Improved placement for slabs etc

Stairs+ nodes now check which side of the face (top/bottom for
horizontal, left/right for vertical placement) is being clicked to
decide on the orientation of the placed node. The aux key can be used
to place the node as if from the opposite side.

When the under node and placed node are the same category (e.g. slab to
slab) then by default the orientation of the under node is copied,
flippingit if placing above or below an upright or upside-down node. The
aux key can be used to place node according to the general (no aux, no
same-cat) rules.

(And yes, this means that it's not possible to place a stair+ node
against another stair+ node as if it were from the other side. Sorry, we
don't have enough special keys.)

stairsplus/common.lua

@@ -15,6 +15,97 @@ local descriptions = {
 	["stair"] = S("%s Stairs"),
 }
 
+-- Extends the standad rotate_node placement so that it takes into account
+-- the side (top/bottom or left/right) of the face being pointed at.
+-- As with the standard rotate_node, sneak can be used to force the perpendicular
+-- placement (wall placement on floor/ceiling, floor/ceiling placement on walls).
+-- Additionally, the aux / sprint / special key can be used to place the node
+-- as if from the opposite side.
+--
+-- When placing a node next to one of the same category (e.g. slab to slab or
+-- stair to stair), the default placement (regardless of sneak) is to copy the
+-- under node's param2, flipping if placed above or below it. The aux key disable
+-- this behavior.
+local wall_right_dirmap = {9, 18, 7, 12}
+local wall_left_dirmap = {11, 16, 5, 14}
+local ceil_dirmap = {20, 23, 22, 21}
+
+stairsplus.rotate_node_aux = function(itemstack, placer, pointed_thing)
+	local sneak = placer and placer:get_player_control().sneak
+	local aux = placer and placer:get_player_control().aux1
+
+	-- namestring for what we are placing, up to the first _ (exclusive)
+	local item_prefix = itemstack:get_name():gsub("_.*$", "")
+	-- namestring for what we are placing against
+	local under = pointed_thing.under
+	local under_node = minetest.get_node(under)
+	local under_prefix = under_node and under_node.name:gsub("_.*$", "")
+
+	local same_cat = item_prefix == under_prefix
+
+	-- standard (floor) facedir, also used for sneak placement against the lower half of the wall
+	local p2 = placer and minetest.dir_to_facedir(placer:get_look_dir()) or 0
+
+	-- check which face and which quadrant we are interested in
+	-- this is used both to check if we're handling parallel placement in the same-category case,
+	-- and in general for sneak placement
+	local face_pos = minetest.pointed_thing_to_face_pos(placer, pointed_thing)
+	local face_off = vector.subtract(face_pos, under)
+	local wallmounted = minetest.dir_to_wallmounted(face_off)
+
+	if same_cat and not aux then
+		p2 = under_node.param2
+		-- flip if placing above or below an upright or upside-down node
+		-- TODO should we also flip when placing next to a side-mounted node?
+		if wallmounted < 2 then
+			if p2 < 4 then
+				p2 = (p2 + 2) % 4
+				p2 = ceil_dirmap[p2 + 1]
+			elseif p2 > 19 then
+				p2 = ceil_dirmap[p2 - 19] - 20
+				p2 = (p2 + 2) % 4
+			end
+		end
+	else
+		-- for same-cat placement, aux is used to disable param2 copying
+		if same_cat then
+			aux = not aux
+		end
+
+		local remap = nil
+
+		-- standard placement against the wall
+		local use_wallmap = (wallmounted > 1 and not sneak) or (wallmounted < 2 and sneak)
+
+		-- standard placement against the ceiling, or sneak placement against the upper half of the wall
+		local use_ceilmap = wallmounted == 1 and not sneak
+		use_ceilmap = use_ceilmap or (wallmounted > 1 and sneak and face_off.y > 0)
+
+		if use_wallmap then
+			local left = (p2 == 0 and face_off.x < 0) or
+				(p2 == 1 and face_off.z > 0) or
+				(p2 == 2 and face_off.x > 0) or
+				(p2 == 3 and face_off.z < 0)
+			if aux then
+				left = not left
+			end
+			remap = left and wall_left_dirmap or wall_right_dirmap
+		elseif use_ceilmap then
+			remap = ceil_dirmap
+		end
+
+		if aux then
+			p2 = (p2 + 2) % 4
+		end
+
+		if remap then
+			p2 = remap[p2 + 1]
+		end
+	end
+
+	return minetest.item_place(itemstack, placer, pointed_thing, p2)
+end
+
 stairsplus.register_single = function(category, alternate, info, modname, subname, recipeitem, fields)
 	local src_def = minetest.registered_nodes[recipeitem] or {}
 	local desc_base = descriptions[category]:format(fields.description)
@@ -42,7 +133,7 @@ stairsplus.register_single = function(category, alternate, info, modname, subnam
 	-- Darken light sources slightly to make up for their smaller visual size
 	def.light_source = math.max(0, (def.light_source or 0) - 1)
 
-	def.on_place = minetest.rotate_node
+	def.on_place = stairsplus.rotate_node_aux
 	def.groups = stairsplus:prepare_groups(fields.groups)
 
 	if category == "slab" then