Best data structure for `Map<u8,usize>`?

I'm looking to store some kind of map between u8 and usize. I'd like it to be as fast as possible, as I'll be going through a lot of these. The obvious option is [usize; 256], using usuze::MAX to represent unset values, but that does waste a lot of memory, which could lead to low memory locality, which wouldn't be ideal. I've also considered Vec<(u8, usize)> which may save space, but is slow to access and by using heap allocation gives up memory locality entirely. A hash map without actually hashing seems like it might be ideal, especially since most commonly I'll have b'a' to b'z' is my values, so a map size of 32 would give no collisions.

Does a better data structure exist for me than just an array?

Can you guarantee it is always <= 64 ?

If so, I'd go with something like this:

pub struct Foo {
  keys: [u8; 64];
  data: [usize; 64];

so all of keys fits in a single cache line. You can use SIMD instructions to figure out if the key is contained; and, if so, make one memory lookup to data

Could you point me at the simd code to use? I've never used simd before actually.

I never implemented this myself. The technique is described in swisstable / hashbrown.

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