`ERFFillPatcher::BuildMask` misses periodic corner images when constructing the coarse/fine mask

[asalmgren]

Location

• Source/BoundaryConditions/ERF_FillPatcher.cpp:126-156
• Source/BoundaryConditions/ERF_FillPatcher.cpp:165-196

Problem

BuildMask augments the fine-grid box list with periodic images by shifting each box along one periodic dimension at a time:

if (m_fgeom.isPeriodic(dim)) {
    ...
    bb_lo.shift(dim,n);
    ...
    bb_hi.shift(dim,-n);
}

It never generates combined shifts such as (+Lx,+Ly) or (-Lx,+Ly).

Why this is a bug

In doubly or triply periodic cases, a fine patch touching a periodic corner should also mask out its diagonal periodic image. Without those combined corner
images, complementIn can classify periodic-corner cells as uncovered coarse/fine-boundary cells, and the patcher can incorrectly interpolate over data t
hat should be treated as fine-covered.

That is most likely to show up near periodic corners in relaxation/set regions.

Suggested patch

Generate periodic image boxes for all combinations of periodic shifts, not only single-axis shifts. One simple approach is to iterate over the 3^N shift
combinations (-1, 0, +1 in each periodic dimension), skip the all-zero shift, and add each shifted image that intersects the bounding box.

--- a/Source/BoundaryConditions/ERF_FillPatcher.cpp
+++ b/Source/BoundaryConditions/ERF_FillPatcher.cpp
@@
-        for (auto& b : bl_mf) {
-            for (int dim = 0; dim < AMREX_SPACEDIM; dim++) {
-                if (m_fgeom.isPeriodic(dim)) {
-                    ...
-                }
-            }
-        }
+        for (auto& b : bl_mf) {
+            for (all periodic shift combinations in {-1,0,+1}^N, excluding the zero shift) {
+                Box bb = b;
+                shift bb by +/- domain length in every selected periodic dimension;
+                bb.enclosedCells();
+                bb.setType(b.ixType());
+                bb &= fba_bnd;
+                if (bb.ok()) { bl_mf_new.push_back(bb); }
+            }
+        }