bitwise: add word halving query

bitwise.hpp

@@ -149,6 +149,29 @@ namespace cruft {
     }
 
 
+    ///////////////////////////////////////////////////////////////////////////
+    namespace bits {
+        inline constexpr std::tuple<u32, u32> halve (u64 val)
+        {
+            return { val >> 32, val & 0xffffffff };
+        }
+
+
+        //-------------------------------------------------------------------------
+        inline constexpr std::tuple<u16, u16> halve (u32 val)
+        {
+            return { val >> 16, val & 0xffff };
+        }
+
+
+        //-------------------------------------------------------------------------
+        inline constexpr std::tuple<u08, u08> halve (u16 val)
+        {
+            return { val >> 8, val & 0xff };
+        }
+    }
+
+
     ///////////////////////////////////////////////////////////////////////////
     /// returns the integral value composed of the bits from `val' in the
     /// inclusive range [lo, hi].