API usage and confusion

[hannesm]

Hey,

I'm not entirely sure I understand the API design. We have two types, ctx and t -- but what is the fundamental difference?

A ctx we can create by either empty or init -- hold on, should the ctx returned by empty be immutable or better not being used?

A t we receive from get, but also from the functions that take care of computing the hash of a string / string list /...

Now, the nicely introduced get_into_bytes is defined on ctx, so if I computed the hash with digest_string, I can't extract it into an existing byte vector.

Would it be possible to simplify the API and only provide a single type t? Since it is private anyways, I'd expect no issues... What do you think?

[hannesm]

I got confused when thinking about this code from mirage-crypto's hmac_drbg RNG:

    let rec go off k v = function
      | 0 -> v
      | 1 ->
        let v = H.hmac_string ~key:k v |> H.to_raw_string in
        let len =
          let rem = len mod H.digest_size in
          if rem = 0 then H.digest_size else rem
        in
        Bytes.unsafe_blit_string v 0 buf off len;
        v
      | i ->
        let v = H.hmac_string ~key:k v |> H.to_raw_string in
        Bytes.unsafe_blit_string v 0 buf off H.digest_size;
        go (off + H.digest_size) k v (pred i)

To avoid further allocations, also a ?len parameter for get_into_bytes would be great.