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 <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);  // NOLINT

[torbjorng (webrtc), 2016/03/02 16:20:50]

Nit: this NOLINT seems unneeded.

[philipel, 2016/03/03 12:21:36]

Done.

+  unsigned long sub = b % M;  // NOLINT
+  if (a < sub)
+    return M - (sub - a);
+  return a - sub;
+}
+
+// Calculates the forward difference between two numbers.
+//
+// Example:
+// uint8_t a = 253;
+// uint8_t b = 2;
+//
+// ForwardDiff(a, b) == 4
+//
+//   252   253   254   255    0     1     2     3
+// #################################################
+// |     |  a  |     |     |     |     |  b  |     |
+// #################################################
+//          |----->----->----->----->----->
+//
+// ForwardDiff(b, a) == 251
+//
+//   252   253   254   255    0     1     2     3
+// #################################################
+// |     |  a  |     |     |     |     |  b  |     |
+// #################################################
+// -->----->                              |----->---
+//
+template <typename T>
+inline T ForwardDiff(T a, T b) {

[torbjorng (webrtc), 2016/03/02 16:20:50]

See comment about ReverseDiff.

[philipel, 2016/03/03 12:21:36]

Done.

+  MOD_OPS_ASSERT_TYPE_IS_UNSIGNED(T);
+  return a <= b ? b - a : std::numeric_limits<T>::max() - (a - b);
+}
+
+// Calculates the reverse difference between two numbers.
+//
+// Example:
+// uint8_t a = 253;
+// uint8_t b = 2;
+//
+// ReverseDiff(a, b) == 251
+//
+//   252   253   254   255    0     1     2     3
+// #################################################
+// |     |  a  |     |     |     |     |  b  |     |
+// #################################################
+//          <-----<-----<-----<-----<-----|
+//
+// ReverseDiff(b, a) == 5
+//
+//   252   253   254   255    0     1     2     3
+// #################################################
+// |     |  a  |     |     |     |     |  b  |     |
+// #################################################
+// ---<-----|                             |<-----<--
+//
+template <typename T>

[torbjorng (webrtc), 2016/03/02 16:20:50]

What's the intended range here?  Presumably 0..255 for uint8_t.

Then, if a=255, b=0 we get a-b=255, sensibly. Now let e.g. increment both,
getting a=0 and b=1. Now we get 255-(1-0) = 254.

That cannot be right, stupid.

I suggest that you make the test code catch that error first, then fix the code.

[philipel, 2016/03/03 12:21:36]

I have now unstupidified the code and implemented unittests. :)

+inline T ReverseDiff(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>

[torbjorng (webrtc), 2016/03/02 16:20:50]

The criteria for "ahead of" is a bit arbitrary. Please document it.

[philipel, 2016/03/03 12:21:36]

Done.

+inline bool AheadOrAt(T a, T b) {
+  MOD_OPS_ASSERT_TYPE_IS_UNSIGNED(T);
+  return ForwardDiff(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_