startup

lane=0
write("Length: ")
length = read()
function moveForward(length)
  for i = 1, length do
        turtle.forward()
  end
end
function switchLanes()
  moveForward(1)
  turtle.turnRight()
  moveForward(2)
  turtle.turnRight()
end
function switchLanes2()
  moveForward(1)
  turtle.turnLeft()
  moveForward(4)
  turtle.turnLeft()
end
function dig()
  turtle.dig()
  moveForward(1)
  turtle.digDown()
end
function home()
    if lane==6 then
        turtle.turnLeft()
        moveForward(18)
        turtle.turnRight()
        lane=0
  end
        if turtle.getFuelLevel() <=1000 then
            turtle.down()
            turtle.down()
            turtle.suckDown()
            for x=1,16,1 do
                turtle.select(x)
                turtle.refuel()
            end
        sleep(120)
    end
end
function roam()
  lane=lane+1
  for i = 1, length do
        if turtle.detect() then
          dig()
        else
         moveForward(1)
        end
  end
  if lane%2==0 then
    switchLanes2()
  else
    switchLanes()
  end
  home()
  sleep(1)
end

turtle.up()
turtle.up()

while true do
  roam()
end