nether/mapgen.lua

--[[

  Nether mod for minetest

  "mapgen.lua" is the modern biomes-based Nether mapgen, which
    requires Minetest v5.1 or greater
  "mapgen_nobiomes.lua" is the legacy version of the mapgen, only used
    in older versions of Minetest or in v6 worlds.


  Copyright (C) 2013 PilzAdam

  Permission to use, copy, modify, and/or distribute this software for
  any purpose with or without fee is hereby granted, provided that the
  above copyright notice and this permission notice appear in all copies.

  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
  WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
  WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR
  BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES
  OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
  WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
  ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
  SOFTWARE.

]]--


-- Parameters

local NETHER_CEILING = nether.DEPTH_CEILING
local NETHER_FLOOR   = nether.DEPTH_FLOOR
local TCAVE = 0.6
local BLEND = 128

-- parameters for central region
local REGION_BUFFER_THICKNESS   = 0.2
local CENTER_REGION_LIMIT       = TCAVE - REGION_BUFFER_THICKNESS -- Netherrack gives way to Deep-Netherrack here
local CENTER_CAVERN_LIMIT       = CENTER_REGION_LIMIT - 0.1       -- Deep-Netherrack gives way to air here
local SURFACE_CRUST_LIMIT       = CENTER_CAVERN_LIMIT * 0.25      -- Crusted-lava at the surface of the lava ocean gives way to liquid lava here
local CRUST_LIMIT               = CENTER_CAVERN_LIMIT * 0.85      -- Crusted-lava under the surface of the lava ocean gives way to liquid lava here
local BASALT_COLUMN_UPPER_LIMIT = CENTER_CAVERN_LIMIT * 0.9       -- Basalt columns may appear between these upper and lower limits
local BASALT_COLUMN_LOWER_LIMIT = CENTER_CAVERN_LIMIT * 0.25      -- This value is close to SURFACE_CRUST_LIMIT so basalt columns give way to "flowing" lava rivers


-- Shared Nether mapgen namespace
-- For mapgen files to share functions and constants
nether.mapgen = {}
local mapgen = nether.mapgen

mapgen.TCAVE                     = TCAVE                     -- const needed in mapgen_mantle.lua
mapgen.BLEND                     = BLEND                     -- const needed in mapgen_mantle.lua
mapgen.CENTER_REGION_LIMIT       = CENTER_REGION_LIMIT       -- const needed in mapgen_mantle.lua
mapgen.CENTER_CAVERN_LIMIT       = CENTER_CAVERN_LIMIT       -- const needed in mapgen_mantle.lua
mapgen.BASALT_COLUMN_UPPER_LIMIT = BASALT_COLUMN_UPPER_LIMIT -- const needed in mapgen_mantle.lua
mapgen.BASALT_COLUMN_LOWER_LIMIT = BASALT_COLUMN_LOWER_LIMIT -- const needed in mapgen_mantle.lua

mapgen.ore_ceiling = NETHER_CEILING - BLEND -- leave a solid 128 node cap of netherrack before introducing ores
mapgen.ore_floor   = NETHER_FLOOR   + BLEND

local debugf = nether.debug

if minetest.read_schematic == nil then
	-- Using biomes to create the Nether requires the ability for biomes to set "node_cave_liquid = air".
	-- This feature was introduced by paramat in b1b40fef1 on 2019-05-19, but we can't test for
	-- it directly. However b2065756c was merged a few months later (in 2019-08-14) and it is easy
	-- to directly test for - it adds minetest.read_schematic() - so we use this as a proxy-test
	-- for whether the Minetest engine is recent enough to have implemented node_cave_liquid=air
	error("This " .. nether.modname .. " mapgen requires Minetest v5.1 or greater, use mapgen_nobiomes.lua instead.", 0)
end

-- Load helper functions for generating the mantle / center region
dofile(nether.path .. "/mapgen_mantle.lua")


-- Misc math functions

-- avoid needing table lookups each time a common math function is invoked
local math_max, math_min, math_abs, math_floor = math.max, math.min, math.abs, math.floor


-- Inject nether_caverns biome

local function override_underground_biomes()
	-- https://forum.minetest.net/viewtopic.php?p=257522#p257522
	-- Q: Is there a way to override an already-registered biome so I can get it out of the
	--    way of my own underground biomes without disturbing the other biomes registered by
	--    default?
	-- A: No, all you can do is use a mod to clear all biomes then re-register the complete
	--    set but with your changes. It has been described as hacky but this is actually the
	--    official way to alter biomes, most mods and subgames would want to completely change
	--    all biomes anyway.
	--    To avoid the engine side of mapgen becoming overcomplex the approach is to require mods
	--    to do slightly more complex stuff in Lua.

