Stairs: Big simplification of slabs combination

Combine slabs if identical based on orientations using a simple lookup
table if the nodes are identical.

Otherwise relies on place_node() to place the node, which properly
handles rotation compared to adjacent nodes already, and can orient
based on look_dir as well.

Initial slabs placed are oriented based on (1) the orientation of
the pointed "face" (assumes nodes are cubic, of course), and uses
the player look direction to orient the node n/e/w/s if the slab
is horizontal or upside-down. If placed against a vertical face,
the slab is placed against the face without rotation around the axis
perpendicular to that vertical face. This allows upside down placement
and vertical placement without screwdriver.

If a slab is placed on top of an upside down slab, or below a normally
placed slab, the rotation is inverted so that no "floating" slab
is created.

Largely based on kilbith's #807 PR. Slab combining and place_node()
usage by sofar.

Since this relies entirely on `on_place` mechanics, this fails to
combine slabs into a plain node if the space *above* is occupied.
This is unavoidable due to the fact that on_place() happens after
the checks required to see if pointed_thing.above is empty or not.

mods/stairs/init.lua

@@ -110,6 +110,11 @@ function stairs.register_stair(subname, recipeitem, groups, images, description,
 end
 
 
+-- Slab facedir to placement 6d matching table
+local slab_trans_dir = {[0] = 8, 0, 2, 1, 3, 4}
+-- Slab facedir when placing initial slab against other surface
+local slab_trans_dir_place = {[0] = 0, 20, 12, 16, 4, 8}
+
 -- Register slabs.
 -- Node will be called stairs:slab_<subname>
 
@@ -129,86 +134,61 @@ function stairs.register_slab(subname, recipeitem, groups, images, description,
 			fixed = {-0.5, -0.5, -0.5, 0.5, 0, 0.5},
 		},
 		on_place = function(itemstack, placer, pointed_thing)
-			if pointed_thing.type ~= "node" then
+			local under = minetest.get_node(pointed_thing.under)
+			local wield_item = itemstack:get_name()
+
+			if under and wield_item == under.name then
+				-- place slab using under node orientation
+				local dir = minetest.dir_to_facedir(vector.subtract(
+					pointed_thing.above, pointed_thing.under), true)
+
+				local p2 = under.param2
+
+				-- combine two slabs if possible
+				if slab_trans_dir[math.floor(p2 / 4)] == dir then
+					if not recipeitem then
+						return itemstack
+					end
+					local player_name = placer:get_player_name()
+					if minetest.is_protected(pointed_thing.under, player_name) and not
+							minetest.check_player_privs(placer, "protection_bypass") then
+						minetest.record_protection_violation(pointed_thing.under,
+							player_name)
+						return
+					end
+					minetest.set_node(pointed_thing.under, {name = recipeitem, param2 = p2})
+					if not minetest.setting_getbool("creative_mode") then
+						itemstack:take_item()
+					end
 					return itemstack
 				end
 
-			-- If it's being placed on an another similar one, replace it with
+				-- Placing a slab on an upside down slab should make it right-side up.
-			-- a full block
+				if p2 >= 20 and dir == 8 then
-			local slabpos = nil
+					p2 = p2 - 20
-			local slabnode = nil
+				-- same for the opposite case: slab below normal slab
-			local p0 = pointed_thing.under
+				elseif p2 <= 3 and dir == 4 then
-			local p1 = pointed_thing.above
+					p2 = p2 + 20
-			local n0 = minetest.get_node(p0)
-			local n1 = minetest.get_node(p1)
-			local param2 = 0
-
-			local n0_is_upside_down = (n0.name == "stairs:slab_" .. subname and
-					n0.param2 >= 20)
-
-			if n0.name == "stairs:slab_" .. subname and not n0_is_upside_down and
-					p0.y + 1 == p1.y then
-				slabpos = p0
-				slabnode = n0
-			elseif n1.name == "stairs:slab_" .. subname then
-				slabpos = p1
-				slabnode = n1
 				end
-			if slabpos then
-				-- Remove the slab at slabpos
-				minetest.remove_node(slabpos)
-				-- Make a fake stack of a single item and try to place it
-				local fakestack = ItemStack(recipeitem)
-				fakestack:set_count(itemstack:get_count())
 
-				pointed_thing.above = slabpos
+				-- else attempt to place node with proper param2
-				local success
+				minetest.item_place_node(ItemStack(wield_item), placer, pointed_thing, p2)
-				fakestack, success = minetest.item_place(fakestack, placer,
+				if not minetest.setting_getbool("creative_mode") then
-					pointed_thing)
+					itemstack:take_item()
-				-- If the item was taken from the fake stack, decrement original
+				end
-				if success then
+				return itemstack
-					itemstack:set_count(fakestack:get_count())
-				-- Else put old node back
 			else
-					minetest.set_node(slabpos, slabnode)
+				-- place slab using look direction of player
-				end
+				local dir = minetest.dir_to_wallmounted(vector.subtract(
-				return itemstack
+					pointed_thing.above, pointed_thing.under), true)
+
+				local rot = slab_trans_dir_place[dir]
+				if rot == 0 or rot == 20 then
+					rot = rot + minetest.dir_to_facedir(placer:get_look_dir())
 				end
 
-			-- Upside down slabs
+				return minetest.item_place(itemstack, placer, pointed_thing, rot)
-			if p0.y - 1 == p1.y then
-				-- Turn into full block if pointing at a existing slab
-				if n0_is_upside_down  then
-					-- Remove the slab at the position of the slab
-					minetest.remove_node(p0)
-					-- Make a fake stack of a single item and try to place it
-					local fakestack = ItemStack(recipeitem)
-					fakestack:set_count(itemstack:get_count())
-
-					pointed_thing.above = p0
-					local success
-					fakestack, success = minetest.item_place(fakestack, placer,
-						pointed_thing)
-					-- If the item was taken from the fake stack, decrement original
-					if success then
-						itemstack:set_count(fakestack:get_count())
-					-- Else put old node back
-					else
-						minetest.set_node(p0, n0)
 			end
-					return itemstack
-				end
-
-				-- Place upside down slab
-				param2 = 20
-			end
-
-			-- If pointing at the side of a upside down slab
-			if n0_is_upside_down and p0.y + 1 ~= p1.y then
-				param2 = 20
-			end
-
-			return minetest.item_place(itemstack, placer, pointed_thing, param2)
 		end,
 	})