Fixed surface handling (again)

This time keeping performance of original design

changelog.txt

@@ -1,4 +1,9 @@
 ---------------------------------------------------------------------------------------------------
+Version: 0.18.2
+Date: 2020-03-20
+  Changes:
+    - Redesigned surface handling again to be more perfomant
+---------------------------------------------------------------------------------------------------
 Version: 0.18.1
 Date: 2020-03-19
   Changes:

control.lua

@@ -2,28 +2,31 @@
 --This mod scans the map for gun-turrets and places alerts when turrets are low. 
 
 script.on_event({defines.events.on_tick}, function (event)
+	--Every minute the surface is rescanned for ammo-turret type entities. This is stored in the global table. 
+	if event.tick%3600 == 0 or global.turret_entities == nil then
+		local planet = game.surfaces
+		for index,_ in pairs(planet) do
+			global.turret_entities[index] = planet[index].find_entities_filtered{type = "ammo-turret"}
+		end
+	end
 	if event.tick%600 == 0 then
 	--Every 10 seconds recheck and give alerts to players for ammo-turret entities on the same force as them. 
-		for index,player in pairs(game.connected_players) do
+		for _,player in pairs(game.connected_players) do
 			
 			GTA_enabled = player.mod_settings["gun-turret-alerts-enabled"].value
 			player_threshold = player.mod_settings["gun-turret-alerts-threshold"].value
-			turret_entities = player.surface.find_entities_filtered{type = "ammo-turret"}
-			if GTA_enabled then
-				for index2, turret_entity in pairs(turret_entities) do
-					if turret_entity.valid then
+			turret_entities = global.turret_entities[player.surface.name]
+
+			if GTA_enabled and turret_entities then
+				for _,turret_entity in pairs(turret_entities) do
+					if turret_entity.valid and turret_entity.force == player.force then
 						inv_var = turret_entity.get_inventory(defines.inventory.turret_ammo)
 						if inv_var.is_empty() then
-							ammo_left = 0
-						else
-							ammo_left = inv_var[1].count
-						end
-						if turret_entity.force == player.force then
-							if ammo_left == 0 then
-								player.add_custom_alert(turret_entity, {type = "item", name = "piercing-rounds-magazine"}, "Out of ammo", true)
-							elseif ammo_left < player_threshold then
-								player.add_custom_alert(turret_entity, {type = "item", name = "firearm-magazine"}, "Ammo low", true)
-							end
+							-- no ammo alert
+							player.add_custom_alert(turret_entity, {type = "item", name = "piercing-rounds-magazine"}, "Out of ammo", true)
+						elseif inv_var[1].count < player_threshold then
+							-- low ammo alert
+							player.add_custom_alert(turret_entity, {type = "item", name = "firearm-magazine"}, "Ammo low", true)
 						end
 					end
 				end

info.json

@@ -1,6 +1,6 @@
 {
     "name": "Gun_Turret_Alerts",
-    "version": "0.18.1",
+    "version": "0.18.2",
     "title": "Gun_Turret_Alerts",
     "author": "Wyrrrd",
     "factorio_version": "0.18",