Fix some signed overflow errors causing undefined behavior (in theory).

BUG=webrtc:5491

Review URL: https://codereview.webrtc.org/1744183002

Cr-Commit-Position: refs/heads/master@{#11832}

webrtc/base/mathutils.h

@@ -12,9 +12,28 @@
 #define WEBRTC_BASE_MATHUTILS_H_
 
 #include <math.h>
+#include <type_traits>
+
+#include "webrtc/base/checks.h"
 
 #ifndef M_PI
 #define M_PI 3.14159265359f
 #endif
 
+// Given two numbers |x| and |y| such that x >= y, computes the difference
+// x - y without causing undefined behavior due to signed overflow.
+template <typename T>
+typename std::make_unsigned<T>::type unsigned_difference(T x, T y) {
+  static_assert(
+      std::is_signed<T>::value,
+      "Function unsigned_difference is only meaningful for signed types.");
+  RTC_DCHECK_GE(x, y);
+  typedef typename std::make_unsigned<T>::type unsigned_type;
+  // int -> unsigned conversion repeatedly adds UINT_MAX + 1 until the number
+  // can be represented as an unsigned. Since we know that the actual
+  // difference x - y can be represented as an unsigned, it is sufficient to
+  // compute the difference modulo UINT_MAX + 1, i.e using unsigned arithmetic.
+  return static_cast<unsigned_type>(x) - static_cast<unsigned_type>(y);
+}
+
 #endif  // WEBRTC_BASE_MATHUTILS_H_