From 7c6aaf7204163464d13e14a4547b48269a70c3d1 Mon Sep 17 00:00:00 2001
From: John Tramm <john.tramm@gmail.com>
Date: Mon, 23 Jun 2025 17:20:36 -0500
Subject: [PATCH] Fix Weight Window Infinite Loop Bug (#3457)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>
---
 include/openmc/particle_data.h |  2 +-
 src/particle.cpp               | 14 ++++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/openmc/particle_data.h b/include/openmc/particle_data.h
index 82b101c227..be7fa74837 100644
--- a/include/openmc/particle_data.h
+++ b/include/openmc/particle_data.h
@@ -47,7 +47,7 @@ struct SourceSite {
   double time {0.0};
   double wgt {1.0};
   int delayed_group {0};
-  int surf_id {0};
+  int surf_id {SURFACE_NONE};
   ParticleType particle;
 
   // Extra attributes that don't show up in source written to file
diff --git a/src/particle.cpp b/src/particle.cpp
index 324276d15f..6cfdf2175a 100644
--- a/src/particle.cpp
+++ b/src/particle.cpp
@@ -104,6 +104,14 @@ void Particle::split(double wgt)
   bank.u = u();
   bank.E = settings::run_CE ? E() : g();
   bank.time = time();
+
+  // Convert signed index to a singed surface ID
+  if (surface() == SURFACE_NONE) {
+    bank.surf_id = SURFACE_NONE;
+  } else {
+    int surf_id = model::surfaces[surface_index()]->id_;
+    bank.surf_id = (surface() > 0) ? surf_id : -surf_id;
+  }
 }
 
 void Particle::from_source(const SourceSite* src)
@@ -140,6 +148,12 @@ void Particle::from_source(const SourceSite* src)
   time() = src->time;
   time_last() = src->time;
   parent_nuclide() = src->parent_nuclide;
+
+  // Convert signed surface ID to signed index
+  if (src->surf_id != SURFACE_NONE) {
+    int index_plus_one = model::surface_map[std::abs(src->surf_id)] + 1;
+    surface() = (src->surf_id > 0) ? index_plus_one : -index_plus_one;
+  }
 }
 
 void Particle::event_calculate_xs()