refactor: stacking of leveled nodes.

extract level transfer between leveled nodes into own function, separate
from code that decides when and how to use that.

mods/bg_api/leveled_node.lua

@@ -4,6 +4,21 @@ local function is_same_kind (pos, kindname)
 	return kindname and node_kindname and kindname == node_kindname
 end
 
+local function stack_attempt (pos, source_level)
+	local node = minetest.get_node(pos)
+	local def = minetest.registered_nodes[node.name]
+	if def.level == level_max then return source_level end
+
+	local level_max = def.level_max
+	local kindname = def.kindname
+
+	local max_diff = level_max - def.level
+	local diff = math.min(max_diff, source_level)
+
+	minetest.swap_node(pos, {name = kindname .. "_" .. (def.level + diff)})
+	return source_level - diff
+end
+
 function blockgame.check_for_stacking (pos)
 	local node = minetest.get_node(pos)
 	if not blockgame.item_matches(node.name, {"group:leveled"}) then return end
@@ -11,21 +26,12 @@ function blockgame.check_for_stacking (pos)
 	local kindname = def.kindname
 	local level_max = def.level_max
 
-	local below = pos + blockgame.vector.dirs.down
+	local below = pos:offset(0, -1, 0)
 	if is_same_kind(below, kindname) then
-		local below_node = minetest.get_node(below)
+		local new_level = stack_attempt(below, def.level)
-		local below_def = minetest.registered_nodes[below_node.name]
 
-		if below_def.level ~= level_max then
+		if new_level ~= def.level then
-			local max_diff = level_max - below_def.level
+			minetest.swap_node(pos, {name = kindname .. "_" .. new_level})
-			local diff = math.min(max_diff, def.level)
-
-			minetest.swap_node(below, {name = kindname .. "_" .. (below_def.level + diff)})
-			if diff == def.level then
-				minetest.remove_node(pos)
-			else
-				minetest.swap_node(pos, {name = kindname .. "_" .. (def.level - diff)})
-			end
 		end
 	end