rndFenceRect

-- arguments: length width
-- length and width of bounding fence rectangle: length width-2 length width-2
-- starts along length with block in front of turtle and turns left after length fence blocks
local args = {...}
local t = turtle
local ts = t.select
local ic = t.getItemCount

print("Fuel: ",t.getFuelLevel()," / ",t.getFuelLimit())

local slotList = {}
local function prepareRandomSlot()
  slotList = {}
  for i=1,16 do
    if ic(i) > 0 then slotList[#slotList+1] = {["slot"]=i,["qty"]=ic(i)} end
  end
end

prepareRandomSlot()

local function updateRandomSlot(slot)
  local index = nil
  for i=1,#slotList do
    if slotList[i].slot == slot then
        slotList[i].qty = slotList[i].qty - 1
        index = i
      break
    end
    if index and slotList[index].qty <= 0 then table.remove(slotList,index) end
  end
end

local function randomSlot()
  if #slotList == 0 then return 1
  else
    return slotList[math.random(#slotList)].slot
  end
end

local function placeFence(slot)
  ts(slot)
  t.digDown()
  t.placeDown()
  updateRandomSlot(slot)
end

local length = args[1] and args[1] or 0
local width = args[2] and args[2] or 0

local function move()
  while t.detect() do
    t.dig()
    sleep(0.5)
  end
  t.forward()
end

local function go()
  local rect = {length,width-1,length-1,width-2}
  for k=1,4 do
    for i=1,rect[k] do
      move()
      placeFence(randomSlot())
    end
    t.turnLeft()
  end
end

go()