Mapgen memory optimisations

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Retranscription of Paramat's forum topic.

Perlin noise objects: Only create once

The noise object is created by minetest.get_perlin_map(). It has to be created inside register_on_generated() to be usable, but only needs to be created once, many mapgen mods create it for every mapchunk, this consumes memory unnecessarily.

Localise the noise object outside register_on_generated() and initialise it to nil. See the code below for how to create it once only. The creation of the perlin noise tables with get_3d_map_flat() etc. is done per mapchunk

-- Initialize noise objects to nil

local nobj_terrain = nil
local nobj_biome = nil

-- ...
-- ...

-- On generated function
minetest.register_on_generated(function(minp, maxp, seed)
   -- ...
   -- ...
   nobj_terrain = nobj_terrain or minetest.get_perlin_map(np_terrain, chulens3d)
   nobj_biome = nobj_biome or minetest.get_perlin_map(np_biome, chulens2d)
   
   nobj_terrain:get_3d_map_flat(minpos3d, nvals_terrain)
   nobj_biome:get2d_map_flat(minpos2d, nvals_biome)
   -- ...
   -- ...
end)

Perlin noise tables: Re-use a single table

The Lua table that stores the noise values for a mapchunk is big, especially for 3D noise (80³ = 512000 values). Many Lua mapgens are creating a new table for every mapchunk, while the previous tables are only cleared out slowly by garbage collection, resulting in a large and unnecessary memory use.

A buffer parameter was added in 0.4.13 to avoid this, a single table is re-used by overwriting the former values.

For each of the functions with an optional `buffer` parameter:  If `buffer` is not
nil, this table will be used to store the result instead of creating a new table.
 
#### Methods
* `get_2d_map_flat(pos, buffer)`: returns a flat `<size.x * size.y>` element array of 2D noise
with values starting at `pos={x=,y=}`
* `get_3d_map_flat(pos, buffer)`: Same as `get_2d_map_flat`, but 3D noise

Localise the noise buffer outside register_on_generated() Use the buffer parameter in get_3d_map_flat() etc.

-- Localise noise buffers
local nvals_terrain = {}
local nvals_biome = {}
-- ...
-- ...
-- On generated function
minetest.register_on_generated(function(minp, maxp, seed)
   -- ...
   -- ...
   nobj_terrain:get3d_map_flat(minpos3d, nvals_terrain)
   nobj_biome:get2d_map_flat(minpos2d, nvals_biome)
   -- ...
   -- ...
end)

Lua voxelmanip table: Re-use a single table

The Lua table that stores the node content IDs for a mapchunk plus the mapblock shell is big (112³ = 1404928 values). Many Lua mapgens are creating a new table for every mapchunk, while the previous tables are only cleared out slowly by garbage collection, resulting in a large and unnecessary memory use.

A buffer parameter was added in 0.4.13 to avoid this, a single table is re-used by overwriting the former values.

In Minetest 0.4.15, a buffer parameter was also added to get_param2_data().

* `get_data([buffer])`: Retrieves the node content data loaded into the `VoxelManip` object
    * returns raw node data in the form of an array of node content IDs
    * if the param `buffer` is present, this table will be used to store the result instead
* `get_param2_data([buffer])`: Gets the raw `param2` data read into the `VoxelManip` object
    * Returns an array (indices 1 to volume) of integers ranging from `0` to `255`
    * If the param `buffer` is present, this table will be used to store the result instead

Localise the data buffer outside register_on_generated(). Use the buffer in get_data() or get_param2_data().

-- Localise data buffer
local data = {}

-- On generated function
minetest.register_on_generated(function(minp, maxp, seed)
   -- ...
   -- ...
   local vm, emin, emax = minetest.get_mapgen_object("voxelmanip")
   local area = VoxelArea:new{MinEdge = emin, MaxEdge = emax}
   vm:get_data(data)
   -- ...
   -- ...
end)