local rows = 20
local dir = 0

function digRow()
  local blocks = rows
	while blocks > 0 do
	  turtle.dig()
		turtle.forward()

		local upTime = 0
		while turtle.detectUp() do
		  turtle.digUp()
			turtle.up()
		  upTime = upTime + 1
		end
		while upTime > 0 do
		  turtle.down()
			upTime = upTime - 1
		end

		if not turtle.detectDown then
		  turtle.placeDown()
		end
	  blocks = blocks - 1
	end
end

while rows > 0 do
  digRow()
	if dir == 0 then
	  turtle.turnLeft()
		turtle.forward()
		turtle.turnLeft()
		dir = 1
	end
	if dir == 1 then
	  turtle.turnRight()
		turtle.forward()
		turtle.turnRight()
		dir = 0
	end
	rows = rows - 1
end