prioritize map decorations to make the generations more deterministic

mods/MAPGEN/mcl_mapgen_core/api.lua

@@ -141,3 +141,58 @@ function mcl_mapgen_core.unregister_generator(id)
 	if rec.needs_param2 then param2 = param2 - 1 end
 	--if rec.needs_level0 then level0 = level0 - 1 end
 end
+
+-- Try to make decorations more deterministic in order, by sorting by rank and name
+-- At least for low-rank this should make map seeds more comparable, but
+-- adding for example a new structure can still change everything that comes
+-- later, because currently decoration blockseeds are incremented sequentially
+-- c.f., https://github.com/minetest/minetest/issues/14919
+local pending_decorations = {}
+
+function mcl_mapgen_core.register_decoration(def, callback)
+	if pending_decorations == nil then
+		-- Please do not register decorations in minetest.register_on_mods_loaded.
+		-- This should usually not happen, but modders may misuse this.
+		-- Nothing really bad should happen though, but the rank is ignored.
+		minetest.log("warning", "Decoration registered after mapgen: "..tostring(def.name))
+		minetest.register_decoration(def)
+		if callback then callback() end
+		return
+	end
+
+	def = table.copy(def) -- defensive deep copy, needed for water lily
+	def.callback = callback
+	pending_decorations[#pending_decorations+1] = def
+end
+
+local function sort_decorations()
+	local keys, map = {}, {}
+	for i, def in pairs(pending_decorations) do
+		-- Name, or fallback names, for better ordering:
+		local name = def.name or def.decoration
+		if not name and type(def.schematic) == "string" then -- filename based
+			local sc = string.split(def.schematic:gsub(".mts",""), "/")
+			name = sc[#sc]
+		end
+		if not name and type(def.schematic) == "table" and def.schematic.data then
+			name = "" -- "serialize" the schematic
+			for _, v in ipairs(def.schematic.data) do
+				if v.name then name = name .. v.name .. ":" end
+			end
+			if name == "" then name = nil end
+		end
+		-- sorting key is: rank, then insertion sequence, then name
+		local key = string.format("%05d:%04d:%s", def.rank or 1000, i, name or "deco")
+		keys[#keys+1] = key
+		map[key] = def
+	end
+	table.sort(keys)
+	for _, key in ipairs(keys) do
+		-- minetest.log("action", "Deco: "..key) -- dump the resulting order
+		minetest.register_decoration(map[key])
+		if map[key].callback then map[key].callback() end
+	end
+	pending_decorations = nil -- as we will not run again
+end
+
+minetest.register_on_mods_loaded(sort_decorations)

mods/MAPGEN/mcl_mapgen_core/v6.lua

@@ -45,6 +45,7 @@ local function register_mgv6_decorations()
 
 	-- Doubletall grass
 	minetest.register_decoration({
+		rank = 1500,
 		deco_type = "schematic",
 		schematic = {
 			size = { x=1, y=3, z=1 },
@@ -81,6 +82,7 @@ local function register_mgv6_decorations()
 		},
 		-- v6 hack: This makes sure large ferns only appear in jungles
 		spawn_by = { "mcl_core:jungletree", "mcl_flowers:fern" },
+		rank = 1510, -- larger than fern
 		num_spawn_by = 1,
 		place_on = {"group:grass_block_no_snow"},
 
@@ -192,6 +194,7 @@ local function register_mgv6_decorations()
 		},
 		-- Small trick to make sure melon spawn in jungles
 		spawn_by = { "mcl_core:jungletree", "mcl_flowers:fern" },
+		rank = 1510, -- larger than fern
 		num_spawn_by = 1,
 		y_min = 1,
 		y_max = 40,
@@ -214,6 +217,7 @@ local function register_mgv6_decorations()
 		y_min = 1,
 		y_max = mcl_vars.overworld_max,
 		decoration = "mcl_flowers:tallgrass",
+		rank = 1500,
 	})
 	minetest.register_decoration({
 		deco_type = "simple",
@@ -230,6 +234,7 @@ local function register_mgv6_decorations()
 		y_min = 1,
 		y_max = mcl_vars.overworld_max,
 		decoration = "mcl_flowers:tallgrass",
+		rank = 1500,
 	})
 
