In particular, empty maps, which make up the majority of instances, and consume memory despite storing no unique data.

Unfortunately, none of the maps are actually empty, and I feel the logic to handle the case of "un-emptying" a map could outweigh the memory savings.

(By the way, since you're here, would you mind if I integrated some of your dormant rendering bugfix PRs into FoamFix? This also implies backporting the fitting ones to earlier Minecraft versions, which Forge most likely won't do.)

I was thinking of possibly having a singleton empty CIMM, with handling a bit like Chunk.NULL_BLOCK_STORAGE. That said, I'm not really sure about the practicality, I'll leave it as your decision.

In response to your question; no, I don't mind if you want to take on bugfixes for backporting.

@bs2609 The singleton empty CIMM would have to turn itself into a non-empty CIMM, however, as entries are added to it as new entity types get spawned. I don't think the practicality is worth it, and I think there are other areas more worth focusing on (for instance, finding large areas of ArrayLists to trim - the latest commit should help that greatly).

I don't think there's any need for an empty CIMM to keep track of types - as it always contains no entities, any query should return false regardless of the entity class. (something like this is what I'm thinking of, if this isn't a good explanation)

ArrayList trimming is certainly an easier win, if there's still more to be had.

@bs2609 Does it always contain no entities, though? That's the thing I'm trying to get at - what do we do once an entity is added?

How about having a wrapper CIMM, that delegates to an internal CIMM instance, which is initialised to the empty singleton and then lazily set to a real map?

Kinda like how Java optimises the backing storage for it's own collections, but one layer above.

@bs2609 Maybe, I'd have to see the CPU performance differences for that though. No point saving a few megabytes if it slows everything down!

(By the way, my take on #4313 -> https://github.com/asiekierka/FoamFix/blob/4a610552bbc2b414a4f380beab555f8eb7fa748b/src/main/java/pl/asie/foamfix/coremod/staging/Patch4313.java )