fix comments in fft unpacking and shifting routines

src_analysis/StructFact.cpp

@@ -523,6 +523,20 @@ void StructFact::ComputeFFT(const MultiFab& variables,
             amrex::ParallelFor(bx,
             [=] AMREX_GPU_DEVICE (int i, int j, int k) noexcept
             {
+                /*
+                  Unpacking rules:
+
+                  For domains from (0,0,0) to (Nx-1,Ny-1,Nz-1)
+
+                  For any cells with i index > Nx/2, these values are complex conjugates of the corresponding
+                  entry where (Nx-i,Ny-j,Nz-k) UNLESS that index is zero, in which case you use 0.
+
+                  e.g. for an 8^3 domain, any cell with i index
+
+                  Cell (6,2,3) is complex conjugate of (2,6,5)
+
+                  Cell (4,1,0) is complex conjugate of (4,7,0)  (note that the FFT is computed for 0 <= i <= Nx/2)
+                */
                 if (i <= bx.length(0)/2) {
                     // copy value
                     realpart(i,j,k) = spectral(i,j,k).real();
@@ -733,18 +747,13 @@ void StructFact::ShiftFFT(MultiFab& dft_out, const Geometry& geom, const int& ze
   /*
     Shifting rules:
 
-    For domains from (0,0,0) to (Nx-1,Ny-1,Nz-1)
-
-    For any cells with i index >= Nx/2, these values are complex conjugates of the corresponding
-    entry where (Nx-i,Ny-j,Nz-k) UNLESS that index is zero, in which case you use 0.
+    For each direction d, shift the data in the +d direction by n_cell[d]/2 and then modulo by n_cell[d].
 
-    e.g. for an 8^3 domain, any cell with i index 
+    e.g. for an 8^3 domain
 
-    Cell (6,2,3) is complex conjugate of (2,6,5)
+    Cell (7,2,3) is shifted to ( (7+4)%8, (2+4)%8, (3+4)%8 ) =  (3,6,7)
 
-    Cell (4,1,0) is complex conjugate of (4,7,0)  (note that the FFT is computed for 0 <= i <= Nx/2)
   */
-
   BoxArray ba_onegrid;
   {
       Box domain = geom.Domain();