Common: Add MathUtil::SignExtend

src/common/math_util.h

@@ -5,6 +5,7 @@
 #pragma once
 
 #include <algorithm>
+#include <cstddef>
 #include <cstdlib>
 #include <type_traits>
 
@@ -16,14 +17,24 @@ inline bool IntervalsIntersect(unsigned start0, unsigned length0, unsigned start
 }
 
 template<typename T>
-inline T Clamp(const T val, const T& min, const T& max)
-{
+inline T Clamp(const T val, const T& min, const T& max) {
     return std::max(min, std::min(max, val));
 }
 
+/// Sign-extends a val of type T that has NBits bits.
+template<size_t NBits, typename T>
+inline T SignExtend(const T val) {
+    static_assert(std::numeric_limits<T>::is_signed, "T must be signed");
+    static_assert(NBits <= 8 * sizeof(T), "NBits larger that bitsize of T");
+
+    using unsignedT = typename std::make_unsigned<T>::type;
+
+    constexpr size_t shift = 8 * sizeof(T) - NBits;
+    return static_cast<T>(static_cast<unsignedT>(val) << shift) >> shift;
+}
+
 template<class T>
-struct Rectangle
-{
+struct Rectangle {
     T left;
     T top;
     T right;