From 4ef3eaa0246a3b3eaf0a9253cfc48793eed5cdc9 Mon Sep 17 00:00:00 2001
From: Hans Johnson <hans-johnson@uiowa.edu>
Date: Sun, 22 Dec 2024 16:50:20 -0600
Subject: [PATCH] STYLE: Prefer compile-time constexpr

The values of m_Factor* are known at compile-
time, so compute them only at compile-time.
---
 .../itkWindowedSincInterpolateImageFunction.h | 16 ++++++-------
 ...tkWindowedSincInterpolateImageFunction.hxx | 23 -------------------
 2 files changed, 8 insertions(+), 31 deletions(-)

diff --git a/Modules/Core/ImageFunction/include/itkWindowedSincInterpolateImageFunction.h b/Modules/Core/ImageFunction/include/itkWindowedSincInterpolateImageFunction.h
index b0a2be4e4d2..09ac2e1b344 100644
--- a/Modules/Core/ImageFunction/include/itkWindowedSincInterpolateImageFunction.h
+++ b/Modules/Core/ImageFunction/include/itkWindowedSincInterpolateImageFunction.h
@@ -46,7 +46,7 @@ class ITK_TEMPLATE_EXPORT CosineWindowFunction
 
 private:
   /** Equal to \f$ \frac{\pi}{2 m} \f$ */
-  static const double m_Factor;
+  static constexpr double m_Factor = itk::Math::pi / (2 * VRadius);
 };
 
 /**
@@ -68,7 +68,7 @@ class ITK_TEMPLATE_EXPORT HammingWindowFunction
 
 private:
   /** Equal to \f$ \frac{\pi}{m} \f$ */
-  static const double m_Factor;
+  static constexpr double m_Factor = itk::Math::pi / VRadius;
 };
 
 /**
@@ -90,7 +90,7 @@ class ITK_TEMPLATE_EXPORT WelchWindowFunction
 
 private:
   /** Equal to \f$ \frac{1}{m^2} \f$ */
-  static const double m_Factor;
+  static constexpr double m_Factor = 1.0 / (VRadius * VRadius);
 };
 
 /**
@@ -112,14 +112,14 @@ class ITK_TEMPLATE_EXPORT LanczosWindowFunction
     if (A == 0.0)
     {
       return static_cast<TOutput>(1.0);
-    }
+    } // namespace Function
     const double z = m_Factor * A;
     return static_cast<TOutput>(std::sin(z) / z);
-  }
+  } // namespace itk
 
 private:
   /** Equal to \f$ \frac{\pi}{m} \f$ */
-  static const double m_Factor;
+  static constexpr double m_Factor = itk::Math::pi / VRadius;
 };
 
 /**
@@ -141,10 +141,10 @@ class ITK_TEMPLATE_EXPORT BlackmanWindowFunction
 
 private:
   /** Equal to \f$ \frac{\pi}{m} \f$ */
-  static const double m_Factor1;
+  static constexpr double m_Factor1 = itk::Math::pi / VRadius;
 
   /** Equal to \f$ \frac{2 \pi}{m} \f$  */
-  static const double m_Factor2;
+  static constexpr double m_Factor2 = 2.0 * itk::Math::pi / VRadius;
 };
 } // namespace Function
 
diff --git a/Modules/Core/ImageFunction/include/itkWindowedSincInterpolateImageFunction.hxx b/Modules/Core/ImageFunction/include/itkWindowedSincInterpolateImageFunction.hxx
index fbf5460a377..b2e49d40eeb 100644
--- a/Modules/Core/ImageFunction/include/itkWindowedSincInterpolateImageFunction.hxx
+++ b/Modules/Core/ImageFunction/include/itkWindowedSincInterpolateImageFunction.hxx
@@ -23,29 +23,6 @@
 
 namespace itk
 {
-
-// Constant definitions for functions
-namespace Function
-{
-template <unsigned int VRadius, typename TInput, typename TOutput>
-const double CosineWindowFunction<VRadius, TInput, TOutput>::m_Factor = itk::Math::pi / (2 * VRadius);
-
-template <unsigned int VRadius, typename TInput, typename TOutput>
-const double HammingWindowFunction<VRadius, TInput, TOutput>::m_Factor = itk::Math::pi / VRadius;
-
-template <unsigned int VRadius, typename TInput, typename TOutput>
-const double WelchWindowFunction<VRadius, TInput, TOutput>::m_Factor = 1.0 / (VRadius * VRadius);
-
-template <unsigned int VRadius, typename TInput, typename TOutput>
-const double LanczosWindowFunction<VRadius, TInput, TOutput>::m_Factor = itk::Math::pi / VRadius;
-
-template <unsigned int VRadius, typename TInput, typename TOutput>
-const double BlackmanWindowFunction<VRadius, TInput, TOutput>::m_Factor1 = itk::Math::pi / VRadius;
-
-template <unsigned int VRadius, typename TInput, typename TOutput>
-const double BlackmanWindowFunction<VRadius, TInput, TOutput>::m_Factor2 = 2.0 * itk::Math::pi / VRadius;
-} // end namespace Function
-
 template <typename TInputImage,
           unsigned int VRadius,
           typename TWindowFunction,