Test

local x = 1

turtle.select(1)
turtle.forward()
for i=1,100 do
	if turtle.detectDown() == false then
		turtle.placeDown()
		turtle.forward()
		if turtle.getItemCount == 1 then
			local x = x + 1
			turtle.select(x)
		end
	else
		turtle.turnRight()
		turtle.forward()
		turtle.turnRight()
		turtle.forward()
	end
end

for i=1,100 do
	if turtle.detectDown() == false then
		turtle.placeDown()
		turtle.forward()
		if turtle.getItemCount == 1 then
			local x = x + 1
			turtle.select(x)
		end
	else
		turtle.turnLeft()
		turtle.forward()
		turtle.turnLeft()
		turtle.forward()
	end
end