Add arithmetic assign operators for endian_wrapper

include/gp/math/boolean/bitops.hpp

@@ -39,13 +39,13 @@ struct endian_wrapper {
 	: value{0} {}
 
 	endian_wrapper(T v) noexcept
-	: value{mode == endian::native ? v : swap_endian(v)} {}
+	: value{adjust(v)} {}
 
 	endian_wrapper(const endian_wrapper& v) noexcept
 	: value{v.value} {}
 
-	endian_wrapper& operator=(T p) noexcept {
-		value = mode == endian::native ? p : swap_endian(p);
+	endian_wrapper& operator=(T v) noexcept {
+		value = adjust(v);
 		return *this;
 	}
 
@@ -55,7 +55,35 @@ struct endian_wrapper {
 	}
 
 	operator T() const noexcept {
-		return mode == endian::native ? value : swap_endian(value);
+		return adjust(value);
+	}
+
+	// arithmetic assignment operators
+
+	endian_wrapper& operator+=(T rhs) noexcept {
+		value = adjust(adjust(value) + rhs);
+		return *this;
+	}
+
+	endian_wrapper& operator+=(endian_wrapper rhs) noexcept {
+		value = adjust(adjust(value) + adjust(rhs.value));
+		return *this;
+	}
+
+	endian_wrapper& operator-=(T rhs) noexcept {
+		value = adjust(adjust(value) - rhs);
+		return *this;
+	}
+
+	endian_wrapper& operator-=(endian_wrapper rhs) noexcept {
+		value = adjust(adjust(value) - adjust(rhs.value));
+		return *this;
+	}
+
+private:
+
+	T adjust(T x) const noexcept {
+		return (mode == endian::native ? x : swap_endian(x));
 	}
 };