[FR] Add digest_jaccard method to MinHash structs or modify existing jaccard methods

[iron3oxide]

It would be really cool if one could store a document and its generated MinHash somewhere (like a database) and check for similarity with other documents at a later stage. As of now, this is not really possible to do, as one would have to regenerate the hash object with the document again since it's not possible to regenerate a MinHash instance from its digest or to get the jaccard distance to a hash in digest form.

I see an easy solution for both CSimHash and RSimHash: add a digest_jaccard method that takes a Vec<u32> digest and spits out the similarity. One could also consider modifying the existing jaccard methods instead, as only the digest of other is ever needed in them, so handing over the full MinHash object seems inefficient. Of course this would be a breaking change, maybe warranting a 0.3.0 release.

I'm happy to implement this, please let me know if this would be a welcome change and which option you'd prefer!

[iron3oxide]

An even cleaner solution might be to structurally separate Hasher and Hash. Say we have RMinHasher which has the fields num_perm, seed and permutations and the added field hash of type MinHash which gets updated with update(). digest() would be dropped as there is no need for it anymore.

MinHash has the field digest/hash_values (however you want to call it) and the method jaccard(). Since num_perm can be easily extracted from the vector size of digest it's not explicitly needed here. This would make for a lightweight hash struct that only stores the digest and is able to compare it to other digests (or MinHash instances if that is needed for interoperability reasons I can't comprehend yet). I'm no algorithm expert, but I assume RMinHash and CMinHash digests are interoperable because the latter has two digest methods, so only one MinHash struct would be needed between the two of them.

I am aware that this constitutes more of a redesign than a simple feature implementation, just wanted to share the idea and hear your thoughts

[beowolx]

@iron3oxide Thanks rite-up, this is a good request you can go ahead and open a PR.