CarpetX: only regrid those fine levels that already catch up

CarpetX/src/schedule.cxx

@@ -1612,6 +1612,44 @@ int Evolve(tFleshConfig *config) {
       for (const auto &leveldata : patchdata.leveldata)
         iteration = min(iteration, leveldata.iteration);
 
+    int min_active_level = 0;
+
+    if (use_subcycling_wip) {
+      // Loop over all levels, in batches that combine levels that don't
+      // subcycle. The level range is [min_level, max_level).
+      for (int min_level = 0, max_level = min_level + 1;
+           min_level < ghext->num_levels();
+           min_level = max_level, max_level = min_level + 1) {
+        // Find end of batch
+        while (max_level < ghext->num_levels()) {
+          bool level_is_subcycling_level = false;
+          for (const auto &patchdata : ghext->patchdata)
+            if (max_level < int(patchdata.leveldata.size()))
+              level_is_subcycling_level |=
+                  patchdata.leveldata.at(max_level).is_subcycling_level;
+          if (level_is_subcycling_level)
+            break;
+          ++max_level;
+        }
+        // Skip this batch of levels if it is not active at the current
+        // iteration
+        rat64 level_iteration = -1;
+        for (const auto &patchdata : ghext->patchdata) {
+          if (min_level < int(patchdata.leveldata.size())) {
+            level_iteration = patchdata.leveldata.at(min_level).iteration;
+            break;
+          }
+        }
+        assert(level_iteration != -1);
+        // Skip those evolved coarse levels when evolving fine levels to catch
+        // up
+        if (level_iteration == iteration) {
+          min_active_level = min_level;
+          break;
+        }
+      };
+    }
+
     // TODO: Move regridding into a function
     if (regrid_every > 0 && cctkGH->cctk_iteration % regrid_every == 0) {
 #pragma omp critical
@@ -1644,7 +1682,7 @@ int Evolve(tFleshConfig *config) {
         }
         patchdata.amrcore->SetMaxGridSize(max_grid_sizes_vec);
 
-        patchdata.amrcore->regrid(0, time);
+        patchdata.amrcore->regrid(min_active_level, time);
 
         const int new_numlevels = patchdata.amrcore->finestLevel() + 1;
         const int max_numlevels = patchdata.amrcore->maxLevel() + 1;