	-- take a copy of all biomes, decorations, and ores. Regregistering a biome changes its ID, so
	-- any decorations or ores using the 'biomes' field must afterwards be cleared and re-registered.
	-- https://github.com/minetest/minetest/issues/9288
	local registered_biomes_copy      = {}
	local registered_decorations_copy = {}
	local registered_ores_copy        = {}

	for old_biome_key, old_biome_def in pairs(minetest.registered_biomes) do
	   registered_biomes_copy[old_biome_key] = old_biome_def
	end
	for old_decoration_key, old_decoration_def in pairs(minetest.registered_decorations) do
	   registered_decorations_copy[old_decoration_key] = old_decoration_def
	end
	for old_ore_key, old_ore_def in pairs(minetest.registered_ores) do
		registered_ores_copy[old_ore_key] = old_ore_def
	 end

	-- clear biomes, decorations, and ores
	minetest.clear_registered_decorations()
	minetest.clear_registered_ores()
	minetest.clear_registered_biomes()

	-- Restore biomes, adjusted to not overlap the Nether
	for biome_key, new_biome_def in pairs(registered_biomes_copy) do
		local biome_y_max, biome_y_min = tonumber(new_biome_def.y_max), tonumber(new_biome_def.y_min)

		if biome_y_max > NETHER_FLOOR and biome_y_min < NETHER_CEILING then
			-- This biome occupies some or all of the depth of the Nether, shift/crop it.
			local spaceOccupiedAbove = biome_y_max - NETHER_CEILING
			local spaceOccupiedBelow = NETHER_FLOOR - biome_y_min
			if spaceOccupiedAbove >= spaceOccupiedBelow or biome_y_min <= -30000 then
				-- place the biome above the Nether
				-- We also shift biomes which extend to the bottom of the map above the Nether, since they
				-- likely only extend that deep as a catch-all, and probably have a role nearer the surface.
				new_biome_def.y_min = NETHER_CEILING + 1
				new_biome_def.y_max = math_max(biome_y_max, NETHER_CEILING + 2)
			else
				-- shift the biome to below the Nether
				new_biome_def.y_max = NETHER_FLOOR - 1
				new_biome_def.y_min = math_min(biome_y_min, NETHER_CEILING - 2)
			end
		end
		minetest.register_biome(new_biome_def)
	end

	-- Restore biome decorations
	for decoration_key, new_decoration_def in pairs(registered_decorations_copy) do
	   minetest.register_decoration(new_decoration_def)
	end
	-- Restore biome ores
	for ore_key, new_ore_def in pairs(registered_ores_copy) do
		minetest.register_ore(new_ore_def)
	 end
 end

-- Shift any overlapping biomes out of the way before we create the Nether biomes
override_underground_biomes()

-- nether:native_mapgen is used to prevent ores and decorations being generated according
-- to landforms created by the native mapgen.
-- Ores and decorations can be registered against "nether:rack" instead, and the lua
-- on_generate() callback will carve the Nether with nether:rack before invoking
-- generate_decorations and generate_ores.
minetest.register_node("nether:native_mapgen", {})

minetest.register_biome({
	name = "nether_caverns",
	node_stone  = "nether:native_mapgen", -- nether:native_mapgen is used here to prevent the native mapgen from placing ores and decorations.
	node_filler = "nether:native_mapgen", -- The lua on_generate will transform nether:native_mapgen into nether:rack then decorate and add ores.
	node_dungeon = "nether:brick",
	node_dungeon_alt = "nether:brick_cracked",
	node_dungeon_stair = "stairs:stair_nether_brick",
	-- Setting node_cave_liquid to "air" avoids the need to filter lava and water out of the mapchunk and
	-- surrounding shell (overdraw nodes beyond the mapchunk).
	-- This feature was introduced by paramat in b1b40fef1 on 2019-05-19, and this mapgen.lua file should only
	-- be run if the Minetest version includes it. The earliest tag made after 2019-05-19 is 5.1.0 on 2019-10-13,
	-- however we shouldn't test version numbers. minetest.read_schematic() was added by b2065756c and merged in
	-- 2019-08-14 and is easy to test for, we don't use it but it should make a good proxy-test for whether the
	-- Minetest version is recent enough to have implemented node_cave_liquid=air
	node_cave_liquid = "air",
	y_max = NETHER_CEILING,
	y_min = NETHER_FLOOR,
	vertical_blend = 0,
	heat_point = 50,
	humidity_point = 50,
})


