Computercraft turtle smart tunneller

-- save this as "tunnel"
-- place turtle at front-left and a chest behind him.
-- put torches in last slot to light up your tunnel
-- usage:   tunnel 20 10
-- will dig a tunnel 20 long and 10x10
-- when inventory is getting full, turtle will return to chest to empty
-- turtle will use any fuel it mines or you can drop lava/coal etc in for him to use


-- grab fuel from all slots
function grabFuel()
 for i=1,15 do
  turtle.select(i)
  turtle.refuel()
 end
end

function digAndMove()
    while (turtle.detect()) do
        turtle.dig(); os.sleep(0.5);
    end
 while (not turtle.forward()) do
  turtle.dig(); turtle.attack(); os.sleep(0.5);
 end
end

function digAndMoveUp()
    while (turtle.detectUp()) do
        turtle.digUp(); os.sleep(0.5);
    end
 while (not turtle.up()) do
    turtle.digUp();
 end
end

function digAndMoveDown()
    while (turtle.detectDown()) do
        turtle.digDown(); os.sleep(0.5);
    end
    while (not turtle.down()) do
    turtle.digDown(); os.sleep(0.5)
 end
end

function returnHome()
  local done=false
  if tonumber(distanceFromChest) >=tonumber(tunlen) then
   done=true
  end
  turtle.turnLeft()
  turtle.turnLeft()
  for d=1,distanceFromChest do
    digAndMove()
  end
  --make sure we are back home
  while not turtle.detect() do
    turtle.forward()
    os.sleep(0.2)
  end

  --unload
  for slot=1, 15, 1 do
    turtle.select(slot);
    turtle.drop();
  end
  if done then
   announce("All done")
   error("done")
  else
   turtle.turnLeft()
   turtle.turnLeft()
   for d=1,distanceFromChest do
    digAndMove()
   end
  end
end

function announce(thing)
print(thing)
end

----------------- START
args={...}
tunlen=args[1]
tunsize=args[2]

if tonumber(tunsize) % 2 ~= 0 then
 tunsize=tonumber(tunsize)+1
end
distanceFromChest=0;
distanceWere=0;

announce("Starting to dig tunnel "..tunlen.." long...")

--find start of mine
while not turtle.detect() do
  turtle.forward()
  os.sleep(0.2)
end

while true do
 if distanceFromChest > tonumber(tunlen) then
  returnHome()
 end
    grabFuel();
    announce("Fuel "..turtle.getFuelLevel().." at "..distanceFromChest.." of "..tunlen);

    --place torch?
    if distanceFromChest % 5 == 1 then
     turtle.turnLeft()
     turtle.dig();
     turtle.select(16);
     turtle.place();
     turtle.turnRight()
    end

    grabFuel();

    --facing bottom left block
     digAndMove()
     distanceFromChest=distanceFromChest + 1;

    turtle.turnRight()

    for bottotop=1,tonumber(tunsize) do
     for leftright=2,tonumber(tunsize) do
      digAndMove()
     end --leftright

     if (bottotop < tonumber(tunsize)) then
       digAndMoveUp()
       turtle.turnLeft()
       turtle.turnLeft()
     else
       turtle.turnRight()
      for down=2,tunsize do
        digAndMoveDown()
      end
     end
     end --botttotop

    local goHome=true
    for inv=1,16 do
      if turtle.getItemCount(inv)==0 then
       goHome=false
      end
     end
    if goHome then
      announce("about to drop off load")
      returnHome()
    end
end