hash collision for non equal collections

[swannodette]

hash collision for non equal collections

[lantiga]

In general, any hash will provide the following guarantee: if A and B have different hashes, then A and B are not equivalent (for some definition of equivalence). They don't however provide you with the converse, i.e. that if A and B have the same hashes then A and B are equivalent. Of course there can be better and worse hashing algorithms, e.g. count is a lousy hash for a vector, while Clojure 1.6 just obtained a performance gain by introducing a new hashing algorithm that leads to fewer collisions.

So you shouldn't rely on hashes as unique keys unless you design for collision, e.g. for associative structures you can use hashes as keys and a list of objects as values (rather than just one object) and then resort to a linear search when you have to disambiguate.

In the case of react component keys, object hashes alone are probably not a good choice.

[eyston]

Thanks. I was a bit surprised by the frequency of collisions so figured I'd ask :)

[lantiga]

Apparently there's ongoing work for supporting the new Clojure 1.6 hashing in ClojureScript: http://dev.clojure.org/jira/browse/CLJS-754
Once this is merged collisions will be a lot fewer.

[swannodette]

Yes hash collisions will be greatly diminished soon, that said I would still not use the hash code as a React key.

[swannodette]

Closing this as it's not really a Mori issue. This will be resolved when it's resolved in ClojureScript itself.

[eyston]

Agreed. I'll take a look at how some other libraries handle the key thing. Thanks for quick responses.