-- Ores and decorations

dofile(nether.path .. "/mapgen_decorations.lua")

minetest.register_ore({
	ore_type       = "scatter",
	ore            = "nether:glowstone",
	wherein        = "nether:rack",
	clust_scarcity = 11 * 11 * 11,
	clust_num_ores = 3,
	clust_size     = 2,
	y_max = mapgen.ore_ceiling,
	y_min = mapgen.ore_floor
})

minetest.register_ore({
	ore_type       = "scatter",
	ore            = "nether:lava_crust", -- crusted lava replaces scattered glowstone in the mantle
	wherein        = "nether:rack_deep",
	clust_scarcity = 16 * 16 * 16,
	clust_num_ores = 4,
	clust_size     = 2,
	y_max = mapgen.ore_ceiling,
	y_min = mapgen.ore_floor
})

minetest.register_ore({
	ore_type       = "scatter",
	ore            = "default:lava_source",
	wherein        = {"nether:rack", "nether:rack_deep"},
	clust_scarcity = 36 * 36 * 36,
	clust_num_ores = 4,
	clust_size     = 2,
	y_max = mapgen.ore_ceiling,
	y_min = mapgen.ore_floor
})

minetest.register_ore({
	ore_type        = "blob",
	ore             = "nether:sand",
	wherein         = "nether:rack",
	clust_scarcity  = 14 * 14 * 14,
	clust_size      = 8,
	y_max = mapgen.ore_ceiling,
	y_min = mapgen.ore_floor
})


-- Mapgen

-- 3D noise

mapgen.np_cave = {
	offset = 0,
	scale = 1,
	spread = {x = 384, y = 128, z = 384}, -- squashed 3:1
	seed = 59033,
	octaves = 5,
	persist = 0.7,
	lacunarity = 2.0,
	--flags = ""
}

-- Buffers and objects we shouldn't recreate every on_generate

local nobj_cave = nil
local nbuf_cave = {}
local dbuf = {}


-- Content ids

local c_air              = minetest.get_content_id("air")
local c_netherrack       = minetest.get_content_id("nether:rack")
local c_netherrack_deep  = minetest.get_content_id("nether:rack_deep")
local c_dungeonbrick     = minetest.get_content_id("nether:brick")
local c_dungeonbrick_alt = minetest.get_content_id("nether:brick_cracked")
local c_netherbrick_slab = minetest.get_content_id("stairs:slab_nether_brick")
local c_netherfence      = minetest.get_content_id("nether:fence_nether_brick")
local c_glowstone        = minetest.get_content_id("nether:glowstone")
local c_lava_source      = minetest.get_content_id("default:lava_source")
local c_lavasea_source   = minetest.get_content_id("nether:lava_source") -- same as lava but with staggered animation to look better as an ocean
local c_lava_crust       = minetest.get_content_id("nether:lava_crust")
local c_native_mapgen    = minetest.get_content_id("nether:native_mapgen")


-- Dungeon excavation functions

function is_dungeon_brick(node_id)
	return node_id == c_dungeonbrick or node_id == c_dungeonbrick_alt
end

function build_dungeon_room_list(data, area)

	local result = {}

	-- Unfortunately gennotify only returns dungeon rooms, not corridors.
	-- We don't need to check for temples because only dungeons are generated in biomes
	-- that define their own dungeon nodes.
	local gennotify = minetest.get_mapgen_object("gennotify")
	local roomLocations = gennotify["dungeon"] or {}

