From d024a3e00842a6535d8d0711a4131e4bee1ee841 Mon Sep 17 00:00:00 2001
From: Paul Romano <paul.k.romano@gmail.com>
Date: Wed, 13 Oct 2021 07:26:43 -0500
Subject: [PATCH] Implementation of C++ Sum1D function

---
 include/openmc/endf.h | 20 ++++++++++++++++++++
 src/endf.cpp          | 26 ++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/include/openmc/endf.h b/include/openmc/endf.h
index 7beb8e452..e580874b4 100644
--- a/include/openmc/endf.h
+++ b/include/openmc/endf.h
@@ -126,6 +126,26 @@ private:
     debye_waller_; //!< Debye-Waller integral divided by atomic mass in [eV^-1]
 };
 
+//==============================================================================
+//! Sum of multiple 1D functions
+//==============================================================================
+
+class Sum1D : public Function1D {
+public:
+  // Constructors
+  explicit Sum1D(hid_t group);
+
+  //! Evaluate each function and sum results
+  //! \param[in] x independent variable
+  //! \return Function evaluated at x
+  double operator()(double E) const override;
+
+  const unique_ptr<Function1D>& functions(int i) const { return functions_[i]; }
+
+private:
+  vector<unique_ptr<Function1D>> functions_; //!< individual functions
+};
+
 //! Read 1D function from HDF5 dataset
 //! \param[in] group HDF5 group containing dataset
 //! \param[in] name Name of dataset
diff --git a/src/endf.cpp b/src/endf.cpp
index b42c8641d..60db3efa4 100644
--- a/src/endf.cpp
+++ b/src/endf.cpp
@@ -271,4 +271,30 @@ double IncoherentElasticXS::operator()(double E) const
   return bound_xs_ / 2.0 * ((1 - std::exp(-4.0 * E * W)) / (2.0 * E * W));
 }
 
+//==============================================================================
+// Sum1D implementation
+//==============================================================================
+
+Sum1D::Sum1D(hid_t group)
+{
+  // Get number of functions
+  int n;
+  read_attribute(group, "n", n);
+
+  // Get each function
+  for (int i = 0; i < n; ++i) {
+    auto dset_name = fmt::format("func_{}", i + 1);
+    functions_.push_back(read_function(group, dset_name.c_str()));
+  }
+}
+
+double Sum1D::operator()(double x) const
+{
+  double result = 0.0;
+  for (auto& func : functions_) {
+    result += (*func)(x);
+  }
+  return result;
+}
+
 } // namespace openmc