FillPatcher time interpolation divides by `m_dt_crse` without zero guard

[asalmgren]

Location

• Source/BoundaryConditions/ERF_FillPatcher.H:130-131
• Source/BoundaryConditions/ERF_FillPatcher.cpp:226,243

Problem

RegisterCoarseData() allows crse_time[1] == crse_time[0] and sets:

m_dt_crse = crse_time[1] - crse_time[0];

Fill() then computes:

fac_new = (time - m_crse_times[0]) / m_dt_crse;

No guard exists for m_dt_crse == 0.

Why this is a bug

This can produce NaN/Inf interpolation factors and corrupt boundary-filled data when coarse snapshots share the same
timestamp.

Suggested patch

Handle zero-interval coarse data explicitly.

--- a/Source/BoundaryConditions/ERF_FillPatcher.H
+++ b/Source/BoundaryConditions/ERF_FillPatcher.H
@@
-    amrex::Real fac_new = (time - m_crse_times[0]) / m_dt_crse;
-    amrex::Real fac_old = 1.0 - fac_new;
+    amrex::Real fac_new = 0.0;
+    amrex::Real fac_old = 1.0;
+    if (amrex::Math::abs(m_dt_crse) > eps) {
+        fac_new = (time - m_crse_times[0]) / m_dt_crse;
+        fac_old = 1.0 - fac_new;
+    }

Optionally add an assert in RegisterCoarseData() if equal coarse times are unsupported.