announcing hasch

I just want to briefly announce hasch, a small library to consistently crypto-hash edn in Clojure and ClojureScript. Practically this means you can create a UUID5 from edn on either platform consistently (patches for CLR or other Clojure hosts welcome).

(uuid ["hello world" {:a 3.14} #{42} '(if true nil \f)]) 
=> #uuid "18f594a3-be77-5449-a0b5-b71552e1d37a"

Compared to just hashing a printed edn-string it respects maps and sets as commutative and uses bitwise XOR to achieve the same hash-value independant of order. Comments and other syntax specific problems are also cleanly avoided as hasch works on the runtime value, e.g. after reading an edn string. Metadata is currently ignored.

We are using it in a cross-platform prototype-level repository system and it works out fine so far, but should not yet be considered production-ready as I want to get critical feedback first and fix the following issue: I am not yet sure whether to extend Clojure protocols or only Clojure collections (this is how it is done in cljs atm.), as both can be considered edn at runtime, but may not satisfy value semantics (e.g. mutable arrays).

cryptography

I am also not a cryptography expert, I tried to get all bits of the edn-data into sha-1 and trust it to avoid collisions then. I encode a flat edn-level type information (e.g. strings and chars are the same type because of JavaScript). You can also use a different hashing algorithm and create similar UUIDs (but this doesn't satisfy the UUID standard yet). Our UUID hash is internally versioned, so it can be fixed 4 times (before hopefully UUID5 will be superseded with something better than SHA-1 anyway).

Don't consider the hash stable yet, but only after some possible initial feedback for the next release(s).

future plans

One design idea is to be able to hash nested collections first with the hash-function before they contribute to the overall hash-value, allowing you to break them out and store them under their nested hash-value, potentially building persistent data-structures which can share values without needing to rehash all data on changes. This is implemented but not used yet. Strings are currently seen as primitives and not hashed that way.

In general we want to rely on it to build a distributed database.

details

I have ported Utf-8 encoding from JavaScript to ClojureScript if you happen to need that. You can also create random UUIDs with:

(uuid) 
=> #uuid "a948572d-4e2a-466d-9963-740824526e7e"

which uses Frank Siebenlist's code in cljs and java.util.UUID/randomUUID in Clojure.

If you have ideas or find any flaws, please open an issue on github. Pull-requests are welcome!