minor refactor of leveled node registration.

they now stack regardless of face targetted, unless there's no space, or
player is sneaking.

mods/bg_api/leveled_node.lua

@@ -1,7 +1,27 @@
 local function is_same_kind (pos, kindname)
 	local node = minetest.get_node(pos)
 	local node_kindname = minetest.registered_items[node.name].kindname
-	return kindname == node_kindname
+	return kindname and node_kindname and kindname == node_kindname
+end
+
+local function transfer_levels (source, target, max)
+	local max_diff = max - target
+	local diff = math.min(max_diff, source)
+	return source - diff, target + diff
+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 source_level, target_level = transfer_levels(source_level, def.level, level_max)
+
+	minetest.swap_node(pos, {name = kindname .. "_" .. (target_level)})
+	return source_level
 end
 
 function blockgame.check_for_stacking (pos)
@@ -11,21 +31,12 @@ 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 below_def = minetest.registered_nodes[below_node.name]
+		local new_level = stack_attempt(below, def.level)
 
-		if below_def.level ~= level_max then
-			local max_diff = level_max - below_def.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
+		if new_level ~= def.level then
+			minetest.swap_node(pos, {name = kindname .. "_" .. new_level})
 		end
 	end
 
@@ -37,6 +48,39 @@ end
 
 
 
+local function on_place (itemstack, placer, pointed)
+	local function fallback_to_default ()
+		return minetest.item_place(itemstack, placer, pointed)
+	end
+
+	local placed_def = minetest.registered_items[itemstack:get_name()]
+	if not placed_def.level_max then return fallback_to_default() end
+
+	if not is_same_kind(pointed.under, placed_def.kindname) then return fallback_to_default() end
+
+	if placer:is_player() and placer:get_player_control().sneak then return fallback_to_default() end
+
+	local above = pointed.under + vector.new(0, 1, 0)
+	local above_node = minetest.get_node(above)
+
+	local target_node = minetest.get_node(pointed.under)
+	local target_def = minetest.registered_items[target_node.name]
+	if target_def.level_max == target_def.level then return fallback_to_default() end
+
+	local source_level, target_level = transfer_levels(placed_def.level, target_def.level, placed_def.level_max)
+	if source_level > 0 and above_node.name ~= "air" then return fallback_to_default() end
+
+	local left_over = stack_attempt(pointed.under, placed_def.level)
+	if above_node.name == "air" then
+		minetest.set_node(pointed.above, {name = target_def.kindname .. "_" .. left_over})
+	end
+	itemstack:set_count(itemstack:get_count() - 1)
+
+	return itemstack
+end
+
+
+
 local function register_layer (kindname, def, level, modname)
 	local def = blockgame.naive_deep_copy(def)
 	def.level = level
@@ -53,10 +97,6 @@ local function register_layer (kindname, def, level, modname)
 		}
 	end
 
-	def.on_construct = blockgame.check_for_stacking
-	def.after_place_node = blockgame.check_for_stacking
-	def.after_land = blockgame.check_for_stacking
-
 	def.groups = def.groups or {}
 	def.groups.leveled = level
 
@@ -74,9 +114,17 @@ function blockgame.register_leveled_node (name, def)
 	local basename, modname, fullname = blockgame.extract_id_parts(name)
 	def.kindname = fullname
 
+	def.on_construct = blockgame.check_for_stacking
+	def.on_place = on_place
+	def.after_place_node = blockgame.check_for_stacking
+	def.after_land = blockgame.check_for_stacking
+
+	def.node_placement_prediction = ""
+
 	for level=1, def.level_max do
 		register_layer(basename, def, level, modname)
 	end
 
 	minetest.register_alias(fullname, fullname .. "_" .. def.level_max)
+	minetest.register_alias(fullname .. "_" .. 0, "air")
 end