Recently I have been asked to share some source code I have written with another PhD student. I have developed and used the methods in that source code for the purposes of my own research. The code will save her approximately 2 months of development/testing. I'm prepared also to offer additional help with data pre/post-processing & writing.
Given the above, am I entitled to ask for co-authorship?
You can discuss with her and her advisor whether your help warrants authorship, but it would be rather unhelpful to refuse her even if she or her advisor declines. I would say, if this is true, (1) Sure! I'd love for more good research to come from my work! and (2) Since my project isn't published yet, we'll need to include a good description in the methods and it probably makes sense for me to be a coauthor.
If the target journal particularly advocates open-data and open-access, you might have to release your work in order for them to get it published. It's important to talk through first to make sure everyone's okay with that. In general it's a fantastic idea since it means that work is not so difficult to replicate, and if you have a valuable tool maybe others will benefit from it. But when you're laboring to finish your own work in time and worried about its perceived novelty, it can seem a little less appealing.
Just have the discussion (in a friendly manner) now so there aren't any unpleasant surprises later.
So, you are giving your colleague some of your research code to use on her own problem. If she ends up using, or trying to use it, then I predict two things.
(a) You will end up having to help her understand the code well enough to use it. This is very likely, amounting to practically a certainty.
(b) You will have to adapt the code to work on her problem. This is still very likely, but not quite as certain.
I'd say (b) definitely entitles you to co-authorship. (a) is not quite so clear. If you just give her the code and don't do anything else (in which case she is unlikely to be able to use the code) then you definitely aren't entitled to co-authorship.
Whatever you do, you could still be denied co-authorship. However, bear in mind that you cannot (in my opinion) reasonably withhold the code (that seems to me contrary to what research is about) but you are not required to work with your colleague on her problem for nothing. Therefore, if you do end up spending significant amounts of time working with your colleague on her problem, then bring up the issue of co-authorship early. If your colleague or her mentors don't want to give you co-authorship, then ask yourself why you would spend time working on this project.
You also say:
I'm prepared also to offer additional help with data pre/post-processing & writing.
This is a separate issue which I'm not addressing here. I have just focused on the involvement which will naturally stem from you providing your source code. If I understand your quote above correctly, this refers to optional assistance you are prepared to provide.
Code is (hopefully) now considered as part of the regular research work. Thus, it can:
But more important, it can be cited, especially when the authors of a paper relied on someone else's code.
What does it mean for you?
In your case, you have several options:
I put a friend of mine (whose code saved me a few days, I would say around 1 week) in the acknowledgements section of a paper of mine. He told me it was already too much, but I didn't want him to go unnoticed/uncited. It was just a matter of being polite and thankful for me...
