From f33ea966b21802e206bb46c04c7aa8e45943e01b Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Sat, 17 Sep 2022 11:10:27 -0500
Subject: [PATCH] Compute interpolation value in loop. Correction to
 from_tab1D.

---
 include/openmc/interpolate.h | 9 +++------
 openmc/filter.py             | 6 +++---
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/include/openmc/interpolate.h b/include/openmc/interpolate.h
index 33167fa2ab..05353f1306 100644
--- a/include/openmc/interpolate.h
+++ b/include/openmc/interpolate.h
@@ -36,9 +36,7 @@ inline double interpolate_log_log(
 inline double interpolate_lagrangian(gsl::span<const double> xs,
   gsl::span<const double> ys, int idx, double x, int order)
 {
-  Expects(order <= 3);
-  std::array<double, 4> coeffs;
-  coeffs.fill(0.0);
+  double output {0.0};
 
   for (int i = 0; i < order + 1; i++) {
     double numerator {1.0};
@@ -49,11 +47,10 @@ inline double interpolate_lagrangian(gsl::span<const double> xs,
       numerator *= (x - xs[idx + j]);
       denominator *= (xs[idx + i] - xs[idx + j]);
     }
-    coeffs[i] = numerator / denominator;
+    output += (numerator / denominator) * ys[i];
   }
 
-  return std::inner_product(
-    coeffs.begin(), coeffs.end(), ys.begin() + idx, 0.0);
+  return output;
 }
 
 double interpolate(gsl::span<const double> xs, gsl::span<const double> ys,
diff --git a/openmc/filter.py b/openmc/filter.py
index 326572c4a4..2af5fe5b79 100644
--- a/openmc/filter.py
+++ b/openmc/filter.py
@@ -1995,11 +1995,11 @@ class EnergyFunctionFilter(Filter):
         if tab1d.n_regions > 1:
             raise ValueError('Only Tabulated1Ds with a single interpolation '
                              'region are supported')
-        interpolation = tab1d.interpolation[0]
-        if interpolation not in cls.INTERPOLATION_SCHEMES.keys():
+        interpolation_val = tab1d.interpolation[0]
+        if interpolation_val not in cls.INTERPOLATION_SCHEMES.keys():
             raise ValueError('Only histogram, linear-linear, linear-log, log-linear, and '
                              'log-log Tabulated1Ds are supported')
-        return cls(tab1d.x, tab1d.y, interpolation)
+        return cls(tab1d.x, tab1d.y, cls.INTERPOLATION_SCHEMES[interpolation_val])
 
     @property
     def energy(self):