableColumn">The index of the <see cref="Table"/> column you are looking for, use <see cref="DataColumn.Ordinal"/></param>
		/// <param name="tableRow">The index of the row in <see cref="Table"/> that you are looking for</param>
		/// <returns></returns>
		public Point? CellToScreen (int tableColumn, int tableRow)
		{
			if(Table == null)
				return null;

			var viewPort = CalculateViewport(Bounds);
				
			var headerHeight = ShouldRenderHeaders()? GetHeaderHeight():0;

			var colHit = viewPort.FirstOrDefault(c=>c.Column.Ordinal == tableColumn);

			// current column is outside the scroll area
			if(colHit == null)
				return null;
				
			// the cell is too far up above the current scroll area
			if(RowOffset > tableRow)
				return null;

			// the cell is way down below the scroll area and off the screen
			if(tableRow > RowOffset + (Bounds.Height - headerHeight))
				return null;
 
			return new Point(colHit.X,tableRow + headerHeight - RowOffset);
		}
		/// <summary>
		/// Updates the view to reflect changes to <see cref="Table"/> and to (<see cref="ColumnOffset"/> / <see cref="RowOffset"/>) etc
		/// </summary>
		/// <remarks>This always calls <see cref="View.SetNeedsDisplay()"/></remarks>
		public void Update()
		{
			if(Table == null) {
				SetNeedsDisplay ();
				return;
			}

			EnsureValidScrollOffsets();
			EnsureValidSelection();

			EnsureSelectedCellIsVisible();

			SetNeedsDisplay ();
		}

		/// <summary>
		/// Updates <see cref="ColumnOffset"/> and <see cref="RowOffset"/> where they are outside the bounds of the table (by adjusting them to the nearest existing cell).  Has no effect if <see cref="Table"/> has not been set.
		/// </summary>
		/// <remarks>Changes will not be immediately visible in the display until you call <see cref="View.SetNeedsDisplay()"/></remarks>
		public void EnsureValidScrollOffsets ()
		{
			if(Table == null){
				return;
			}

			ColumnOffset = Math.Max(Math.Min(ColumnOffset,Table.Columns.Count -1),0);
			RowOffset = Math.Max(Math.Min(RowOffset,Table.Rows.Count -1),0);
		}


		/// <summary>
		/// Updates <see cref="SelectedColumn"/> and <see cref="SelectedRow"/> where they are outside the bounds of the table (by adjusting them to the nearest existing cell).  Has no effect if <see cref="Table"/> has not been set.
		/// </summary>
		/// <remarks>Changes will not be immediately visible in the display until you call <see cref="View.SetNeedsDisplay()"/></remarks>
		public void EnsureValidSelection ()
		{
			if(Table == null){
				return;
			}

			SelectedColumn = Math.Max(Math.Min(SelectedColumn,Table.Columns.Count -1),0);
			SelectedRow = Math.Max(Math.Min(SelectedRow,Table.Rows.Count -1),0);
		}

		/// <summary>
		/// Updates scroll offsets to ensure that the selected cell is visible.  Has no effect if <see cref="Table"/> has not been set.
		/// </summary>
		/// <remarks>Changes will not be immediately visible in the display until you call <see cref="View.SetNeedsDisplay()"/></remarks>
		public void EnsureSelectedCellIsVisible ()
		{
			if(Table == null || Table.Columns.Count <= 0){
				return;
			}

			var columnsToRender = CalculateViewport (Bounds).ToArray();
			var headerHeight = ShouldRenderHeaders()? GetHeaderHeight() : 0;

			//if we have scrolled too far to the left 
			if (SelectedColumn < columnsToRender.Min (r => r.Column.Ordinal)) {
				ColumnOffset = SelectedColumn;
			}

			//if we have scrolled too far to the right
			if (SelectedColumn > columnsToRender.Max (r=> r.Column.Ordinal)) {
				ColumnOffset = SelectedColumn;
			}

			//if we have scrolled too far down
			if (SelectedRow >= RowOffset + (Bounds.Height - headerHeight)) {
				RowOffset = SelectedRow;
			}
			//if we have scrolled too far up
			if (SelectedRow < RowOffset) {
				RowOffset = SelectedRow;
			}
		}

		/// <summary>
		/// Invokes the <see cref="SelectedCellChanged"/> event
		/// </summary>
		protected virtual void OnSelectedCellChanged(SelectedCellChangedEventArgs args)
		{
			SelectedCellChanged?.Invoke(args);
		}

		/// <summary>
		/// Calculates which columns should be rendered given the <paramref name="bounds"/> in which to display and the <see cref="ColumnOffset"/>
		/// </summary>
		/// <param name="bounds"></param>
		/// <param name="padding"></param>
		/// <returns></returns>
		private IEnumerable<ColumnToRender> CalculateViewport (Rect bounds, int padding = 1)
		{
			if(Table == null)
				yield break;
			
			int usedSpace = 0;

			//if horizontal space is required at the start of the line (before the first header)
			if(Style.ShowVerticalHeaderLines || Style.ShowVerticalCellLines)
				usedSpace+=1;
			
			int availableHorizontalSpace = bounds.Width;
			int rowsToRender = bounds.Height;

			// reserved for the headers row
			if(ShouldRenderHeaders())
				rowsToRender -= GetHeaderHeight(); 

			bool first = true;

			foreach (var col in Table.Columns.Cast<DataColumn>().Skip (ColumnOffset)) {

				int startingIdxForCurrentHeader = usedSpace;
				var colStyle = Style.GetColumnStyleIfAny(col);

