Make sure ancient debris is not generated exposed to air

mods/MAPGEN/mcl_biomes/init.lua

@@ -3920,7 +3920,7 @@ end
 -- Decorations in non-Overworld dimensions
 local function register_dimension_decorations()
 	--[[ NETHER ]]
-	-- TODO: Nether 
+	-- TODO: Nether
 
 	--[[ THE END ]]
 

mods/MAPGEN/mcl_debrisgen/init.lua

@@ -0,0 +1,44 @@
+local c_debris = minetest.get_content_id("mcl_nether:ancient_debris")
+local c_netherrack = minetest.get_content_id("mcl_nether:netherrack")
+local c_air = minetest.get_content_id("air")
+
+local facedir = {
+  vector.new(0, 0, 1),
+  vector.new(0, 1, 0),
+  vector.new(1, 0, 0),
+  vector.new(0, 0, -1),
+  vector.new(0, -1, 0),
+  vector.new(-1, 0, 0),
+}
+
+minetest.register_on_generated(function(minp, maxp)
+  if maxp.y < mcl_vars.mg_nether_min or minp.y > mcl_vars.mg_nether_max then
+    return
+  end
+
+  local vm, emin, emax = minetest.get_mapgen_object("voxelmanip")
+  local data = vm:get_data()
+  local area = VoxelArea:new({MinEdge = emin, MaxEdge = emax})
+
+  for idx in area:iter(minp.x, math.max(minp.y, mcl_vars.mg_nether_min), minp.z, maxp.x, math.min(maxp.y, mcl_vars.mg_nether_max), maxp.z) do
+    if data[idx] == c_debris then
+      local pos = area:position(idx)
+      local exposed = false
+      for _, dir in pairs(facedir) do
+        if data[area:indexp(vector.add(pos, dir))] == c_air then
+          exposed = true
+          break
+        end
+      end
+      if exposed then
+        data[idx] = c_netherrack
+      end
+    end
+  end
+
+  vm:set_data(data)
+  vm:calc_lighting()
+  vm:update_liquids()
+  vm:write_to_map()
+end)
+

mods/MAPGEN/mcl_debrisgen/mod.conf

@@ -0,0 +1,4 @@
+name = mcl_debrisgen
+author = Fleckenstein
+description = Make sure ancient debris is not generated exposed to air
+depends = mcl_mapgen_core, mcl_nether