A looong time ago I wrote my first blog post - on a now defunct website - about a VecMap where I did exactly that. Sort when needed and full flat array. That said flat_map as coined by Google is an acronym for swiss tables. See [1]. I.e. exactly what Rust's standard library `HashMap` is, also the one being tested here.

[1] https://github.com/abseil/abseil-cpp/blob/23b9b75217721040f5...