	-- Excavation should still know to stop if a cave or corridor has removed the dungeon wall.
	-- See MapgenBasic::generateDungeons in mapgen.cpp for max room sizes.
	local maxRoomSize = 18
	local maxRoomRadius = math.ceil(maxRoomSize / 2)

	local xStride, yStride, zStride = 1, area.ystride, area.zstride
	local minEdge, maxEdge = area.MinEdge, area.MaxEdge

	for _, roomPos in ipairs(roomLocations) do

		if area:containsp(roomPos) then -- this safety check does not appear to be necessary, but lets make it explicit

			local room_vi = area:indexp(roomPos)
			--data[room_vi] = minetest.get_content_id("default:torch") -- debug

			local startPos = vector.new(roomPos)
			if roomPos.y + 1 <= maxEdge.y and data[room_vi + yStride] == c_air then
				-- The roomPos coords given by gennotify are at floor level, but whenever possible we
				-- want to be performing searches a node higher than floor level to avoids dungeon chests.
				startPos.y = startPos.y + 1
				room_vi = area:indexp(startPos)
			end

			local bound_min_x = math_max(minEdge.x, roomPos.x - maxRoomRadius)
			local bound_min_y = math_max(minEdge.y, roomPos.y - 1) -- room coords given by gennotify are on the floor
			local bound_min_z = math_max(minEdge.z, roomPos.z - maxRoomRadius)

			local bound_max_x = math_min(maxEdge.x, roomPos.x + maxRoomRadius)
			local bound_max_y = math_min(maxEdge.y, roomPos.y + maxRoomSize) -- room coords given by gennotify are on the floor
			local bound_max_z = math_min(maxEdge.z, roomPos.z + maxRoomRadius)

			local room_min = vector.new(startPos)
			local room_max = vector.new(startPos)

			local vi = room_vi
			while room_max.y < bound_max_y and data[vi + yStride] == c_air do
				room_max.y = room_max.y + 1
				vi = vi + yStride
			end

			vi = room_vi
			while room_min.y > bound_min_y and data[vi - yStride] == c_air do
				room_min.y = room_min.y - 1
				vi = vi - yStride
			end

			vi = room_vi
			while room_max.z < bound_max_z and data[vi + zStride] == c_air do
				room_max.z = room_max.z + 1
				vi = vi + zStride
			end

			vi = room_vi
			while room_min.z > bound_min_z and data[vi - zStride] == c_air do
				room_min.z = room_min.z - 1
				vi = vi - zStride
			end

			vi = room_vi
			while room_max.x < bound_max_x and data[vi + xStride] == c_air do
				room_max.x = room_max.x + 1
				vi = vi + xStride
			end

			vi = room_vi
			while room_min.x > bound_min_x and data[vi - xStride] == c_air do
				room_min.x = room_min.x - 1
				vi = vi - xStride
			end

			local roomInfo = vector.new(roomPos)
			roomInfo.minp = room_min
			roomInfo.maxp = room_max
			result[#result + 1] = roomInfo
		end
	end

	return result;
end

-- Only partially excavates dungeons, the rest is left as an exercise for the player ;)
-- (Corridors and the parts of rooms which extend beyond the emerge boundary will remain filled)
function excavate_dungeons(data, area, rooms)

	local vi, node_id

	-- any air from the native mapgen has been replaced by netherrack, but
	-- we don't want this inside dungeons, so fill dungeon rooms with air
	for _, roomInfo in ipairs(rooms) do

		local room_min = roomInfo.minp
		local room_max = roomInfo.maxp

		for z = room_min.z, room_max.z do
			for y = room_min.y, room_max.y do
				vi = area:index(room_min.x, y, z)
				for x = room_min.x, room_max.x do
					node_id = data[vi]
					if node_id == c_netherrack or node_id == c_netherrack_deep then data[vi] = c_air end
					vi = vi + 1
				end
			end
		end
	end
end

