Add minimal API

[cschwan]

Commit c5f4b29a3da816cb7d534352f66d6e84afb98d81 adds a minimal API, as outlined in #12, which I copied from the lhapdf crate. There are a few things to do:

• [x] allow different backends to be used, ideally depending on the setname specification string. In practice we probably want to have an internal backend and LHAPDF. This was added in commits 0cff0026555817c11ef279cfb6b783406bd8589f and de8f95d2cb3066588008f2faf054b7d05de1b55a.
• [ ] automatic fetching and caching of files (this is #7 and concerns also #8).

[cschwan]

@AleCandido @scarlehoff you can add more backends now. To do that copy the file partons/src/noop.rs, give it a different name and fill the methods with meaningful implementations. Finally you'll need to add the corresponding lines in partons/src/frontend.rs here:

https://github.com/AleCandido/partons/blob/de8f95d2cb3066588008f2faf054b7d05de1b55a/partons/src/frontend.rs#L36-L39

and here

https://github.com/AleCandido/partons/blob/de8f95d2cb3066588008f2faf054b7d05de1b55a/partons/src/frontend.rs#L68-L71

The functions PdfSet::new and PdfSetEnum::new should then select between the different backends depending on the selected set or PDF (still TBD).

[alecandido]

Given the interest, I will soon restart working on automatic fetching and caching.

I was about to propose you the noop.rs as well, and to move there the former fake_lhapdf.hpp. But I see you don't need any suggestion :D Thanks for this part, it looks perfect to me :)

[cschwan]

I'm a little bit unhappy about partons/src/fake_lhapdf.rs, which is a copy of noop.rs and nothing else, but it's necessary because every struct implementing Pdf or PdfSet must be a different type; I can live with it.

We should decide how the backend selection works, which right now is very dumb for Pdf

https://github.com/AleCandido/partons/blob/dc585d3a5d58ffdbfd0ace911b6b514d59adcd3a/partons/src/frontend.rs#L57-L60

and PdfSet:

https://github.com/AleCandido/partons/blob/dc585d3a5d58ffdbfd0ace911b6b514d59adcd3a/partons/src/frontend.rs#L87-L90

One easy way would be to

• allow a prefix, for instance LHAPDF:NNPDF40_nnlo_as_01180 would load NNPDF4.0 using LHAPDF and PARTONS:NNPDF40_nnlo_as_01180 would use our internal backend, and
• use the 'default' backend if the prefix is missing. This could be our internal backend.

Another option is to have different backends name PDF sets differently, but I don't like it that much.

[alecandido]

use the 'default' backend if the prefix is missing. This could be our internal backend.

Apply a default is perfectly fine. To select the others, I would pass an optional argument, i.e. Option<BackendsEnum> in Rust (in Python it can really be optional).

Since I already added a configuration file, we can add an option also there to select a different backend. But I would do at a later stage.