From 1e436577ea6d202e2124dee318f212fe043b62e9 Mon Sep 17 00:00:00 2001
From: Paul Romano <paul.k.romano@gmail.com>
Date: Fri, 7 Aug 2020 10:51:28 -0500
Subject: [PATCH] Use pair instead of struct as return type of find_temperature

---
 include/openmc/nuclide.h | 12 +++++-----
 src/nuclide.cpp          | 49 +++++++++++++++++-----------------------
 2 files changed, 27 insertions(+), 34 deletions(-)

diff --git a/include/openmc/nuclide.h b/include/openmc/nuclide.h
index 0155c318f4..69bb50cc8f 100644
--- a/include/openmc/nuclide.h
+++ b/include/openmc/nuclide.h
@@ -7,6 +7,7 @@
 #include <array>
 #include <memory> // for unique_ptr
 #include <unordered_map>
+#include <utility> // for pair
 #include <vector>
 
 #include <gsl/gsl>
@@ -35,11 +36,6 @@ public:
     std::vector<double> energy;
   };
 
-  struct InterpResult {
-    gsl::index i;  //!< Index in tabulated data
-    double f;      //!< Interpolation factor between i and i+1
-  };
-
   // Constructors/destructors
   Nuclide(hid_t group, const std::vector<double>& temperature);
   ~Nuclide();
@@ -120,7 +116,11 @@ public:
 private:
   void create_derived(const Function1D* prompt_photons, const Function1D* delayed_photons);
 
-  InterpResult find_temperature(double T) const;
+  //! Determine temperature index and interpolation factor
+  //
+  //! \param[in] T Temperature in [K]
+  //! \return Temperature index and interpolation factor
+  std::pair<gsl::index, double> find_temperature(double T) const;
 
   static int XS_TOTAL;
   static int XS_ABSORPTION;
diff --git a/src/nuclide.cpp b/src/nuclide.cpp
index 0d69adb8ec..02b0ed3653 100644
--- a/src/nuclide.cpp
+++ b/src/nuclide.cpp
@@ -910,49 +910,40 @@ void Nuclide::calculate_urr_xs(int i_temp, Particle& p) const
 
 }
 
-Nuclide::InterpResult Nuclide::find_temperature(double T) const
+std::pair<gsl::index, double> Nuclide::find_temperature(double T) const
 {
   Expects(T >= 0.0);
 
-  // Make sure value is within bounds
-  double T_min = kTs_.front() / K_BOLTZMANN;
-  double T_max = kTs_.back() / K_BOLTZMANN;
-  if (T < T_min || T > T_max) {
-    throw std::out_of_range{fmt::format(
-      "Temperature out of range (min={:.1f}, max={:.1f}).", T_min, T_max
-    )};
-  }
-
   // Determine temperature index
-  InterpResult res;
+  gsl::index i_temp = 0;
+  double f = 0.0;
   double kT = K_BOLTZMANN * T;
+  gsl::index n = kTs_.size();
   switch (settings::temperature_method) {
   case TemperatureMethod::NEAREST:
     {
       double max_diff = INFTY;
-      for (gsl::index t = 0; t < kTs_.size(); ++t) {
+      for (gsl::index t = 0; t < n; ++t) {
         double diff = std::abs(kTs_[t] - kT);
         if (diff < max_diff) {
-          res.i = t;
+          i_temp = t;
           max_diff = diff;
         }
       }
     }
-    res.f = 0.0;
     break;
 
   case TemperatureMethod::INTERPOLATION:
     // Find temperatures that bound the actual temperature
-    for (res.i = 0; res.i < kTs_.size() - 1; ++res.i) {
-      if (kTs_[res.i] <= kT && kT < kTs_[res.i + 1]) break;
-    }
-    res.f = (kT - kTs_[res.i]) / (kTs_[res.i + 1] - kTs_[res.i]);
+    while (kTs_[i_temp + 1] < kT && i_temp + 1 < n - 1) ++i_temp;
+
+    // Determine interpolation factor
+    f = (kT - kTs_[i_temp]) / (kTs_[i_temp + 1] - kTs_[i_temp]);
   }
 
-  Ensures(res.i >= 0 && res.i < kTs_.size());
-  Ensures(res.f >= 0.0 && res.f <= 1.0);
+  Ensures(i_temp >= 0 && i_temp < n);
 
-  return res;
+  return {i_temp, f};
 }
 
 double Nuclide::collapse_rate(int MT, double temperature, gsl::span<const double> energy,
@@ -967,17 +958,19 @@ double Nuclide::collapse_rate(int MT, double temperature, gsl::span<const double
   const auto& rx = reactions_[i_rx];
 
   // Determine temperature index
-  auto res = this->find_temperature(temperature);
+  gsl::index i_temp;
+  double f;
+  std::tie(i_temp, f) = this->find_temperature(temperature);
 
   // Get reaction rate at lower temperature
-  const auto& grid_low = grid_[res.i].energy;
-  double rr_low = rx->collapse_rate(res.i, energy, flux, grid_low);
+  const auto& grid_low = grid_[i_temp].energy;
+  double rr_low = rx->collapse_rate(i_temp, energy, flux, grid_low);
 
-  if (res.f > 0.0) {
+  if (f > 0.0) {
     // Interpolate between reaction rate at lower and higher temperature
-    const auto& grid_high = grid_[res.i + 1].energy;
-    double rr_high = rx->collapse_rate(res.i + 1, energy, flux, grid_high);
-    return rr_low + res.f*(rr_high - rr_low);
+    const auto& grid_high = grid_[i_temp + 1].energy;
+    double rr_high = rx->collapse_rate(i_temp + 1, energy, flux, grid_high);
+    return rr_low + f*(rr_high - rr_low);
   } else {
     // If interpolation factor is zero, return reaction rate at lower temperature
     return rr_low;