add rfftfreq function

Author: cval26

src/fftpack.f90

@@ -7,7 +7,7 @@ module fftpack
     public :: zffti, zfftf, zfftb
     public :: fft, ifft
     public :: fftshift, ifftshift
-    public :: fftfreq
+    public :: fftfreq, rfftfreq
 
     public :: dffti, dfftf, dfftb
     public :: rfft, irfft
@@ -181,6 +181,11 @@ pure module function fftfreq(n) result(out)
             integer, dimension(n) :: out
         end function fftfreq
 
+        pure module function rfftfreq(n) result(out)
+            integer, intent(in) :: n
+            integer, dimension(n) :: out
+        end function rfftfreq
+
     end interface
 
     !> Version: experimental

src/fftpack_utils.f90

@@ -32,4 +32,29 @@ pure module function fftfreq(n) result(out)
         end if
     end function fftfreq
 
+    !> Returns an integer array with the frequency values involved in the
+    !> performed real FFT, ordered in the standard way (zero first, then
+    !> positive frequencies, then, if applicable, the negative one).
+    pure module function rfftfreq(n) result(out)
+        integer, intent(in) :: n
+        integer, dimension(n) :: out
+        integer :: i
+
+        out(1) = 0
+        if (n == 1) return
+
+        if (mod(n,2) == 0) then  !> n even, smallest n = 2
+            do i = 2, n-2, 2
+                out(i) = out(i-1) + 1
+                out(i+1) = out(i)
+            end do
+            out(n) = -n/2
+        else  !> n odd, smallest n = 3
+            do i = 2, n-1, 2
+                out(i) = out(i-1) + 1
+                out(i+1) = out(i)
+            end do
+        end if
+    end function rfftfreq
+
 end submodule fftpack_utils