CODEWITHSUNDEEP
arrayslua
angryInsomniac
angryInsomniac

Reputation: 859

Memory management in Lua

I am making a level editor for a simple game in lua and the tiles are represented by integers in a 2d array, when I read the level description in from a file, it may so happen that this 2d array is sparsely populated, how does lua manage memory ? will it kep those holes in the array or will it be smart about it and not waste any space?

Upvotes: 1

Views: 640

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473272

The question itself is irrelevant in a practical sense. You have one of two cases:

  1. Your tilemaps are reasonably small.
  2. Your tilemaps are big enough such that compression is important for fitting in memory constraints.

If #1 is the case, then you shouldn't care. It doesn't matter how memory efficient Lua is or isn't, because your tilemaps aren't big enough for it to ever matter.

If #2 is the case, then you shouldn't care either. Why? Because if fitting in memory is important to you, and you're likely to run out, then you shouldn't leave it to the vagaries of how Lua happens to manage the memory for arrays.

If memory is important, you should build a specialized data structure that Lua can use, but is written in C. That way, you can have explicit control over memory management; your tilemaps will therefore take up as much or as little memory as you choose for them to.

As for the actual question, it rather depends on how you build your "array". Lua tables are associative arrays by nature, but their implementation is split between an "array part" and a "table part". In general though, if you store elements sparsely, then the elements will be sparsely stored in memory (for some definition of "sparse"). As long as you don't do something silly like:

for i = 1, max_table_size do
  my_tilemap[i] = 0
end

Then again, you may want to do that for performance reasons. This ensures that you have a big array rather than a sparse table. Since the array elements are references rather than values, they only take up maybe 16 bytes per element. Once you decide to put something real in an entry (an actual tile), you can. Indexing into the array would be fast in this case, though since the table part is a hash-table, it's not exactly slow.

Upvotes: 3

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