Implemented a new sequence number unwrapper in sequence_number_util.h.

webrtc/rtc_base/mod_ops.h

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 41 matching lines @@
 //          |----->----->----->----->----->
 //
 // ForwardDiff(y, x) == 251
 //
 //   252   253   254   255    0     1     2     3
 // #################################################
 // |     |  x  |     |     |     |     |  y  |     |
 // #################################################
 // -->----->                              |----->---
 //
+// If M > 0 then wrapping occurs at M, if M == 0 then wrapping occurs at the
+// largest value representable by T.
 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);
   RTC_DCHECK_LT(b, M);
   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>
+inline T ForwardDiff(T a, T b) {
+  return ForwardDiff<T, 0>(a, b);
+}
+
 // Calculates the reverse difference between two wrapping numbers.
 //
 // Example:
 // uint8_t x = 253;
 // uint8_t y = 2;
 //
 // ReverseDiff(y, x) == 5
 //
 //   252   253   254   255    0     1     2     3
 // #################################################
 // |     |  x  |     |     |     |     |  y  |     |
 // #################################################
 //          <-----<-----<-----<-----<-----|
 //
 // ReverseDiff(x, y) == 251
 //
 //   252   253   254   255    0     1     2     3
 // #################################################
 // |     |  x  |     |     |     |     |  y  |     |
 // #################################################
 // ---<-----|                             |<-----<--
 //
+// If M > 0 then wrapping occurs at M, if M == 0 then wrapping occurs at the
+// largest value representable by T.
 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);
   RTC_DCHECK_LT(b, M);
   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>
+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_