libcamera: base: utils: Add abs_diff() utility function

The abs_diff() function computes the absolute difference of two
elements. This may seem trivial at first, but can lead to unexpected
results when operating on unsigned operands. A common implementation
of the absolute difference of two unsigned int (used through the
libcamera code base) is

	std::abs(static_cast<int>(a - b))

but doesn't return the expected result when either a or b is larger than
UINT_MAX / 2 due to overflows. The abs_diff() function offers a safe
alternative.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>

include/libcamera/base/utils.h

@@ -346,6 +346,15 @@ public:
 	}
 };
 
+template<typename T>
+decltype(auto) abs_diff(const T &a, const T &b)
+{
+	if (a < b)
+		return b - a;
+	else
+		return a - b;
+}
+
 } /* namespace utils */
 
 #ifndef __DOXYGEN__