Add stat:sum field to MCPL files for proper weight normalization (#3522)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

tests/cpp_unit_tests/test_mcpl_stat_sum.cpp

@@ -0,0 +1,108 @@
+#include <catch2/catch_test_macros.hpp>
+#include <cstdio>
+#include <string>
+#include <vector>
+
+#include "openmc/bank.h"
+#include "openmc/mcpl_interface.h"
+
+// Test the MCPL stat:sum functionality (issue #3514)
+TEST_CASE("MCPL stat:sum field")
+{
+  // Check if MCPL interface is available
+  if (!openmc::is_mcpl_interface_available()) {
+    SKIP("MCPL library not available");
+  }
+
+  SECTION("stat:sum field is written to MCPL files")
+  {
+    // Create a temporary filename
+    std::string filename = "test_stat_sum.mcpl";
+
+    // Create some test particles
+    std::vector<openmc::SourceSite> source_bank(100);
+    std::vector<int64_t> bank_index = {0, 100}; // 100 particles total
+
+    // Initialize test particles
+    for (int i = 0; i < 100; ++i) {
+      source_bank[i].particle = openmc::ParticleType::neutron;
+      source_bank[i].r = {i * 0.1, i * 0.2, i * 0.3};
+      source_bank[i].u = {0.0, 0.0, 1.0};
+      source_bank[i].E = 2.0e6; // 2 MeV
+      source_bank[i].time = 0.0;
+      source_bank[i].wgt = 1.0;
+    }
+
+    // Write the MCPL file
+    openmc::write_mcpl_source_point(filename.c_str(), source_bank, bank_index);
+
+    // Verify the file was created
+    FILE* f = std::fopen(filename.c_str(), "r");
+    REQUIRE(f != nullptr);
+    std::fclose(f);
+
+    // Read the file back to check stat:sum
+    // Note: This would require mcpl_open_file and checking the header
+    // Since we can't easily read MCPL headers in C++ without the full MCPL API,
+    // we rely on the Python test to verify the actual content
+
+    // Clean up
+    std::remove(filename.c_str());
+  }
+
+  SECTION("stat:sum uses correct particle count")
+  {
+    std::string filename = "test_count.mcpl";
+
+    // Test with different particle counts
+    std::vector<int> test_counts = {1, 10, 100, 1000};
+
+    for (int count : test_counts) {
+      std::vector<openmc::SourceSite> source_bank(count);
+      std::vector<int64_t> bank_index = {0, count};
+
+      // Initialize particles
+      for (int i = 0; i < count; ++i) {
+        source_bank[i].particle = openmc::ParticleType::neutron;
+        source_bank[i].r = {0.0, 0.0, 0.0};
+        source_bank[i].u = {0.0, 0.0, 1.0};
+        source_bank[i].E = 1.0e6;
+        source_bank[i].time = 0.0;
+        source_bank[i].wgt = 1.0;
+      }
+
+      // Write MCPL file
+      openmc::write_mcpl_source_point(
+        filename.c_str(), source_bank, bank_index);
+
+      // The stat:sum should equal count (verified by Python test)
+      // Here we just verify the file was created successfully
+      FILE* f = std::fopen(filename.c_str(), "r");
+      REQUIRE(f != nullptr);
+      std::fclose(f);
+
+      // Clean up
+      std::remove(filename.c_str());
+    }
+  }
+
+  SECTION("stat:sum handles empty particle bank")
+  {
+    std::string filename = "test_empty.mcpl";
+
+    // Create empty particle bank
+    std::vector<openmc::SourceSite> source_bank;
+    std::vector<int64_t> bank_index = {0};
+
+    // This should still create a valid MCPL file with stat:sum = 0
+    openmc::write_mcpl_source_point(filename.c_str(), source_bank, bank_index);
+
+    // Verify file was created
+    FILE* f = std::fopen(filename.c_str(), "r");
+    REQUIRE(f != nullptr);
+    std::fclose(f);
+
+    // Clean up
+    std::remove(filename.c_str());
+  }
+}