Improve the placement logic for slabs, etc (minetest-mods#160)

* related minetest-mods#166
* backported minetest-mods@ab91ad9
* merged with "Fix stairs placement over oddly-shaped nodes"
* backported minetest-mods@8a14250

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