Implemented more general version of ForwardDiff/RevereseDiff.

webrtc/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.
  */
 
 #ifndef WEBRTC_BASE_MOD_OPS_H_
 #define WEBRTC_BASE_MOD_OPS_H_
 
 #include <limits>
 #include <type_traits>
 
 #include "webrtc/base/checks.h"
 
-#define MOD_OPS_ASSERT_TYPE_IS_UNSIGNED(T)              \
-  static_assert(std::numeric_limits<T>::is_integer &&   \
-                    !std::numeric_limits<T>::is_signed, \
-                "Type must be of unsigned integer.")
-
 namespace webrtc {
 
 template <unsigned long M>                                    // NOLINT
 inline unsigned long Add(unsigned long a, unsigned long b) {  // NOLINT
   RTC_DCHECK_LT(a, M);
   unsigned long t = M - b % M;  // NOLINT
   unsigned long res = a - t;    // NOLINT
   if (t > a)
     return res + M;
   return res;
 }
 
 template <unsigned long M>                                         // NOLINT
 inline unsigned long Subtract(unsigned long a, unsigned long b) {  // NOLINT
   RTC_DCHECK_LT(a, M);
   unsigned long sub = b % M;  // NOLINT
   if (a < sub)
     return M - (sub - a);
   return a - sub;
 }
 
-// Calculates the forward difference between two numbers.
+// Calculates the forward difference between two wrapping numbers.
 //
 // Example:
 // uint8_t x = 253;
 // uint8_t y = 2;
 //
 // ForwardDiff(x, y) == 4
 //
 //   252   253   254   255    0     1     2     3
 // #################################################
 // |     |  x  |     |     |     |     |  y  |     |
 // #################################################
 //          |----->----->----->----->----->
 //
 // ForwardDiff(y, x) == 251
 //
 //   252   253   254   255    0     1     2     3
 // #################################################
 // |     |  x  |     |     |     |     |  y  |     |
 // #################################################
 // -->----->                              |----->---
 //
+template <typename T, T M>
+inline T 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) {
-  MOD_OPS_ASSERT_TYPE_IS_UNSIGNED(T);
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
   return b - a;
 }
 
-// Calculates the reverse difference between two numbers.
+// 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  |     |
 // #################################################
 // ---<-----|                             |<-----<--
 //
+template <typename T, T M>
+inline T 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) {
-  MOD_OPS_ASSERT_TYPE_IS_UNSIGNED(T);
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
   return a - b;
 }
 
 // Test if the sequence number a is ahead or at sequence number b.
 // If the two sequence numbers are at max distance from each other
 // then the sequence number with highest value is considered to
 // be ahead.
 template <typename T>
 inline bool AheadOrAt(T a, T b) {
-  MOD_OPS_ASSERT_TYPE_IS_UNSIGNED(T);
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
   const T maxDist = std::numeric_limits<T>::max() / 2 + T(1);
   if (a - b == maxDist)
     return b < a;
   return ForwardDiff(b, a) < maxDist;
 }
 
 // Test if sequence number a is ahead of sequence number b.
 template <typename T>
 inline bool AheadOf(T a, T b) {
-  MOD_OPS_ASSERT_TYPE_IS_UNSIGNED(T);
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
   return a != b && AheadOrAt(a, b);
 }
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_BASE_MOD_OPS_H_