Implemented a new sequence number unwrapper in sequence_number_util.h.

webrtc/rtc_base/mod_ops.h

Index: webrtc/rtc_base/mod_ops.h
diff --git a/webrtc/rtc_base/mod_ops.h b/webrtc/rtc_base/mod_ops.h
index 211f2e3553aa0e3672c872a53c6c04170e49cea4..5c988f9331eae506b371167bc65bd3074075606b 100644
--- a/webrtc/rtc_base/mod_ops.h
+++ b/webrtc/rtc_base/mod_ops.h
@@ -60,7 +60,7 @@ inline unsigned long Subtract(unsigned long a, unsigned long b) {  // NOLINT
 // -->----->                              |----->---
 //
 template <typename T, T M>
-inline T ForwardDiff(T a, T b) {
+inline typename std::enable_if<(M > 0), T>::type ForwardDiff(T a, T b) {
   static_assert(std::is_unsigned<T>::value,
                 "Type must be an unsigned integer.");
   RTC_DCHECK_LT(a, M);
@@ -68,13 +68,18 @@ inline T ForwardDiff(T a, T b) {
   return a <= b ? b - a : M - (a - b);
 }
 
-template <typename T>
-inline T ForwardDiff(T a, T b) {
+template <typename T, T M>
+inline typename std::enable_if<(M == 0), T>::type ForwardDiff(T a, T b) {
   static_assert(std::is_unsigned<T>::value,
                 "Type must be an unsigned integer.");
   return b - a;
 }
 
+template <typename T, typename M = void>
+inline T ForwardDiff(T a, T b) {
+  return ForwardDiff<T, 0>(a, b);
+}
+
 // Calculates the reverse difference between two wrapping numbers.
 //
 // Example:
@@ -98,7 +103,7 @@ inline T ForwardDiff(T a, T b) {
 // ---<-----|                             |<-----<--
 //
 template <typename T, T M>
-inline T ReverseDiff(T a, T b) {
+inline typename std::enable_if<(M > 0), T>::type ReverseDiff(T a, T b) {
   static_assert(std::is_unsigned<T>::value,
                 "Type must be an unsigned integer.");
   RTC_DCHECK_LT(a, M);
@@ -106,30 +111,28 @@ inline T ReverseDiff(T a, T b) {
   return b <= a ? a - b : M - (b - a);
 }
 
-template <typename T>
-inline T ReverseDiff(T a, T b) {
+template <typename T, T M>
+inline typename std::enable_if<(M == 0), T>::type ReverseDiff(T a, T b) {
   static_assert(std::is_unsigned<T>::value,
                 "Type must be an unsigned integer.");
   return a - b;
 }
 
+template <typename T, typename M = void>
+inline T ReverseDiff(T a, T b) {
+  return ReverseDiff<T, 0>(a, b);
+}
+
 // Calculates the minimum distance between to wrapping numbers.
 //
 // The minimum distance is defined as min(ForwardDiff(a, b), ReverseDiff(a, b))
-template <typename T, T M>
+template <typename T, T M = 0>
 inline T MinDiff(T a, T b) {
   static_assert(std::is_unsigned<T>::value,
                 "Type must be an unsigned integer.");
   return std::min(ForwardDiff<T, M>(a, b), ReverseDiff<T, M>(a, b));
 }
 
-template <typename T>
-inline T MinDiff(T a, T b) {
-  static_assert(std::is_unsigned<T>::value,
-                "Type must be an unsigned integer.");
-  return std::min(ForwardDiff(a, b), ReverseDiff(a, b));
-}
-
 }  // namespace webrtc
 
 #endif  // WEBRTC_RTC_BASE_MOD_OPS_H_