Speed up group filtering code

This improves the startup time dramatically by lowering the algorithm complexity.
Mainly noticeable with large inventory sizes.

Written by Test_User, post-edited by SmallJoker

api.lua

@@ -43,7 +43,7 @@ minetest.after(0.01, function()
 	end
 	table.sort(ui.items_list)
 	ui.items_list_size = #ui.items_list
-	print("Unified Inventory. inventory size: "..ui.items_list_size)
+	print("Unified Inventory. Inventory size: "..ui.items_list_size)
 	for _, name in ipairs(ui.items_list) do
 		local def = minetest.registered_items[name]
 		-- Simple drops
@@ -133,19 +133,61 @@ minetest.after(0.01, function()
 			end
 		end
 	end
+
+	-- Step 1: group-indexed lookup table for items
+	local spec_matcher = {}
+	for _, name in ipairs(ui.items_list) do
+		-- we only need to care about groups, exact items are handled separately
+		for group, value in pairs(minetest.registered_items[name].groups) do
+			if value and value ~= 0 then
+				if not spec_matcher[group] then
+					spec_matcher[group] = {}
+				end
+				spec_matcher[group][name] = true
+			end
+		end
+	end
+
+	-- Step 2: Find all matching items for the given spec (groups)
+	local function get_matching_spec_items(specname)
+		if specname:sub(1,6) ~= "group:" then
+			return { specname }
+		end
+
+		local accepted = {}
+		for i, group in ipairs(specname:sub(7):split(",")) do
+			if i == 1 then
+				-- First step: Copy all possible item names in this group
+				for name, _ in pairs(spec_matcher[group] or {}) do
+					accepted[name] = true
+				end
+			else
+				-- Perform filtering
+				if spec_matcher[group] then
+					for name, _ in pairs(accepted) do
+						accepted[name] = spec_matcher[group][name]
+					end
+				else
+					-- No matching items
+					return {}
+				end
+			end
+		end
+		return accepted
+	end
+
 	for _, recipes in pairs(ui.crafts_for.recipe) do
 		for _, recipe in ipairs(recipes) do
 			local ingredient_items = {}
 			for _, spec in pairs(recipe.items) do
-				local matches_spec = ui.canonical_item_spec_matcher(spec)
+				-- Get items that fit into this spec (group or item name)
-				for _, name in ipairs(ui.items_list) do
+				local specname = ItemStack(spec):get_name()
-					if matches_spec(name) then
+				for item_name, _ in pairs(get_matching_spec_items(specname)) do
-						ingredient_items[name] = true
+					ingredient_items[item_name] = true
-					end
 				end
 			end
 			for name, _ in pairs(ingredient_items) do
-				if ui.crafts_for.usage[name] == nil then
+				if not ui.crafts_for.usage[name] then
 					ui.crafts_for.usage[name] = {}
 				end
 				table.insert(ui.crafts_for.usage[name], recipe)