From 8344431ea06fa9612694d4dcaef00e6e93f8be77 Mon Sep 17 00:00:00 2001
From: Gavin Ridley <gavin.keith.ridley@gmail.com>
Date: Mon, 24 Apr 2023 13:19:29 -0400
Subject: [PATCH] fix my incorrect MPI code

---
 src/message_passing.cpp | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/src/message_passing.cpp b/src/message_passing.cpp
index 1630e52c0..374c1aa72 100644
--- a/src/message_passing.cpp
+++ b/src/message_passing.cpp
@@ -20,21 +20,19 @@ extern "C" bool openmc_master()
 vector<int64_t> calculate_parallel_index_vector(int64_t size)
 {
   vector<int64_t> result;
-  result.reserve(n_procs + 1);
+  result.resize(n_procs + 1);
+  result[0] = 0;
 
 #ifdef OPENMC_MPI
-  result.resize(n_procs);
-  result[0] = 0;
-  vector<int64_t> bank_size(n_procs + 1);
 
   // Populate the result with cumulative sum of the number of
   // surface source banks per process
-  MPI_Scan(&size, bank_size.data(), 1, MPI_INT64_T, MPI_SUM, intracomm);
-  MPI_Allgather(bank_size.data() + 1, 1, MPI_INT64_T, result.data(), 1,
-    MPI_INT64_T, intracomm);
+  int64_t scan_total;
+  MPI_Scan(&size, &scan_total, 1, MPI_INT64_T, MPI_SUM, intracomm);
+  MPI_Allgather(
+    &scan_total, 1, MPI_INT64_T, result.data() + 1, 1, MPI_INT64_T, intracomm);
 #else
-  result.push_back(0);
-  result.push_back(size);
+  result[1] = size;
 #endif
 
   return result;