In an earlier post, I mentioned that we needed an object-oriented modern Fortran library for multidimensional linear interpolation. Well, here it is. I call it finterp, and it is available on GitHub. It can be used for 1D-6D interpolation/extrapolation of data on a regular grid (i.e., not scattered). It has a similar object-oriented interface as my bspline-fortran library, with `initialize`

, `evaluate`

, and `destroy`

methods like so:

type(linear_interp_3d) :: interp
call interp%initialize(xvec,yvec,zvec,fmat,iflag)
call interp%evaluate(x,y,z,f)
call interp%destroy()

For example, the low-level 3D interpolation `evaluate`

routine is:

pure subroutine interp_3d(me,x,y,z,fxyz)
implicit none
class(linear_interp_3d),intent(inout) :: me
real(wp),intent(in) :: x
real(wp),intent(in) :: y
real(wp),intent(in) :: z
real(wp),intent(out) :: fxyz !! Interpolated \( f(x,y,z) \)
integer,dimension(2) :: ix, iy, iz
real(wp) :: p1, p2, p3
real(wp) :: q1, q2, q3
integer :: mflag
real(wp) :: fx11, fx21, fx12, fx22, fxy1, fxy2
call dintrv(me%x,x,me%ilox,ix(1),ix(2),mflag)
call dintrv(me%y,y,me%iloy,iy(1),iy(2),mflag)
call dintrv(me%z,z,me%iloz,iz(1),iz(2),mflag)
q1 = (x-me%x(ix(1)))/(me%x(ix(2))-me%x(ix(1)))
q2 = (y-me%y(iy(1)))/(me%y(iy(2))-me%y(iy(1)))
q3 = (z-me%z(iz(1)))/(me%z(iz(2))-me%z(iz(1)))
p1 = one-q1
p2 = one-q2
p3 = one-q3
fx11 = p1*me%f(ix(1),iy(1),iz(1)) + q1*me%f(ix(2),iy(1),iz(1))
fx21 = p1*me%f(ix(1),iy(2),iz(1)) + q1*me%f(ix(2),iy(2),iz(1))
fx12 = p1*me%f(ix(1),iy(1),iz(2)) + q1*me%f(ix(2),iy(1),iz(2))
fx22 = p1*me%f(ix(1),iy(2),iz(2)) + q1*me%f(ix(2),iy(2),iz(2))
fxy1 = p2*( fx11 ) + q2*( fx21 )
fxy2 = p2*( fx12 ) + q2*( fx22 )
fxyz = p3*( fxy1 ) + q3*( fxy2 )
end subroutine interp_3d

The finterp library is released under a permissive BSD-3 license. As far as I can tell, it is unique among publicly-available Fortran codes in providing linear interpolation for up to 6D data sets. There used to be a Fortran 77 library called REGRIDPACK for regridding 1D-4D data sets (it had the option to use linear or cubic interpolation independently for each dimension). Written by John C. Adams at the National Center for Atmospheric Research in 1999, it used to be hosted at the UCAR website, but the link is now dead. You can still find it on the internet though, as part of other projects (for example here, where it is being used via a Python wrapper). But the licensing is unclear to me.

The dearth of easy-to-find, easy-to-use, high-quality open source modern Fortran libraries is a big problem for the long-term future of the language. Fortran users don’t seem to be as interested in promoting their language as users of some of the newer programming languages are. There is a group of us working to change this, but we’ve got a long way to go. For example, Julia, a programming language only four years old, already has a ton of libraries, some of which are just wrappers to Fortran 77 code that no one’s ever bothered to modernize. They even have a yearly conference. An in depth article about this state of affairs is a post for another day.

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