Implement spiral square, but it's untested.

It would also be awesome to customise the directional plane upon which 
the spiral is generated. It might actually be possible without melting 
my brain I think....


Looking at http://www.mathematische-basteleien.de/spiral.htm it should 
be possible to go this for circles too. But there's 1 particular 
function in Vector2.js that we need to port to Vector3.lua that we 
haven't yet which we'd need for that.....

worldeditadditions/lib/spiral_square.lua

@@ -0,0 +1,83 @@
+local wea = worldeditadditions
+local Vector3 = wea.Vector3
+
+-- ███████ ██████  ██ ██████   █████  ██
+-- ██      ██   ██ ██ ██   ██ ██   ██ ██
+-- ███████ ██████  ██ ██████  ███████ ██
+--      ██ ██      ██ ██   ██ ██   ██ ██
+-- ███████ ██      ██ ██   ██ ██   ██ ███████
+-- 
+-- ███████  ██████  ██    ██  █████  ██████  ███████
+-- ██      ██    ██ ██    ██ ██   ██ ██   ██ ██
+-- ███████ ██    ██ ██    ██ ███████ ██████  █████
+--      ██ ██ ▄▄ ██ ██    ██ ██   ██ ██   ██ ██
+-- ███████  ██████   ██████  ██   ██ ██   ██ ███████
+--             ▀▀
+
+--- Creates a square spiral that fills the defined region.
+-- @param	pos1			Vector3		The 1st position of the defined region.
+-- @param	pos2			Vector3		The 2nd position of the defined region.
+-- @param	target_node		Vector3		The *normalised* name of the node to use to build the square spiral with.
+-- @param	interval		number		The distance between the walls of the spiral.
+-- @param	acceleration=0	number		Increate the interval by this number every time we hit a corner of the square spiral.
+-- @returns	bool,number|string			A success boolean value, followed by either the number of the nodes set or an error message string.
+function worldeditadditions.spiral_square(pos1, pos2, target_node, interval, acceleration)
+	if not acceleration then acceleration = 0 end
+	
+	pos1, pos2 = Vector3.sort(pos1, pos2)
+	local volume = pos2:subtract(pos1)
+	local volume_half = volume:divide(2)
+	
+	print("DEBUG:spiral_square | pos1: "..pos1..", pos2: "..pos2..", target_node: "..target_node)
+	
+	
+	-- Fetch the nodes in the specified area
+	local manip, area = worldedit.manip_helpers.init(pos1, pos2)
+	local data = manip:get_data()
+	
+	local node_id = minetest.get_content_id(target_node)
+	
+	local count = 0 -- The number of nodes replaced
+	
+	local centre = pos2:subtract(pos1):floor()
+	
+	local pos_current = centre:clone()
+	local pos_corner_last = pos_current:clone()
+	local side_length = 0
+	local direction = Vector3.new(1, 0, 0)
+	local side_length_max = interval
+	
+	while pos_current:is_contained(pos1, pos2) do
+		
+		for y = pos2.y, pos1.y, -1 do
+			data[area:index(pos_current.x, y, pos_current.z)] = node_id
+			count = count + 1
+		end
+		
+		pos_current = pos_current:add(direction)
+		side_length = side_length + 1
+		
+		if side_length > side_length_max then
+			side_length_max = side_length_max + interval + acceleration
+			
+			if direction.x == 0 and direction.z == 1 then
+				direction.x = 1
+				direction.z = 0
+			elseif direction.x == 1 and direction.z == 0 then
+				direction.x = 0
+				direction.z = -1
+			elseif direction.x == 0 and direction.z == -1 then
+				direction.x = -1
+				direction.z = 0
+			else
+				direction.x = 0
+				direction.z = 1
+			end
+		end
+	end
+	
+	-- Save the modified nodes back to disk & return
+	worldedit.manip_helpers.finish(manip, data)
+	
+	return true, count
+end