Proof of concept build

Spawning is slow, but almost no CPU overhead :)

init.lua

@@ -1,8 +1,62 @@
+local timer = 0
+local passive_only = false
+spawnlite = {}
+spawnlite.passive = {{name = "boats:boat",size = {x=1,y=1,z=1}}}
+spawnlite.agressive = {{name = "boats:boat",size = {x=1,y=1,z=1}}}
+
+local function is_space(pos,size)
+	local x,y,z = 1,0,0
+	if size.x*size.y*size.z == 1 then
+		return true
+	end
+	for i=2,size.x*sixe.y*size.z do
+		if x > size.x then
+			x = 0
+			y = y + 1
+		end
+		if y > size.y then
+			y = 0
+			z = z + 1
+		end
+		if minetest.get_node({x=pos.x+x,y=nodes[i]+y,z=pos.z+z}).name ~= "air" then
+			return false
+		end
+		x = x + 1
+	end
+	return true
+end
+
 minetest.register_globalstep(function(dtime)
+	timer = timer + dtime
+	if timer < 1 then
+		return
+	end
+	timer = 0
 	local players = minetest.get_connected_players()
+	local rand_x = math.random(0,19) - 10
+	local rand_z = math.random(0,19) - 10
 	for i=1,#players do
 		local pos = players[i]:getpos()
-		local nodes = minetest.find_nodes_in_area_under_air({x=pos.x,y=pos.y-80,z=pos.z},{x=pos.x,y=pos.y+80,z=pos.z},{"group:cracky","group:crumbly","group:snappy"})
+		local nodes = minetest.find_nodes_in_area_under_air({x=pos.x+rand_x,y=pos.y-80,z=pos.z+rand_z}
+			,{x=pos.x+rand_x,y=pos.y+80,z=pos.z+rand_z},{"group:cracky","group:crumbly","group:snappy"})
+		for i=1,#nodes do
+			local lightlevel = minetest.get_node_light({x=pos.x+rand_x,y=nodes[i].y+1,z=pos.z+rand_z})
+			local mob = nil
+			if lightlevel < 8 then
+				if passive_only then
+					break
+				end
+				mob = spawnlite.agressive[math.random(1,#spawnlite.agressive)]
+			else
+				if minetest.get_node(nodes[i]).name ~= "default:sand" then
+					break
+				end
+				mob = spawnlite.passive[math.random(1,#spawnlite.passive)]
+			end
+			if is_space({x=pos.x+rand_x,y=nodes[i].y+1,z=pos.z+rand_z},mob.size) then
+				minetest.add_entity({x=pos.x+rand_x,y=nodes[i].y+1,z=pos.z+rand_z},mob.name)
+			end
+		end
 	end
 end)