excavateSM

function excavate(size, max_depth)

	local depth = 0

	local xPos, zPos = 0, 0
	local xDir, zDir = 0, 1

	local goTo -- Filled in further down
	local refuel -- Filled in further down

	local function unload()
		print("Unloading items...")
		for n = 3, 16 do
			local nCount = turtle.getItemCount(n)
			if nCount > 0 then
				turtle.select(n)
				turtle.dropDown()
			end
		end
		turtle.select(3)
	end

	local function returnSupplies()
		local x, y, z, xd, zd = xPos, depth, zPos, xDir, zDir
		print("Returning to surface...")
		goTo(0, 0, -1, 0, -1)
		local fuelNeeded = 2 * (x + y + z) + 1
		if not refuel(fuelNeeded) then
			unload()
			print("Waiting for fuel")
			while not refuel(fuelNeeded) do
				os.pullEvent("turtle_inventory")
			end
		else
			unload()
		end
		print("Resuming mining...")
		goTo(0, 0, 0, 0, -1)
		goTo(x, y, z, xd, zd)
	end

	local function collect()
		local bFull = true
		for n = 3, 16 do
			local nCount = turtle.getItemCount(n)
			if nCount == 0 then
				bFull = false
			end
		end
		if bFull then
			print("No empty slots left")
			return false
		end
		return true
	end

	function refuel(amount)
		local fuelLevel = turtle.getFuelLevel()
		if fuelLevel == "unlimited" then
			return true
		end
		local needed = amount or (xPos + zPos + depth + 2)
		if turtle.getFuelLevel() < needed then
			turtle.select(1)
			while turtle.getItemCount(1) > 0 and turtle.getFuelLevel() < needed do
				turtle.refuel(1)
			end
			if turtle.getFuelLevel() >= needed then
				turtle.select(1)
				return true
			end
			return false
		end
		return true
	end

	local function tryForwards()
		if not refuel() then
			print("Not enough Fuel")
			returnSupplies()
		end
		while not turtle.forward() do
			if turtle.detect() then
				if turtle.dig() then
					if not collect() then
						returnSupplies()
					end
				else
					return false
				end
			elseif turtle.attack() then
				if not collect() then
					returnSupplies()
				end
			else
				sleep(0.5)
			end
		end
		xPos = xPos + xDir
		zPos = zPos + zDir
		return true
	end

	local function tryDown()
		if not refuel() then
			print("Not enough Fuel")
			returnSupplies()
		end
		while not turtle.down() do
			if turtle.detectDown() then
				if turtle.digDown() then
					if not collect() then
						returnSupplies()
					end
				else
					return false
				end
			elseif turtle.attackDown() then
				if not collect() then
					returnSupplies()
				end
			else
				sleep(0.5)
			end
		end
		depth = depth + 1
		if math.fmod(depth, 10) == 0 then
			print("Descended "..depth.." metres.")
		end
		return true
	end

	local function turnLeft()
		turtle.turnLeft()
		xDir, zDir = -zDir, xDir
	end

	local function turnRight()
		turtle.turnRight()
		xDir, zDir = zDir, -xDir
	end

	function goTo(x, y, z, xd, zd)
		while depth > y do
			if turtle.up() then
				depth = depth - 1
			elseif turtle.digUp() or turtle.attackUp() then
				collect()
			else
				sleep(0.5)
			end
		end
		if xPos > x then
			while xDir ~= -1 do
				turnLeft()
			end
			while xPos > x do
				if turtle.forward() then
					xPos = xPos - 1
				elseif turtle.dig() or turtle.attack() then
					collect()
				else
					sleep(0.5)
				end
			end
		elseif xPos < x then
			while xDir ~= 1 do
				turnLeft()
			end
			while xPos < x do
				if turtle.forward() then
					xPos = xPos + 1
				elseif turtle.dig() or turtle.attack() then
					collect()
				else
					sleep(0.5)
				end
			end
		end
		if zPos > z then
			while zDir ~= -1 do
				turnLeft()
			end
			while zPos > z do
				if turtle.forward() then
					zPos = zPos - 1
				elseif turtle.dig() or turtle.attack() then
					collect()
				else
					sleep(0.5)
				end
			end
		elseif zPos < z then
			while zDir ~= 1 do
				turnLeft()
			end
			while zPos < z do
				if turtle.forward() then
					zPos = zPos + 1
				elseif turtle.dig() or turtle.attack() then
					collect()
				else
					sleep(0.5)
				end
			end
		end
		while depth < y do
			if turtle.down() then
				depth = depth + 1
			elseif turtle.digDown() or turtle.attackDown() then
				collect()
			else
				sleep(0.5)
			end
		end
		while zDir ~= zd or xDir ~= xd do
			turnLeft()
		end
	end

	if not refuel() then
		print("Out of Fuel")
		return
	end

	print("Excavating...")

	local alternate = 0
	local done = false
	while not done do
		for n = 1, size do
			for m = 1, size-1 do
				if not tryForwards() then
					done = true
					break
				end
			end
			if done then
				break
			end
			if n<size then
				if math.fmod(n + alternate, 2) == 0 then
					turnLeft()
					if not tryForwards() then
						done = true
						break
					end
					turnLeft()
				else
					turnRight()
					if not tryForwards() then
						done = true
						break
					end
					turnRight()
				end
			end
		end
		if done then
			break
		end
		if size > 1 then
			if math.fmod(size, 2) == 0 then
				turnRight()
			else
				if alternate == 0 then
					turnLeft()
				else
					turnRight()
				end
				alternate = 1 - alternate
			end
		end

		if (not tryDown() or depth >= max_depth) then
			done = true
			break
		end
	end

	print("Returning to surface...")

	-- Return to where we started
	goTo(0, 0, 0, 0, -1)
	goTo(0, 0, 0, 0, 1)

end