Only check the parts of the map where players are

Cosmic Realm’s server splits every map into chunks of 16 by 16 tiles. Enemies only run when a player is within 3 chunks of them. Everything else sleeps, which is how a big realm stays cheap.

The slow part was finding where the players were. Every tick, in every world, the server checked every chunk of the map for a player. A 2048 by 2048 realm has 16,384 chunks. With ten players online, almost all of those checks found nothing. It also built a new set for the answer each time, which is more garbage for .NET to clean up.

The fix

Stop searching and keep count. Every chunk has a counter of how many things are in it, and the map keeps a set of the chunks that aren’t empty:

void Occupy(int x, int y) { var i = ChunkIndex(x, y); if (_chunkCounts[i]++ == 0) _occupied.Add(i); }
void Vacate(int x, int y) { var i = ChunkIndex(x, y); if (--_chunkCounts[i] == 0) _occupied.Remove(i); }

Entering, leaving or moving between chunks updates the counters. A chunk joins the set when its count goes from 0 to 1, and leaves when it drops back to 0. Finding the players is now a walk over only those chunks: ten players means at most ten chunks, however big the map is. Each world also reuses one set for the result instead of making a new one every tick.

Tests check that the new version gives exactly the same answer as the old full scan, including after hundreds of random moves.

Same idea, other places

The same pass found two more cases:

• Projectiles were looked up by a Tuple key, which allocates on every add, remove and lookup. Now the key is one long, with the owner’s id and the bullet’s id packed together.
• Always-visible enemies were found by searching every enemy and object in every world, every 100 ms. Now they add themselves to a small list when they spawn.

What I adopted

If the server asks the same question every tick, keep the answer up to date as things change instead of searching again. It costs a counter update on each move and saves a full search on every tick.