prioritize map decorations to make the generations more deterministic

mods/MAPGEN/mcl_mapgen_core/api.lua

@@ -169,3 +169,54 @@ 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 priority and name
+-- At least for low-priority 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 minetest_register_decoration = minetest.register_decoration
+local pending_decorations = {}
+function mcl_mapgen_core.register_decoration(def, callback)
+	if pending_decorations == nil then
+		minetest.log("warning", "Decoration registered after mapgen: "..tostring(def.name))
+		minetest_register_decoration(def)
+		if callback ~= nil then callback() end
+		return
+	end
+	def = table.copy(def) -- defensive deep copy, needed for water lily
+	def.callback = callback
+	table.insert(pending_decorations, def)
+end
+local function sort_decorations()
+	local keys, map = {}, {}
+	for i, def in pairs(pending_decorations) do
+		local name = def.name
+		-- we try to generate fallback names to make order more deterministic
+		name = name or (def.decoration and string.format("%s:%04d", def.decoration, i))
+		if not name and type(def.schematic) == "string" then
+			local sc = string.split(def.schematic, "/")
+			name = string.format("%s:%04d", sc[#sc], i)
+		end
+		if not name and type(def.schematic) == "table" and def.schematic.data then
+			name = ""
+			for _, v in ipairs(def.schematic.data) do
+				if v.name then name = name .. v.name .. ":" end
+			end
+			name = name .. string.format("%04d", i)
+		end
+		name = name or string.format("%04d", i)
+		local prio = (def.priority or 1000) + i/1000
+		local key = string.format("%08.3f:%s", prio, name)
+		table.insert(keys, key)
+		map[key] = def
+	end
+	table.sort(keys)
+	for _, key in ipairs(keys) do
+		-- minetest.log("action", "Deco: "..key)
+		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({
+		priority = 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" },
+		priority = 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" },
+		priority = 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",
+		priority = 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",
+		priority = 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,
+			priority = 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,
+			priority = 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",
+		priority = 1500,
 	})
 
 	local mushrooms = {"mcl_mushrooms:mushroom_red", "mcl_mushrooms:mushroom_brown"}

mods/MAPGEN/mcl_structures/api.lua

@@ -326,7 +326,6 @@ 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
 	flags = def.flags or "place_center_x, place_center_z, force_placement"
@@ -335,6 +334,7 @@ function mcl_structures.register_structure(name,def,nospawn) --nospawn means it
 		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,
+				priority = def.priority or (def.terrain_feature and 900) or 100, -- run before regular decorations
 				deco_type = "schematic",
 				schematic = EMPTY_SCHEMATIC,
 				place_on = def.place_on,

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",{
+	priority = 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)