Wall builder

local function forward()
  while not turtle.forward() do
    print("Blocked or out of fuel")
  end
end

local slot = 1

turtle.select(1)

local function checkSlot()
  local loopCount = 0
  while turtle.getItemCount(slot) == 0 do
    loopCount = loopCount + 1
    slot = slot + 1
    if slot > 16 then
      slot = 1
    end
    turtle.select(slot)

    if loopCount > 15 then
      print("Waiting for more building materials.")
      sleep(10)
    end
  end
end

while true do
  turtle.placeUp()
  checkSlot()
  if turtle.detect() then
    turtle.turnLeft()
    turtle.turnLeft()
    turtle.down()
  else
    forward()
  end
end