ComputerCraft turtle to patrol a rectangular area delimited by obsidian blocks

patrolturtle.lua

shell.run('clear')
print("Please put obsidian in Slot 1 and mark the corners with obsidian.")
print("Go clockwise? (y/N)")
local clockwise = io.read():upper() == "Y"
local obsidian = 1

print("Going "..(clockwise and "clockwise" or "counter-clockwise")..".")

while true do
	turtle.select(obsidian)
	repeat
		for i = 1, 3 do
			turtle.attack()
			turtle.attackUp()
		end

		turtle.forward()
	until turtle.compareDown()
	if clockwise then
		turtle.turnRight()
	else
		turtle.turnLeft()
	end
end