Cache tiers rather than recalculating them constantly

code/modules/research/techweb/_techweb.dm

@@ -19,6 +19,7 @@
 	var/max_bomb_value = 0
 	var/organization = "Third-Party"							//Organization name, used for display.
 	var/last_bitcoins = 0										//Current per-second production, used for display only.
+	var/list/tiers = list()										//Assoc list, datum = number, 1 is available, 2 is all reqs are 1, so on
 
 /datum/techweb/New()
 	for(var/i in SSresearch.techweb_nodes_starting)
@@ -161,6 +162,24 @@
 			hidden_nodes -= N.id
 	return TRUE
 
+/datum/techweb/proc/update_tiers(datum/techweb_node/base)
+	var/list/current = list(base)
+	while (current.len)
+		var/list/next = list()
+		for (var/node_ in current)
+			var/datum/techweb_node/node = node_
+			var/tier = 0
+			if (!researched_nodes[node.id])  // researched is tier 0
+				for (var/id in node.prereq_ids)
+					var/prereq_tier = tiers[node.prerequisites[id]]
+					tier = max(tier, prereq_tier + 1)
+
+			if (tier != tiers[node])
+				tiers[node] = tier
+				for (var/id in node.unlocks)
+					next += node.unlocks[id]
+		current = next
+
 /datum/techweb/proc/update_node_status(datum/techweb_node/node, autoupdate_consoles = TRUE)
 	var/researched = FALSE
 	var/available = FALSE
@@ -189,6 +208,7 @@
 		else
 			if(visible)
 				visible_nodes[node.id] = node
+	update_tiers(node)
 	if(autoupdate_consoles)
 		for(var/v in consoles_accessing)
 			var/obj/machinery/computer/rdconsole/V = v