endian: refactor endian conversion

endian.hpp

@@ -24,13 +24,6 @@
 #include <type_traits>
 
 namespace util {
-    //-------------------------------------------------------------------------
-    // Uses the TIFF header values. Just because. Don't rely on this.
-    enum class endian : uint16_t {
-        BIG    = 0x4D4D,
-        LITTLE = 0x4949,
-    };
-
     //-------------------------------------------------------------------------
     template <typename T>
     constexpr T
@@ -147,9 +140,52 @@ namespace util {
     template <typename T> constexpr T ltoh (T v) { return v; }
 #endif
 
+    template <typename T> T btol (T t) { return bswap (t); }
+    template <typename T> T ltob (T t) { return bswap (t); }
+
+    namespace endian {
+        //---------------------------------------------------------------------
+        // Uses the TIFF header values. Just because. Don't rely on this.
+        enum class scheme : uint16_t {
+            BIG    = 0x4D4D,
+            LITTLE = 0x4949,
+
+            big = BIG,
+            little = LITTLE,
+
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+            native = little,
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+            native = big
+#else
+#error Unhandled endianness
+#endif
+        };
+
+
+        template <scheme DstV, scheme SrcV>
+        struct converter;
+
+        template <> struct converter<scheme::little,scheme::big> {
+            template <typename T>
+            static T convert (T t) { return btol (t); }
+        };
+
+        template <> struct converter<scheme::big,scheme::little> {
+             template <typename T>
+             static T convert (T t) { return ltob (t); }
+        };
+
+        template <scheme DstV, scheme SrcV, typename T>
+        T convert (T t)
+        {
+            return converter<DstV,SrcV>::convert (t);
+        }
+    }
+
     //-------------------------------------------------------------------------
     struct from_endian {
-        explicit from_endian (endian _endian):
+        explicit from_endian (endian::scheme _endian):
             src (_endian)
         { ; }
 
@@ -168,7 +204,7 @@ namespace util {
             else
                 uint = v;
 
-            uint = (src == endian::LITTLE) ? ltoh (uint) : btoh (uint);
+            uint = (src == endian::scheme::LITTLE) ? ltoh (uint) : btoh (uint);
 
             if (std::is_signed<T>::value)
                 return T (sint);
@@ -176,7 +212,7 @@ namespace util {
                 return T (uint);
         }
 
-        endian src;
+        endian::scheme src;
     };
 }
 #endif