CC TreeFarm

local args = {...}

local width = tonumber(args[1]) or 12
local height = tonumber(args[2]) or width

local start = turtle.loc

local function makeItem(name, metadata)
  return { name=name, metadata=metadata }
end

local treeType = 0 -- Oak

local trunk = makeItem("minecraft:log", treeType)
local sapling = makeItem("minecraft:sapling", treeType)
local leaves = makeItem("minecraft:leaves", 4 + treeType)

local function isMatch(a, b)
  return a.name == b.name and a.metadata == b.metadata
end

local function chopAndPlantOne()
  -- Done if this is outside the planting area
  local x = turtle.loc.x - start.x + 1
  local z = turtle.loc.z - start.z + 1
  if x <= 2 or x + 2 > width or z <= 2 or z + 2 > height then
    return
  end

  -- Done if there is already a sapling planted
  local _, down = turtle.inspectDown()
  if isMatch(down, sapling) then
    return
  end

  -- Not a sapling, so dig it out
  turtle.digDown()

  -- Replant if we have extra saplings
  if turtle.getItemCount(1) > 1 then
    turtle.select(1)
    turtle.placeDown()
  end
end

local function chopAndPlant(n)
  for i=1,n do
    turtle.digUp()
    chopAndPlantOne()
    turtle.suckDown()
    turtle.dig()
    turtle.forward()
  end
end

local function chopAll(n)
  for i=1,n do
    turtle.digUp()
    turtle.digDown()
    turtle.dig()
    turtle.forward()
  end
end

local layerCount = 0

local function chopLayer()
  local turnParity = layerCount + width;

  local layer = (turtle.loc.y - 1 - start.y)/3
  if layer > 0 then
    layerCount = layerCount + 1
  else
    layerCount = 0
  end

  -- Chop and plant on the first layer, otherwise just shop
  local chop = layer == 0 and chopAndPlant or chopAll

  for i=1,width do
    local turn = (i + turnParity) % 2 == 0 and turtle.lt or turtle.rt
    if i > 1 then
      turn()
      chop(1)
      turn()
    end
    chop(height-1)
  end

  -- About face
  turtle.rt()
  turtle.rt()
end

local function chopUp(n)
  for i=1,n do
    turtle.digUp()
    turtle.up()
  end
end

local function chopDown(n)
  for i=1,n do
    turtle.digDown()
    turtle.down()
  end
end

----------
-- Main --
----------

local requiredFuel = width*height*2 + 6

chopUp(1)

while true do
  -- TODO: inventory stuff

  -- Check fuel
  if turtle.getFuelLevel() < requiredFuel then
    print "Not enough fuel"
    -- TODO: refuel
    break
  end

  chopUp(3)
  chopLayer()

  chopDown(3)
  chopLayer()

  -- stop after one pass for now
  break
end

chopDown(1)