STYLE: Clean-up ByteSwapper, using private SwapBytes helper function

Aims to reduce code duplication. Inspired by a suggestion from Bradley Lowekamp
at https://github.com/InsightSoftwareConsortium/ITK/pull/4841/files#r1751777344

.github/workflows/itk_dict.txt

@@ -277,6 +277,7 @@ btw
 buf
 bugfix
 bw
+byteswap
 byu
 bzip
 calc

Modules/Core/Common/include/itkByteSwapper.h

@@ -192,6 +192,11 @@ class ITK_TEMPLATE_EXPORT ByteSwapper : public Object
   SwapWrite8Range(const void * ptr, BufferSizeType num, OStreamType * fp);
 
 private:
+  /** Swaps the bytes of the specified argument in-place. Assumes that its number of bytes is either 2, 4, or 8.
+   * Otherwise, it throws an exception. */
+  static void
+  SwapBytes(T &);
+
   static constexpr bool m_SystemIsBigEndian{
 #ifdef CMAKE_WORDS_BIGENDIAN
     true

Modules/Core/Common/include/itkByteSwapper.hxx

@@ -28,7 +28,8 @@
 #ifndef itkByteSwapper_hxx
 #define itkByteSwapper_hxx
 #include "itkMakeUniqueForOverwrite.h"
-#include <algorithm> // For swap.
+#include <algorithm> // For for_each_n and swap.
+#include <cstddef>   // For byte.
 #include <memory>
 #include <cstring>
 
@@ -44,49 +45,19 @@ template <typename T>
 void
 ByteSwapper<T>::SwapFromSystemToBigEndian([[maybe_unused]] T * p)
 {
-  if constexpr (!m_SystemIsBigEndian)
+  if constexpr (!m_SystemIsBigEndian && sizeof(T) > 1)
   {
-    switch (sizeof(T))
-    {
-      case 1:
-        return;
-      case 2:
-        Self::Swap2(p);
-        return;
-      case 4:
-        Self::Swap4(p);
-        return;
-      case 8:
-        Self::Swap8(p);
-        return;
-      default:
-        itkGenericExceptionMacro("Cannot swap number of bytes requested");
-    }
+    SwapBytes(*p);
   }
 }
 
 template <typename T>
 void
 ByteSwapper<T>::SwapRangeFromSystemToBigEndian([[maybe_unused]] T * p, [[maybe_unused]] BufferSizeType num)
 {
-  if constexpr (!m_SystemIsBigEndian)
+  if constexpr (!m_SystemIsBigEndian && sizeof(T) > 1)
   {
-    switch (sizeof(T))
-    {
-      case 1:
-        return;
-      case 2:
-        Self::Swap2Range(p, num);
-        return;
-      case 4:
-        Self::Swap4Range(p, num);
-        return;
-      case 8:
-        Self::Swap8Range(p, num);
-        return;
-      default:
-        itkGenericExceptionMacro("Cannot swap number of bytes requested");
-    }
+    std::for_each_n(p, num, [](T & element) { SwapBytes(element); });
   }
 }
 
@@ -127,49 +98,19 @@ template <typename T>
 void
 ByteSwapper<T>::SwapFromSystemToLittleEndian([[maybe_unused]] T * p)
 {
-  if constexpr (m_SystemIsBigEndian)
+  if constexpr (m_SystemIsBigEndian && sizeof(T) > 1)
   {
-    switch (sizeof(T))
-    {
-      case 1:
-        return;
-      case 2:
-        Self::Swap2(p);
-        return;
-      case 4:
-        Self::Swap4(p);
-        return;
-      case 8:
-        Self::Swap8(p);
-        return;
-      default:
-        itkGenericExceptionMacro("Cannot swap number of bytes requested");
-    }
+    SwapBytes(*p);
   }
 }
 
 template <typename T>
 void
 ByteSwapper<T>::SwapRangeFromSystemToLittleEndian([[maybe_unused]] T * p, [[maybe_unused]] BufferSizeType num)
 {
-  if constexpr (m_SystemIsBigEndian)
+  if constexpr (m_SystemIsBigEndian && sizeof(T) > 1)
   {
-    switch (sizeof(T))
-    {
-      case 1:
-        return;
-      case 2:
-        Self::Swap2Range(p, num);
-        return;
-      case 4:
-        Self::Swap4Range(p, num);
-        return;
-      case 8:
-        Self::Swap8Range(p, num);
-        return;
-      default:
-        itkGenericExceptionMacro("Cannot swap number of bytes requested");
-    }
+    std::for_each_n(p, num, [](T & element) { SwapBytes(element); });
   }
 }
 
@@ -369,6 +310,33 @@ ByteSwapper<T>::SwapWrite8Range(const void * ptr, BufferSizeType num, OStreamTyp
     }
   }
 }
+
+
+// The following member function is private:
+
+template <typename T>
+void
+ByteSwapper<T>::SwapBytes(T & value)
+{
+  static constexpr size_t numberOfBytes = sizeof(T);
+
+  // Historically (from ITK v1.2.0, March 2003) the following number of bytes are supported:
+  if constexpr (numberOfBytes == 2 || numberOfBytes == 4 || numberOfBytes == 8)
+  {
+    // When the value is an integer, the following code is equivalent to `value = std::byteswap(value)`, in C++26:
+    auto * const bytes = reinterpret_cast<std::byte *>(&value);
+
+    for (size_t i{}; i < (numberOfBytes / 2); ++i)
+    {
+      std::swap(bytes[i], bytes[(numberOfBytes - 1) - i]);
+    }
+  }
+  else
+  {
+    itkGenericExceptionMacro("Cannot swap number of bytes requested");
+  }
+}
+
 } // end namespace itk
 
 #endif