From 852f92780c9c82dae7f5fbcfff91165510719f90 Mon Sep 17 00:00:00 2001
From: Satoshi Misumi <18492524+lzpel@users.noreply.github.com>
Date: Wed, 22 Jul 2026 00:45:41 +0900
Subject: [PATCH] Fix M_PI being unavailable where _USE_MATH_DEFINES comes too
 late (#4023)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>
---
 examples/custom_source/source_ring.cpp              |  1 +
 .../parameterized_source_ring.cpp                   |  1 +
 src/external/quartic_solver.cpp                     |  2 +-
 src/mesh.cpp                                        | 13 ++++++-------
 src/plot.cpp                                        |  3 +--
 5 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/examples/custom_source/source_ring.cpp b/examples/custom_source/source_ring.cpp
index 5ab531a4a..bce6c27eb 100644
--- a/examples/custom_source/source_ring.cpp
+++ b/examples/custom_source/source_ring.cpp
@@ -1,3 +1,4 @@
+#define _USE_MATH_DEFINES
 #include <cmath>  // for M_PI
 #include <memory> // for unique_ptr
 
diff --git a/examples/parameterized_custom_source/parameterized_source_ring.cpp b/examples/parameterized_custom_source/parameterized_source_ring.cpp
index 9756e1b97..3aabf3581 100644
--- a/examples/parameterized_custom_source/parameterized_source_ring.cpp
+++ b/examples/parameterized_custom_source/parameterized_source_ring.cpp
@@ -1,3 +1,4 @@
+#define _USE_MATH_DEFINES
 #include <cmath>
 #include <memory>
 #include <unordered_map>
diff --git a/src/external/quartic_solver.cpp b/src/external/quartic_solver.cpp
index 915020ffa..71bdb25c3 100644
--- a/src/external/quartic_solver.cpp
+++ b/src/external/quartic_solver.cpp
@@ -1,5 +1,5 @@
-#include <algorithm>
 #define _USE_MATH_DEFINES // to make M_PI declared in Intel and MSVC compilers
+#include <algorithm>
 #include <cmath>
 #include <complex>
 #include <cstdlib>
diff --git a/src/mesh.cpp b/src/mesh.cpp
index 89d1ff24b..b05560d2a 100644
--- a/src/mesh.cpp
+++ b/src/mesh.cpp
@@ -1,11 +1,10 @@
 #include "openmc/mesh.h"
 #include <algorithm> // for copy, equal, min, min_element
 #include <cassert>
-#include <cstdint>        // for uint64_t
-#include <cstring>        // for memcpy
-#define _USE_MATH_DEFINES // to make M_PI declared in Intel and MSVC compilers
-#include <cmath>          // for ceil
-#include <cstddef>        // for size_t
+#include <cmath>   // for ceil
+#include <cstddef> // for size_t
+#include <cstdint> // for uint64_t
+#include <cstring> // for memcpy
 #include <limits>
 #include <numeric> // for accumulate
 #include <string>
@@ -1829,7 +1828,7 @@ StructuredMesh::MeshIndex CylindricalMesh::get_indices(
   } else {
     mapped_r[1] = std::atan2(r.y, r.x);
     if (mapped_r[1] < 0)
-      mapped_r[1] += 2 * M_PI;
+      mapped_r[1] += 2 * PI;
   }
 
   MeshIndex idx = StructuredMesh::get_indices(mapped_r, in_mesh);
@@ -2123,7 +2122,7 @@ StructuredMesh::MeshIndex SphericalMesh::get_indices(
     mapped_r[1] = std::acos(r.z / mapped_r.x);
     mapped_r[2] = std::atan2(r.y, r.x);
     if (mapped_r[2] < 0)
-      mapped_r[2] += 2 * M_PI;
+      mapped_r[2] += 2 * PI;
   }
 
   MeshIndex idx = StructuredMesh::get_indices(mapped_r, in_mesh);
diff --git a/src/plot.cpp b/src/plot.cpp
index 96df1c5be..5c4eb9460 100644
--- a/src/plot.cpp
+++ b/src/plot.cpp
@@ -1,7 +1,6 @@
 #include "openmc/plot.h"
 
 #include <algorithm>
-#define _USE_MATH_DEFINES // to make M_PI declared in Intel and MSVC compilers
 #include <cmath>
 #include <cstdio>
 #include <fstream>
@@ -1328,7 +1327,7 @@ std::pair<Position, Direction> RayTracePlot::get_pixel_ray(
   int horiz, int vert) const
 {
   // Compute field of view in radians
-  constexpr double DEGREE_TO_RADIAN = M_PI / 180.0;
+  constexpr double DEGREE_TO_RADIAN = PI / 180.0;
   double horiz_fov_radians = horizontal_field_of_view_ * DEGREE_TO_RADIAN;
   double p0 = static_cast<double>(pixels()[0]);
   double p1 = static_cast<double>(pixels()[1]);