ellipse

-- drawElipse <semi_major_axis> <semi_minor_axis>
-- place turtle facing semi-minor axis in the center
sMa, sma = {...}
stage = 1
running = true
selectedItem = 1

--[[stages 1=started, moving toward sma
           2=moving right, back
           3=moving left, back
           4=moving left, forward
           5=moving right, forward]]--
           1
location = {x=0, y=0}
foci = {1={x=0, y=0}, 2={x=0,y=0}}

function findFoci()
	dF = math.sqrt((sMa*sMa)-(sma*sma))
	foci[1].x = math.floor(dF)
	foci[2].x = -(math.floor(dF)
end

function move(dir)
-- dir = 1 forward, 2 = right, 3 = back, 4 = left
-- note: turtle will ALWAYS face the same direction
	if dir == 1 then
		if turtle.forward() then
		  location.y = location.y + 1
		end
	elsif dir == 2 then
	  turtle.turnRight()
	  if turtle.forward() then
	  	location.x = location.x + 1
	  end
	  turtle.turnLeft()
	 elseif dir == 3 then
	 	if turtle.back() then
	 		location.y = location.y - 1
	 	end
	 elseif dir == 4 then
	 	turtle.turnLeft()
	 		if turtle.forward() then
	 			location.x = location.x - 1
	 		end
	 	turtle.turnRight()
	 end

end

function focalDistance()
  dx1 = location.x - foci[1].x
  dx2 = location.x - foci[2].x
  dy1 = location.y - foci[1].y
  dy2 = location.y - foci[2].y

  df1 = math.sqrt(dx1*dx1 + dy1*dy1)
  df2 = math.sqrt(dx2*dx2 + dy2*dy2)

  return math.floor(df1+df2)
end

function chooseItem()
  if turtle.getItemCount(selectedItem) > 2 then
    selectedItem = selectedItem + 1
  end
  turtle.select(selectedItem)
end

function main()
  chooseItem()
  if stage == 1 then
    if focalDistance() < (sMa * 2) then
      move(1)
    else
      turtle.placeUp()
      stage = 2
      return
    end
  elseif stage == 2 then
    move(2)
    if focalDistance() > (sMa * 2) then
      move(3)
    end
    turtle.placeUp()

    if location.x == 0 then
      stage = 3
      return
    end
  elseif stage == 3 then
    move( 3)
    if focalDistance() > (2*sMa) then
      move(4)
    end
    turtle.placeUp()

    if location.y == 0 then
      stage = 4
      return
    end
  elseif stage == 4 then
    move(4)
    if focalDistance() == (2*sMa) then
      move(1)
    end
    turtle.placeUp()
    if location.y == 0 then
      stage = 5
      return
    end
  elseif stage == 5
    move(1)
    if focalDistance() > (2*sMa) then
      move(2)
    end
    turtle.placeUp()
    if location.y == 0 then
      stage = 6 return
    end
  elseif stage == 6 then
    move(3)
    if location.x == 0 and location.y == 0 then
      running = false
    end

  end
end

while running do
  parallel.waitForAny(main)
end