ERFA Wrapping Extension
This year I am applying for Google Summer of Code under OpenAstronomy umbrella organization to Astropy. My project idea is Implement Public API for ERFA. You can view my proposal in the previous post here.
In this blog post, I am going to explain about ERFA and its use in Astropy.
ERFA is a C library containing key algorithms for astronomy and is based on the SOFA library published by the International Astronomical Union (IAU). It implements high-quality astronomical routines for coordinate, time etc. More about ERFA can be seen here.
_erfa is a python interface, not publically exposed, which wraps around original C functions, creating and exposing vectorized versions of them.
Main parts in
_erfa are Jinja 2 templates
core.py.templ. These combined with
erfa_generator.py generates the wrapper code, splitting the wrappers into python portion
core.py and C portion
The main goal of my project is to pull out pull this wrapper out of astropy and instead make a standalone Python wrapper library so that any project can use it without needing Astropy.
To get to know the extent to which the modifications in existing wrapper would be needed I tried to do some quick hacks on it.
astropy._erfa wraps only from
Astronomy section and
SphericalCartesian subsections under
VectorMatrix section from the original erfa library. This could be checked in
To test how the current infrastructure works if it is wrapped to all the sections of original erfa library I modified above code to always
and compiled the whole project again. And it failed. The error was:
astropy/_erfa/core.c: In function ‘Py_cr’: astropy/_erfa/core.c:5376:9: error: too few arguments to function ‘eraCr’ eraCr(*_r); ^ In file included from astropy/_erfa/core.c:14:0: cextern/erfa/erfa.h:376:6: note: declared here void eraCr(double r, double c); ^ error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
Above error suggests that
eraCr function was not wrapped correctly, which would need changing the templates.
To test for other remaining sections I modified the code to
This time, it compiled successfully and the whole package was set up.
I tried to use some functions like
eraPmp etc. and they all worked correctly. But it all needs to be checked carefully.
From this little exercise, I was able to somewhat understand the workflow in
_erfa and relate to the changes the project would be needing.