 	-- Seagrass and kelp
@@ -256,6 +261,7 @@ local function register_mgv6_decorations()
 			y_min = mcl_vars.overworld_min,
 			y_max = 0,
 			decoration = "mcl_ocean:seagrass_"..mat,
+			rank = 1500,
 		})
 		minetest.register_decoration({
 			deco_type = "simple",
@@ -276,6 +282,7 @@ local function register_mgv6_decorations()
 			y_min = mcl_vars.overworld_min,
 			y_max = -5,
 			decoration = "mcl_ocean:seagrass_"..mat,
+			rank = 1500,
 		})
 
 		minetest.register_decoration({
@@ -356,6 +363,7 @@ local function register_mgv6_decorations()
 		y_min = 1,
 		y_max = mcl_vars.overworld_max,
 		decoration = "mcl_flowers:tallgrass",
+		rank = 1500,
 	})
 
 	local mushrooms = {"mcl_mushrooms:mushroom_red", "mcl_mushrooms:mushroom_brown"}

mods/MAPGEN/mcl_structures/API.md

@@ -22,6 +22,7 @@ If nospawn is truthy the structure will not be placed by mapgen and the decorati
 					-- called before placement. denies placement when returning falsy.
 	after_place = function(pos,def,pr)
 					-- executed after successful placement
+	rank = int, -- decorations are generated by increasing rank. Default for structures is 100, terrain features 900
 	sidelen = int, --length of one side of the structure. used for foundations.
 	solid_ground = bool, -- structure requires solid ground
 	make_foundation = bool, -- a foundation is automatically built for the structure. needs the sidelen param

mods/MAPGEN/mcl_structures/api.lua

@@ -326,15 +326,15 @@ function mcl_structures.place_structure(pos, def, pr, blockseed, rot)
 end
 
 local EMPTY_SCHEMATIC = { size = {x = 0, y = 0, z = 0}, data = { } }
-
 function mcl_structures.register_structure(name,def,nospawn) --nospawn means it will not be placed by mapgen decoration mechanism
 	if mcl_structures.is_disabled(name) then return end
 	local flags = def.flags or "place_center_x, place_center_z, force_placement"
 	def.name = name
 	if not nospawn and def.place_on then
 		minetest.register_on_mods_loaded(function() --make sure all previous decorations and biomes have been registered
-			def.deco = minetest.register_decoration({
-				name = "mcl_structures:deco_"..name,
+			mcl_mapgen_core.register_decoration({
+				name = "mcl_structures:"..name,
+				rank = def.rank or (def.terrain_feature and 900) or 100, -- run before regular decorations
 				deco_type = "schematic",
 				schematic = EMPTY_SCHEMATIC,
 				place_on = def.place_on,
@@ -347,10 +347,13 @@ function mcl_structures.register_structure(name,def,nospawn) --nospawn means it
 				biomes = def.biomes,
 				y_max = def.y_max,
 				y_min = def.y_min
-			})
-			def.deco_id = minetest.get_decoration_id("mcl_structures:deco_"..name)
-			minetest.set_gen_notify({decoration=true}, { def.deco_id })
-			--catching of gennotify happens in mcl_mapgen_core
+				},
+				function()
+					def.deco_id = minetest.get_decoration_id("mcl_structures:"..name)
+					minetest.set_gen_notify({decoration=true}, { def.deco_id })
+					--catching of gennotify happens in mcl_mapgen_core
+				end
+			)
 		end)
 	end
 	mcl_structures.registered_structures[name] = def

mods/MAPGEN/mcl_terrain_features/init.lua

@@ -134,6 +134,7 @@ local function get_fallen_tree_schematic(pos,pr)
 end
 
 mcl_structures.register_structure("fallen_tree",{
+	rank = 1100, -- after regular trees
 	place_on = {"group:grass_block"},
 	terrain_feature = true,
 	noise_params = {
@@ -151,12 +152,9 @@ mcl_structures.register_structure("fallen_tree",{
 	y_min = minetest.get_mapgen_setting("water_level"),
 	on_place = function(pos,def,pr)
 		local air_p1 = vector.offset(pos,-def.sidelen/2,1,-def.sidelen/2)
-		local air_p2 = vector.offset(pos,def.sidelen/2,1,def.sidelen/2)
+		local air_p2 = vector.offset(air_p1,def.sidelen-1,0,def.sidelen-1)
 		local air = minetest.find_nodes_in_area(air_p1,air_p2,{"air"})
-		if #air < ( def.sidelen * def.sidelen ) / 2 then
-			return false
-		end
-		return true
+		return #air >= (def.sidelen * def.sidelen) / 2
 	end,
 	place_func = function(pos,def,pr)
 		local schem=get_fallen_tree_schematic(pos,pr)