Add XorShiftRng::extract_seed

[tomaka]

Useful for serializing the PRNG and reload it later.

[jacwah]

I'm not sure seed is the correct term here. As far as I know, the seed is the inital state, not the current one. Maybe extract_state would be a better name.

Generally, I think a way to serialize the state of all RNGs is important. Perhaps we should explore ways to do this in a consistent way across all algorithms. Serde could maybe be useful.

However, the issue with serialization is that it breaks encapsulation. As soon as we merge something like this PR, we can no longer change struct fields without breaking user's code.

[tomaka]

Serde could maybe be useful.

Serde would be way overkill and create a semver hell.
This just about adding one function per struct that returns an array.

[dagit]

I found this PR while reading this thread: https://www.reddit.com/r/roguelikedev/comments/6mk4k4/serializing_the_prng/

There the OP wants to serialize the PRNG state in a game.

I agree that the name should be changed to extract_state. I would love to see a similar function for the other PRNGs too, but for the ones that reseed from external entropy sources that may require additional API changes. Specially to deal with the case where the goal is to get the exact same sequence given some PRNG state (common in simulations).

[dhardy]

Serialization should be provided by #189, which I think is the main motivation here.

[tomaka]

Do we really need a PR that adds 874 lines of code just to give access to four u32s? :-/

[dhardy]

It's not to give us four u32s, it's to allow full serialisation and deserialisation of all types. Your code isn't anywhere near so comprehensive (and can't be along this approach, because most PRNGs don't let you directly set their internal state).

Besides, once serde support for wrapping types and generalisation over constants allows proper handling of large arrays, it should be possible to reduce all of this to #[derive(Serialize, Deserialize)].

[dhardy]

We have Serde support now, and it doesn't require 874 lines of code.