It looks like FFTPACK has "double precision" hard coded in. Would the maintainers be open to accepting a pull request that made this configurable? I would:

• change all instances of "double precision" to "real(rk)" and do the same for complex
• add a new file, that contains "integer, parameter :: rk = kind(1.0d0)"
• include that new file in all the old fortran77 files, so that the kind being used could be changed at compile time in exactly one place.

We could use a module instead of "include", but I'm not familiar enough with old-style loose functions to know if that would work.

The end result would be that the functions are still only provided for one kind, but rather than that kind being hard-coded to double precision, the user could choose the kind themselves. And just to be clear, I'm offering to do this, not requesting that someone else do it (though that's fine too, of course).

So what do we think? Does that involve too many changes to the old code? Or is it ok?