_G.facing = "south"

local rightOf = {
	south = "west",
	west = "north",
	north = "east",
	east = "south"
}

local leftOf = {
	west = "south",
	north = "west",
	east = "north",
	south = "east"
}

local opposite = {
	west = "east",
	east = "west",
	south = "north",
	north = "south"
}

function step()
--	exists, block = turtle.inspectDown()
	--if not exists then
		repeat
			turtle.down()
			exists, block = turtle.inspectDown()
		until exists
--	end
	if block.name == "minecraft:rail" then
		shape = block.state.shape
		if shape  == "ascending_" .. _G.facing then
			turtle.up()
			turtle.forward()
		elseif string.match(shape, _G.facing) or shape == "ascending_" .. opposite[_G.facing] then
			turtle.forward()
		elseif string.match(shape, leftOf[_G.facing]) then
			_G.facing = leftOf[_G.facing]
			turtle.turnLeft()
		elseif string.match(shape, rightOf[_G.facing]) then
			_G.facing = rightOf[_G.facing]
			turtle.turnRight()
		else
			error("i'm lost\n".. shape)
		end
		return false
	elseif block.name == "minecraft:iron_block" then
		return "done"
	else
		return "lost"
	end
end

function follow_rail(init_dir)
	_G.facing = init_dir
	repeat
		state = step()
	until state
	print(state)
end

print("what direction am i facing?")
follow_rail(read())