[somat]

This is a perpetual problem in computer science, people want a hash function, then decide that the random function very nearly does what they want. then use the random function as a hash function and are dumbfounded that it turns out there is no hard specification for how it works internally. The random function has no guarantee of result across versions, system, time. Hell I would even be harsh enough to say that any reproducibility in random is an accident of implementation. random should always be considered the non-deterministic function, hash is the deterministic function.

[hansvm]

> hash is the deterministic function

In most languages and libraries, hashing is still only deterministic within a given run of the program. Authors generally have no qualms "fixing" implementations over time, and some systems introduce a salt to the hash intentionally to help protect web programmers from DOS "CVEs".

If you want reproducible RNG, you need to write it yourself.

System's based on Jax's notion of splitting a PRNG are often nice. If your splitting function takes in inputs (salts, seeds, whatever you want to call them), you can gain the property that sub-branches are reproducible even if you change the overall input to have more or fewer components.

[skybrian]

I don’t think you need to write it yourself, since there are also libraries that implement specific algorithms for pseudo-random number generators, if if you’re worried, you could pin or vendor the dependency.

But it’s true that if the algorithm isn’t specified then the implementation is free to change it, and probably should change it sometimes, to ensure calls don’t depend on it being fixed.

(Reproducibility is normally about what happens when you use the same version of everything.)

[FooBarBizBazz]

The article has absolutely nothing to do with "hashes vs. PRNGs". Literally nothing. It's all about the linear algebra and floating point operations that happen (in this case) to be used to transform those numbers. It has nothing to do with PRNGs.

Also, PRNG and hash functions are extremely similar, and there's no reason why one should be deterministic and the other not. They're both built out of simple integer and bit operations. If a certain PRNG is implementation-defined, that's a detail of the specific (possibly standard) library you've chosen; it's nothing fundamental.