Fix stat:sum to use total source particles, not file particle count

Per issue #3514 requirements, stat:sum should contain the original number of source particles in the simulation, not the number written to the file. Changed implementation to calculate: (n_batches - n_inactive) * n_particles instead of using bank_index.back() This ensures proper weight normalization when merging MCPL files.
paulromano · Aug 8, 2025 · Aug 6, 2025 · Aug 6, 2025 · Aug 6, 2025 · Aug 6, 2025
commit a255618c4b4387bc0714fe7a204169a0689f1062
diff --git a/src/mcpl_interface.cpp b/src/mcpl_interface.cpp
@@ -527,13 +527,17 @@ void write_mcpl_source_point(const char* filename, span<SourceSite> source_bank,
     if (file_id) {
       // Update stat:sum with actual particle count before closing (issue #3514)
       // This represents the original number of source particles in the
-      // simulation
+      // simulation (not the number of particles in the file)
       if (g_mcpl_api->hdr_add_stat_sum) {
-        // If bank_index is empty, statistics are unavailable, use -1
-        int64_t total_particles = bank_index.empty() ? -1 : bank_index.back();
+        // Calculate total source particles from active batches
+        // Per issue #3514: this should be the original number of source particles,
+        // not the number written to the file
+        int64_t total_source_particles = 
+          static_cast<int64_t>(settings::n_batches - settings::n_inactive) * 
+          settings::n_particles;
         // Update with actual count - this overwrites the initial -1 value
         g_mcpl_api->hdr_add_stat_sum(
-          file_id, "openmc_np1", static_cast<double>(total_particles));
+          file_id, "openmc_np1", static_cast<double>(total_source_particles));
       }
 
       g_mcpl_api->close_outfile(file_id);