GridFixtureSystem Optimizations

[Aidenkrz]

About the PR

Optimizes GridFixtureSystem in a few hot paths that were doing more work (and allocating more garbage) than they really needed to.

Ended up doing a bunch more fuckery to SharedGridFixtureSystem as well after looking through it as well, refer to technical details for info on that.

Why / Balance

One of my maintainers pointed out that there was some evil LINQ garbage in this system that called .Sum() a bajillion times whenever CheckSplits ran. Not the end of the world, but also easily avoidable. This cleans that up and makes the whole thing scale way better without changing behavior. Also noticed system was allocating new collections constantly, so i changed that as well.

Technical details

• The BFS scratch structures (_splitFrontier queue and _splitGrids list) are now reused as class fields instead of being allocated every CheckSplits call.

• The sort comparator was doing repeated LINQ .Sum() calls, resulting in something like
  $O(N \times M \times \log N)$ work.
  This is now replaced with a precomputed gridSizes dictionary, bringing it down to
  $O(N \times M + N \times \log N)$.

  As a rough example:

  • For N = 50 grids with M = 100 tiles each, this cuts the comparator work by ~90%.
  • For N = 200 grids with M = 250 tiles each, it’s closer to a ~97% reduction.

  I didn't do any real benchmarking as I don't have dottrace, but I mean, the math checks out.

• In CreateNodes, the tile HashSet now only tracks non-empty tiles (capacity set to chunk.FilledTiles) instead of blindly allocating for all 256 chunk positions.

• Cross-chunk neighbor detection now bails early for interior tiles, since only edge tiles can possibly have neighbors in adjacent chunks.

• MapGridComponent is now passed through the full call chain as a nullable parameter with TryComp/Comp fallback.

• Grid fixture diffing in UpdateFixture now cross-references old and new fixtures by ID instead of using a full Fixture.Equals comparison. The old approach always failed because content (e.g. ShuttleSystem.OnGridFixtureChange) modifies fixture density after creation, causing every fixture to be destroyed and recreated on every tile change. The new approach compares shapes only via EqualsApprox, preserving content-set properties like density. This also eliminated a redundant shape check that existed solely as a workaround for the broken equality comparison, and fixed a latent bug where chunk.Fixtures (a HashSet<string>) was being modified during foreach enumeration.

• During grid splits, entity reparenting now does a single broadphase query per node using the union bounding box of all its tiles, instead of one query per tile. Entities are filtered by converting their position to a tile index and checking membership in a HashSet<Vector2i>. This reduces broadphase queries from O(tiles) to O(1) per node.

[Aidenkrz]

accidentally left the PR title as the commit name and cant change it now 👎

[ArtisticRoomba]

github stays winning

[Tayrtahn]

Apparently you can still change it from the mobile app. You can also just tell me what title you'd like and I'll change it for you.

[Aidenkrz]

Apparently you can still change it from the mobile app. You can also just tell me what title you'd like and I'll change it for you.

Thanks I used GitHub mobile

[moonheart08]

Looking for footage of this being tested, ideally, just as a "this obviously works". The changes lgtm though.

[DrSmugleaf]

Outside of DungeonJob it seems like the only other caller to CheckSplits is SharedMapSystem.RegenerateCollision, which seem fine at a glance

[Aidenkrz]

grid.mp4