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On 20.04.2015 20:02, Francis Avila wrote:
There's no contract, but strings, keywords, and symbols should hash
the same, and collections of these (vectors, lists, maps, sets)
should hash the same.
It's difficult to hash numbers the same between Clojure and
Clojurescript.
Clojurescript numbers are all doubles (because JS), so they should
in theory hash the same as Clojure doubles. Clojure hashes doubles
using Java's Double.hashCode(), which relies on knowing the exact
bits of the double. These bits are not available in Javascript (at
least not easily or without using typedarrays). I'm also not sure
if this particular implementation of hashCode is part of the Java
spec (i.e. implemented the same by all JDKs and JVMs.)
Of course in practice the same clojure form (as read) will not hash
the same in clj and cljs because clj uses longs most of the time.
You could take a hybrid approach where integers in cljs are hashed
like longs in Clojure. This works up to 52 bits, but longs with
more bits than that are not representable in Clojurescript. This is
an approach I was pursuing in my murmur3 hashing implementation for
cljs:
http://dev.clojure.org/jira/browse/CLJS-754
https://github.com/favila/clojurescript/blob/murmur3/src/cljs/cljs/core.cljs#L
https://github.com/favila/clojurescript/blob/murmur3/src/cljs/cljs/murmur3.cljs#L61
In practice you will hash the same most of the time if you most
deal with integer numbers, but any doubles, bigdecimals, or large
integers will still hash differently.
This is what happens in clojurescript now:
(js-mod (Math/floor o) 2147483647)
Hi,
I am the author of hasch (1) for cryptographic cross-platform hashing,
which Herwig mentioned. The numeric types were a problem for me, too,
as Francis describes. I guess that all the corner cases are difficult
to catch, so I decided to treat all numbers like doubles in edn data.
Still this is fragile due to floating point arithmetic differences to
JVM integer arithmetic... JavaScript is really bad for numerical tasks
sadly and I don't see a lightweight work-around. So once you
numerically calculate the same thing on both runtimes and expect the
hashed results to be the same, there could be trouble.
Additionally there is no Character type in JavaScript, so you have to
treat them like Strings as well. Normally different types with the
same content should hash differently, except for seqs and vectors, to
my current understanding.
My implementation is just a recursive hashing scheme protocol, which
allows to swap the hash function, so if you want something more
lightweight than sha512, you could also use a cross-platform murmur
implementation with hasch maybe. Maps and Sets are hashed elementwise
and XORed afterwards, which might cause performance problems in your
case. I haven't found a cheaper way to ensure against malicious
collisions.
Cheers,
Christian
(1) https://github.com/ghubber/hasch
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