' version 16-10-2016
' compile with: fbc -s gui

' a cell size of 4 x 4 pixels is used
' In FreeBASIC the 0,0 is the top left corner

ScreenRes 400,400,8 ' give a 100 by 100 field
Dim As UByte Ptr p = ScreenPtr
If p = 0 Then End ' p does not point to screen

Palette 0,       0,   0,   0      ' index 0 = black
Palette 255,   255, 255, 255      ' index 225 = white

Line (0, 0) - (799, 799), 255, bf   ' draw box and fill it with white color

Dim As Integer count, offset, x = 199, y = 199
Dim As UByte col   ' = color
' direction, 0 = up, 1 = right, 2 = down, 3 = left
Dim As UByte d     ' d = 0, looking up

Do
  offset = x + y * 400
  col = p[offset]

  If col = 0 Then
    d = (d -1) And 3
  Else
    d = (d +1) And 3
  EndIf

  col = col Xor 255 ' flip the color

  ScreenLock        ' don't update screen while we are drawing

  ' draw a 4*4 block and paint it with palette color [0 | 255]
  Line (x, y) - (x +3, y -3), col, bf

  ScreenUnLock    ' allow screen update's

  'Sleep 100       ' slow the program down if needed

  ' true = 0, false = -1
  If (d And 1) = 1 Then
    x = x + (d = 1) * 4 - (d = 3) * 4
  Else
    y = y - (d = 0) * 4 + (d = 2) * 4
  End If

  count += 1
  ' update step count window title bar
  WindowTitle "Langton's ant step: " + Str(count)

  ' has user clicked on close window "X" then end program
  If InKey = Chr(255) + "k" Then End

Loop Until x < 1 Or x > 398 Or y < 1 Or y > 398

' display total count in window title bar
WindowTitle "Langton's ant has left the field in " + Str(count) + " steps"

' empty keyboard buffer
While InKey <> "" : Wend
'Print : Print "hit any key to end program"
Sleep
End