Sparse Components (non-movable components)

[Ukendio]

Non-movable components, that when inserted goes into the sparse set storage. The sparse component itself does not need to be moved to the destination archetype during insertion.

Benefits

• Doesn't need to move the sparse component itself so slightly faster removals and insertions
• Fast iteration of single sparse components since it should just iterate over the densely packed pool/column

Drawbacks

• Explicit storage specification
• Queries of sparse components are always slower than archetype queries except for two scenarios:

1. if the query is just interested in a single sparse component (just iterate the dense array of the sparse set)
2. if fragmentation is very high (like 1-2 entities per archetype, and lots of archetypes)

[Ukendio]

After a lot of deliberation, I think that keeping a sparse container such as HashMap<Entity, T> besides the ECS has been the best opt-in "sparse components". The pros of this is that it has the fastest possible O(1) add/remove because it doesn't require copying movable component values in the old archetype and doesn't require the introduction to new constructs or concepts. The biggest downside here is that it doesn't integrate into the ECS so you can't query with them. This doesn't feel too bad though since you wouldn't want to be iterating sparse components in the first place because that would be a naive linear iteration.