Move cauldron nodebox preset table outside of function, to be built once instead of with each function call.

Skip intermediate variable for returns.
Added mcl_cauldrons.get_substance() to get substance from cauldron.
Added explicit list of cauldron substances.

mods/ITEMS/mcl_cauldrons/api.lua

@@ -8,6 +8,14 @@ mcl_cauldrons.BUCKET_FILL_LEVELS = 3
 mcl_cauldrons.BOTTLE_FILL_LEVELS = 1
 
 
+-- list of substances accepted by cauldron.
+-- N.B. hopefully this will be obsoleted by the liquids API.
+mcl_cauldrons.substances = {
+	"cauldron_water",
+	"cauldron_river_water",
+}
+
+
 -- Find cauldron from registered cauldrons -> string, table
 --  `substance` == nil for any cauldron content (incl. empty), "" = only empty.
 --  `fill_level` == nil for any/first fill level satisfying `substance`.
@@ -43,6 +51,25 @@ function mcl_cauldrons.has_substance(node, substance)
 	return (minetest.get_item_group(node.name, substance) ~= 0)
 end
 
+-- list of supported substances -> table
+function mcl_cauldrons.list_substances()
+	return mcl_cauldron.substances
+end
+
+-- Get substance inside cauldron -> string or nil
+function mcl_cauldrons.get_substance(node)
+	local nodename = node.name
+	-- data-driven to avoid if-cascade.
+	-- until liquids API is available, the substance name matches the group key in the node definition (e.g. "cauldron_water").
+	for i, probe_key in ipairs(mcl_cauldrons.substances) do
+		if minetest.get_item_group(node.name, probe_key) ~= 0 then
+			return probe_key
+		end
+	end
+	-- claim cauldron is empty.
+	return nil
+end
+
 -- Get maximum number of levels available for cauldron node -> number
 -- Optionally specify `substance` (mainly to determine a change in cauldron substance, e.g. filling a full one with bucket of something else)
 -- substance == nil to ignore substance criteria
@@ -91,11 +118,7 @@ end
 function mcl_cauldrons.set_level(node, fill_level, substance)
 	if substance == nil then
 		-- determine current substance.
-		if mcl_cauldrons.has_substance(node, "cauldron_water") then
-			substance = "cauldron_water"
-		elseif mcl_cauldrons.has_substance(node, "cauldron_river_water") then
-			substance = "cauldron_river_water"
-		end
+		substance = mcl_cauldrons.get_substance(node)
 	end
 	local nodename, nodedef = mcl_cauldrons.find_cauldron(substance, fill_level)
 	local newnode = { name = nodename, param1 = node.param1, param2 = node.param2 }
@@ -109,8 +132,7 @@ function mcl_cauldrons.fill_levels(node, change_levels, substance)
 	local nodedef = mcl_cauldrons.registered_cauldrons[node.name]
 	local callback = nodedef._mcl_on_fill
 	-- use callback if available, else revert.
-	local newnode = callback and callback(node, change_levels, substance) or node
-	return newnode
+	return callback and callback(node, change_levels, substance) or node
 end
 
 -- drain counterpart to fill_levels()
@@ -118,8 +140,7 @@ function mcl_cauldrons.drain_levels(node, change_levels, substance)
 	local nodedef = mcl_cauldrons.registered_cauldrons[node.name]
 	local callback = nodedef._mcl_on_drain
 	-- use callback if available, else revert.
-	local newnode = callback and callback(node, change_levels, substance) or node
-	return newnode
+	return callback and callback(node, change_levels, substance) or node
 end
 
 

mods/ITEMS/mcl_cauldrons/register.lua

@@ -2,13 +2,14 @@ local S = minetest.get_translator(minetest.get_current_modname())
 
 
 -- Convenience function because the cauldron nodeboxes are very similar
+local floors_table = {
+	[0]=-0.1875,  -- empty
+	[1]=1/16,  -- 1/3 filled
+	[2]=4/16,  -- 2/3 filled
+	[3]=7/16,  -- full
+}
+
 local create_cauldron_nodebox = function(water_level)
-	local floors_table = {
-		[0]=-0.1875,  -- empty
-		[1]=1/16,  -- 1/3 filled
-		[2]=4/16,  -- 2/3 filled
-		[3]=7/16,  -- full
-	}
 	local floor_y = floors_table[water_level]
 	return {
 		type = "fixed",