-- Since we already know where all the rooms and their walls are, and have all the nodes stored
-- in a voxelmanip already, we may as well add a little Nether flair to the dungeons found here.
function decorate_dungeons(data, area, rooms)

	local xStride, yStride, zStride = 1, area.ystride, area.zstride
	local minEdge, maxEdge = area.MinEdge, area.MaxEdge

	for _, roomInfo in ipairs(rooms) do

		local room_min, room_max = roomInfo.minp, roomInfo.maxp
		local room_size = vector.distance(room_min, room_max)

		if room_size > 10 then
			local room_seed = roomInfo.x + 3 * roomInfo.z + 13 * roomInfo.y
			local window_y  = roomInfo.y + math_min(2, room_max.y - roomInfo.y - 1)

			if room_seed % 3 == 0 and room_max.y < maxEdge.y then
				-- Glowstone chandelier (feel free to replace with a fancy schematic)
				local vi = area:index(roomInfo.x, room_max.y + 1, roomInfo.z)
				if is_dungeon_brick(data[vi]) then data[vi] = c_glowstone end

			elseif room_seed % 4 == 0 and room_min.y > minEdge.y
				   and room_min.x > minEdge.x and room_max.x < maxEdge.x
				   and room_min.z > minEdge.z and room_max.z < maxEdge.z then
				-- lava well (feel free to replace with a fancy schematic)
				local vi = area:index(roomInfo.x, room_min.y, roomInfo.z)
				if is_dungeon_brick(data[vi - yStride]) then
					data[vi - yStride] = c_lava_source
					if data[vi - zStride] == c_air then data[vi - zStride] = c_netherbrick_slab end
					if data[vi + zStride] == c_air then data[vi + zStride] = c_netherbrick_slab end
					if data[vi - xStride] == c_air then data[vi - xStride] = c_netherbrick_slab end
					if data[vi + xStride] == c_air then data[vi + xStride] = c_netherbrick_slab end
				end
			end

			-- Barred windows
			if room_seed % 7 < 5 and room_max.x - room_min.x >= 4 and room_max.z - room_min.z >= 4
			   and window_y >= minEdge.y and window_y + 1 <= maxEdge.y
			   and room_min.x > minEdge.x and room_max.x < maxEdge.x
			   and room_min.z > minEdge.z and room_max.z < maxEdge.z then
				--data[area:indexp(roomInfo)] = minetest.get_content_id("default:mese_post_light") -- debug

				-- Until whisper glass is added, every window will be made of netherbrick fence (rather
				-- than material depending on room_seed)
				local window_node = c_netherfence

				local vi_min = area:index(room_min.x - 1, window_y, roomInfo.z)
				local vi_max = area:index(room_max.x + 1, window_y, roomInfo.z)
				local locations = {-zStride, zStride, -zStride + yStride, zStride + yStride}
				for _, offset in ipairs(locations) do
					if is_dungeon_brick(data[vi_min + offset]) then data[vi_min + offset] = window_node end
					if is_dungeon_brick(data[vi_max + offset]) then data[vi_max + offset] = window_node end
				end
				vi_min = area:index(roomInfo.x, window_y, room_min.z - 1)
				vi_max = area:index(roomInfo.x, window_y, room_max.z + 1)
				locations = {-xStride, xStride, -xStride + yStride, xStride + yStride}
				for _, offset in ipairs(locations) do
					if is_dungeon_brick(data[vi_min + offset]) then data[vi_min + offset] = window_node end
					if is_dungeon_brick(data[vi_max + offset]) then data[vi_max + offset] = window_node end
				end
			end

			-- Weeds on the floor once Nether weeds are added
		end
	end
end


local yblmin = NETHER_FLOOR   + BLEND * 2
local yblmax = NETHER_CEILING - BLEND * 2
-- At both the top and bottom of the Nether, as set by NETHER_CEILING and NETHER_FLOOR,
-- there is a 128 deep cap of solid netherrack, followed by a 128-deep blending zone
-- where Nether caverns may start to appear.
-- The solid zones and blending zones are achieved by adjusting the np_cave noise to be
-- outside the range where caverns form, this function returns that adjustment.
--
-- Returns two values: the noise limit adjustment for nether caverns, and the
-- noise limit adjustment for the central region / mantle caverns
mapgen.get_mapgenblend_adjustments = function(y)

	-- floorAndCeilingBlend will normally be 0, but shifts toward 1 in the
	-- blending zone, and goes higher than 1 in the solid zone between the
	-- blending zone and the end of the nether.
	local floorAndCeilingBlend = 0
	if y > yblmax then floorAndCeilingBlend = ((y - yblmax) / BLEND) ^ 2 end
	if y < yblmin then floorAndCeilingBlend = ((yblmin - y) / BLEND) ^ 2 end

