blockPlacer.lua

local function find()
  for i = 1, 16 do
    if turtle.getItemCount(i) > 0 then
      turtle.select(i)
      print("Selected " .. tostring(i))
      return
    end
  end
  error("No items.")
end

local oldplc = turtle.place
function turtle.place()
  find()
  return oldplc()
end

while true do
  print("Checking")
  if not turtle.inspect() then
    print("No block, placing")
    turtle.place()
  end
  print("ok")
  os.sleep(5)
end