				// is there enough space for this column (and it's data)?
				usedSpace += CalculateMaxCellWidth (col, rowsToRender,colStyle) + padding;

				// no (don't render it) unless its the only column we are render (that must be one massively wide column!)
				if (!first && usedSpace > availableHorizontalSpace)
					yield break;

				// there is space
				yield return new ColumnToRender(col, startingIdxForCurrentHeader);
				first=false;
			}
		}

		private bool ShouldRenderHeaders()
		{
			if(Table == null || Table.Columns.Count == 0)
				return false;

		    return Style.AlwaysShowHeaders || rowOffset == 0;
		}

		/// <summary>
		/// Returns the maximum of the <paramref name="col"/> name and the maximum length of data that will be rendered starting at <see cref="RowOffset"/> and rendering <paramref name="rowsToRender"/>
		/// </summary>
		/// <param name="col"></param>
		/// <param name="rowsToRender"></param>
		/// <param name="colStyle"></param>
		/// <returns></returns>
		private int CalculateMaxCellWidth(DataColumn col, int rowsToRender,ColumnStyle colStyle)
		{
			int spaceRequired = col.ColumnName.Sum(c=>Rune.ColumnWidth(c));

			// if table has no rows
			if(RowOffset < 0)
				return spaceRequired;


			for (int i = RowOffset; i < RowOffset + rowsToRender && i < Table.Rows.Count; i++) {

				//expand required space if cell is bigger than the last biggest cell or header
				spaceRequired = Math.Max (spaceRequired, GetRepresentation(Table.Rows [i][col],colStyle).Sum(c=>Rune.ColumnWidth(c)));
			}

			// Don't require more space than the style allows
			if(colStyle != null){

				// enforce maximum cell width based on style
				if(spaceRequired > colStyle.MaxWidth) {
					spaceRequired = colStyle.MaxWidth;
				}

				// enforce minimum cell width based on style
				if(spaceRequired < colStyle.MinWidth) {
					spaceRequired = colStyle.MinWidth;
				}
			}
			
			// enforce maximum cell width based on global table style
			if(spaceRequired > MaxCellWidth)
				spaceRequired = MaxCellWidth;


			return spaceRequired;
		}

		/// <summary>
		/// Returns the value that should be rendered to best represent a strongly typed <paramref name="value"/> read from <see cref="Table"/>
		/// </summary>
		/// <param name="value"></param>
		/// <param name="colStyle">Optional style defining how to represent cell values</param>
		/// <returns></returns>
		private string GetRepresentation(object value,ColumnStyle colStyle)
		{
			if (value == null || value == DBNull.Value) {
				return NullSymbol;
			}

			return colStyle != null ? colStyle.GetRepresentation(value): value.ToString();
		}
	}

	/// <summary>
	/// Describes a desire to render a column at a given horizontal position in the UI
	/// </summary>
	internal class ColumnToRender {

		/// <summary>
		/// The column to render
		/// </summary>
		public DataColumn Column {get;set;}

		/// <summary>
		/// The horizontal position to begin rendering the column at
		/// </summary>
		public int X{get;set;}

		public ColumnToRender (DataColumn col, int x)
		{
			Column = col;
			X = x;
		}
	}

	/// <summary>
	/// Defines the event arguments for <see cref="TableView.SelectedCellChanged"/> 
	/// </summary>
	public class SelectedCellChangedEventArgs : EventArgs
	{
		/// <summary>
		/// The current table to which the new indexes refer.  May be null e.g. if selection change is the result of clearing the table from the view
		/// </summary>
		/// <value></value>
		public DataTable Table {get;}


		/// <summary>
		/// The previous selected column index.  May be invalid e.g. when the selection has been changed as a result of replacing the existing Table with a smaller one
		/// </summary>
		/// <value></value>
		public int OldCol {get;}


		/// <summary>
		/// The newly selected column index.
		/// </summary>
		/// <value></value>
		public int NewCol {get;}


		/// <summary>
		/// The previous selected row index.  May be invalid e.g. when the selection has been changed as a result of deleting rows from the table
		/// </summary>
		/// <value></value>
		public int OldRow {get;}


		/// <summary>
		/// The newly selected row index.
		/// </summary>
		/// <value></value>
		public int NewRow {get;}

		/// <summary>
		/// Creates a new instance of arguments describing a change in selected cell in a <see cref="TableView"/>
		/// </summary>
		/// <param name="t"></param>
		/// <param name="oldCol"></param>
		/// <param name="newCol"></param>
		/// <param name="oldRow"></param>
		/// <param name="newRow"></param>
		public SelectedCellChangedEventArgs(DataTable t, int oldCol, int newCol, int oldRow, int newRow)
		{
			Table = t;
			OldCol = oldCol;
			NewCol = newCol;
			OldRow = oldRow;
			NewRow = newRow;
		}
	}

	/// <summary>
	/// Describes a selected region of the table
	/// </summary>
	public class TableSelection{

		/// <summary>
		/// Corner of the <see cref="Rect"/> where selection began
		/// </summary>
		/// <value></value>
		public Point Origin{get;}

		/// <summary>
		/// Area selected
		/// </summary>
		/// <value></value>
		public Rect Rect { get; }

		/// <summary>
		/// Creates a new selected area starting at the origin corner and covering the provided rectangular area
		/// </summary>
		/// <param name="origin"></param>
		/// <param name="rect"></param>
		public TableSelection(Point origin, Rect rect)
		{
			Origin = origin;
			Rect = rect;
		}

	}
}