	-- the nether caverns exist when np_cave noise is greater than TCAVE, so
	-- to fade out the nether caverns, adjust TCAVE upward.
	local tcave_adj               = floorAndCeilingBlend

	-- the central regions exists when np_cave noise is below CENTER_REGION_LIMIT,
	-- so to fade out the mantle caverns adjust CENTER_REGION_LIMIT downward.
	local centerRegionLimit_adj = -(CENTER_REGION_LIMIT * floorAndCeilingBlend)

	return tcave_adj, centerRegionLimit_adj
end



-- On-generated function

local tunnelCandidate_count = 0
local tunnel_count = 0
local total_chunk_count = 0
local function on_generated(minp, maxp, seed)

	if minp.y > NETHER_CEILING or maxp.y < NETHER_FLOOR then
		return
	end

	local vm, emerge_min, emerge_max = minetest.get_mapgen_object("voxelmanip")
	local area = VoxelArea:new{MinEdge=emerge_min, MaxEdge=emerge_max}
	local data = vm:get_data(dbuf)

	local x0, y0, z0 = minp.x, math_max(minp.y, NETHER_FLOOR),   minp.z
	local x1, y1, z1 = maxp.x, math_min(maxp.y, NETHER_CEILING), maxp.z

	local yCaveStride = x1 - x0 + 1
	local zCaveStride = yCaveStride * yCaveStride
	local chulens = {x = yCaveStride, y = yCaveStride, z = yCaveStride}

	nobj_cave = nobj_cave or minetest.get_perlin_map(mapgen.np_cave, chulens)
	local nvals_cave = nobj_cave:get_3d_map_flat(minp, nbuf_cave)

	local dungeonRooms = build_dungeon_room_list(data, area)
	local abs_cave_noise, abs_cave_noise_adjusted

	local contains_nether = false
	local contains_shell  = false
	local contains_mantle = false
	local contains_ocean = false


	for y = y0, y1 do -- Y loop first to minimise tcave & lava-sea calculations

		local sea_level, cavern_limit_distance = mapgen.find_nearest_lava_sealevel(y) -- function provided by mapgen_mantle.lua
		local above_lavasea = y > sea_level
		local below_lavasea = y < sea_level

		local tcave_adj, centerRegionLimit_adj = mapgen.get_mapgenblend_adjustments(y)
		local tcave   = TCAVE + tcave_adj
		local tmantle = CENTER_REGION_LIMIT + centerRegionLimit_adj -- cavern_noise_adj already contains central_region_limit_adj, so tmantle is only for comparisons when cavern_noise_adj hasn't been added to the noise value
		local cavern_noise_adj =
			CENTER_REGION_LIMIT * (cavern_limit_distance * cavern_limit_distance * cavern_limit_distance) -
			centerRegionLimit_adj -- cavern_noise_adj gets added to noise value instead of added to the limit np_noise is compared against, so subtract centerRegionLimit_adj instead of adding

		for z = z0, z1 do
			local vi = area:index(x0, y, z) -- Initial voxelmanip index
			local ni = (z - z0) * zCaveStride + (y - y0) * yCaveStride + 1
			local noise2di = 1 + (z - z0) * yCaveStride

			for x = x0, x1 do

				local cave_noise = nvals_cave[ni]

				if cave_noise > tcave then
					-- Prime region
					-- This was the only region in initial versions of the Nether mod.
					-- It is the only region which portals from the surface will open into.
					data[vi] = c_air
					contains_nether = true

				elseif -cave_noise > tcave then
					-- Secondary/spare region
					-- This secondary region is unused until someone decides to do something cool or novel with it.
					-- Reaching here would require the player to first find and journey through the central region,
					-- as it's always separated from the Prime region by the central region.

					data[vi] = c_netherrack -- For now I've just left this region as solid netherrack instead of air.

