Simple Lumberjack Turtle

local saplingSlot = 13
local bonemealSlot = 14
local fuelSlot = 15
local logSlot = 16
local issue = false

function checkFuel ()
  if turtle.getFuelLevel() < 6 then
    if turtle.getItemCount(fuelSlot) > 1 then
      turtle.select(fuelSlot)
      while turtle.getFuelLevel() < 6 do
        turtle.refuel(1)
      end
    else
      print("Fuel Reserves too low. Exiting.")
      issue = true
    end
  end
end

function backDown ()
  checkFuel()
  while turtle.down() do
    checkFuel()
  end
end

function chopTree ()
  checkFuel()
  turtle.select(logSlot)
  while turtle.compare() do
    checkFuel()
    turtle.dig()
    while not turtle.up() do
      turtle.digUp()
    end
    turtle.select(logSlot)
  end
  backDown()
  if turtle.getItemCount(saplingSlot) > 1 then
    turtle.select(saplingSlot)
    turtle.place()
  else
    print("Low on saplings. Exiting.")
    issues = true
  end
end

backDown()

turtle.select(logSlot)
if not turtle.compare() then
  turtle.dig()
  turtle.select(saplingSlot)
  turtle.place()
end

while not issues do
  turtle.select(logSlot)
  if not turtle.compare() then
    if turtle.getItemCount(bonemealSlot) > 1 then
      turtle.select(bonemealSlot)
      turtle.place()
    end
  else
    chopTree()
  end
end