Implement the NackModule as part of the new jitter buffer.

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, \
+               "ModNum type must be of unsigned integer.")
+
+namespace webrtc {
+
+template <size_t M> inline size_t Add(size_t a , size_t b) {
+  size_t add = b % M;
+  if (a + add < a)
+    return M - a + add;
+  return (a + add) % M;
+}
+
+template <size_t M> inline size_t Subtract(size_t a, size_t b) {
+  size_t sub = b % M;
+  if (a < sub)
+    return M - (sub - a);
+  return a - sub;
+}
+
+template <typename T> inline T PositiveDiff(T a, T b) {
+  MOD_OPS_ASSERT_TYPE_IS_UNSIGNED(T);
+  return a <= b ? b - a : std::numeric_limits<T>::max() - (a - b);
+}
+
+template <typename T> inline T NegativeDiff(T a, T b) {
+  MOD_OPS_ASSERT_TYPE_IS_UNSIGNED(T);
+  return a < b ? std::numeric_limits<T>::max() - (b - a) : a - b;
+}
+
+template <typename T> inline bool AheadOrAt(T a, T b) {
+  MOD_OPS_ASSERT_TYPE_IS_UNSIGNED(T);
+  return PositiveDiff(b, a) < std::numeric_limits<T>::max() / 2;
+}
+
+template <typename T> inline bool AheadOf(T a, T b) {
+  MOD_OPS_ASSERT_TYPE_IS_UNSIGNED(T);
+  return a != b && AheadOrAt(a, b);
+}
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_BASE_MOD_OPS_H_