					-- Only set contains_nether to true here if you want tunnels created between the secondary region
					-- and the central region.
					--contains_nether = true
					--data[vi] = c_air
				else
					-- netherrack walls and/or center region/mantle
					abs_cave_noise = math_abs(cave_noise)

					-- abs_cave_noise_adjusted makes the center region smaller as distance from the lava ocean
					-- increases, we do this by pretending the abs_cave_noise value is higher.
					abs_cave_noise_adjusted = abs_cave_noise + cavern_noise_adj

					if abs_cave_noise_adjusted >= CENTER_CAVERN_LIMIT then

						local id = data[vi] -- Check existing node to avoid removing dungeons
						if id == c_air or id == c_native_mapgen then
							if abs_cave_noise < tmantle then
								data[vi] = c_netherrack_deep
							else
								-- the shell seperating the mantle from the rest of the nether...
								data[vi] = c_netherrack -- excavate_dungeons() will mostly reverse this inside dungeons
								contains_shell = true
							end
						end

					elseif above_lavasea then
						data[vi] = c_air
						contains_mantle = true
					elseif abs_cave_noise_adjusted < SURFACE_CRUST_LIMIT or (below_lavasea and abs_cave_noise_adjusted < CRUST_LIMIT) then
						data[vi] = c_lavasea_source
						contains_ocean = true
					else
						data[vi] = c_lava_crust
						contains_ocean = true
					end
				end

				ni = ni + 1
				vi = vi + 1
				noise2di = noise2di + 1
			end
		end
	end

	if contains_mantle or contains_ocean then
		mapgen.add_basalt_columns(data, area, minp, maxp) -- function provided by mapgen_mantle.lua
	end

	if contains_nether and contains_mantle then
		tunnelCandidate_count = tunnelCandidate_count + 1
		local success = mapgen.excavate_tunnel_to_center_of_the_nether(data, area, nvals_cave, minp, maxp) -- function provided by mapgen_mantle.lua
		if success then tunnel_count = tunnel_count + 1 end
	end
	total_chunk_count = total_chunk_count + 1
	if total_chunk_count % 50 == 0 then
		debugf(
			"%s of %s chunks contain both nether and lava-sea (%s%%), %s chunks generated a pathway (%s%%)",
			tunnelCandidate_count,
			total_chunk_count,
			math_floor(tunnelCandidate_count * 100 / total_chunk_count),
			tunnel_count,
			math_floor(tunnel_count * 100 / total_chunk_count)
		)
	end

	-- any air from the native mapgen has been replaced by netherrack, but we
	-- don't want netherrack inside dungeons, so fill known dungeon rooms with air.
	excavate_dungeons(data, area, dungeonRooms)
	decorate_dungeons(data, area, dungeonRooms)

	vm:set_data(data)

	minetest.generate_ores(vm)
	minetest.generate_decorations(vm)

	vm:set_lighting({day = 0, night = 0}, minp, maxp)
	vm:calc_lighting()
	vm:update_liquids()
	vm:write_to_map()
end


-- use knowledge of the nether mapgen algorithm to return a suitable ground level for placing a portal.
-- player_name is optional, allowing a player to spawn a remote portal in their own protected areas.
function nether.find_nether_ground_y(target_x, target_z, start_y, player_name)
	local nobj_cave_point = minetest.get_perlin(mapgen.np_cave)
	local air = 0 -- Consecutive air nodes found

	local minp_schem, maxp_schem = nether.get_schematic_volume({x = target_x, y = 0, z = target_z}, nil, "nether_portal")
	local minp = {x = minp_schem.x, y = 0, z = minp_schem.z}
	local maxp = {x = maxp_schem.x, y = 0, z = maxp_schem.z}

	for y = start_y, math_max(NETHER_FLOOR + BLEND, start_y - 4096), -1 do
		local nval_cave = nobj_cave_point:get3d({x = target_x, y = y, z = target_z})

		if nval_cave > TCAVE then -- Cavern
			air = air + 1
		else -- Not cavern, check if 4 nodes of space above
			if air >= 4 then
				local portal_y = y + 1
				-- Check volume for non-natural nodes
				minp.y = minp_schem.y + portal_y
				maxp.y = maxp_schem.y + portal_y
				if nether.volume_is_natural_and_unprotected(minp, maxp, player_name) then
					return portal_y
				else -- Restart search a little lower
					nether.find_nether_ground_y(target_x, target_z, y - 16, player_name)
				end
			else -- Not enough space, reset air to zero
				air = 0
			end
		end
	end

	return math_max(start_y, NETHER_FLOOR + BLEND) -- Fallback
end

-- We don't need to be gen-notified of temples because only dungeons will be generated
-- if a biome defines the dungeon nodes
minetest.set_gen_notify({dungeon = true})

minetest.register_on_generated(on_generated)