Moved sequence number specific operations from mod_ops.h

to sequence_number_util.h

Also in this CL:
  - Implemented a MinDiff function which finds the smallest diff of two
    wrapping numbers.
  - Implemented comparators for sequence numbers.

BUG=
R=mflodman@webrtc.org, tommi@webrtc.org, torbjorng@webrtc.org

Review URL: https://codereview.webrtc.org/1814753002 .

Cr-Commit-Position: refs/heads/master@{#12083}

webrtc/modules/video_coding/sequence_number_util.h

@@ -0,0 +1,110 @@
+/*
+ *  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_MODULES_VIDEO_CODING_SEQUENCE_NUMBER_UTIL_H_
+#define WEBRTC_MODULES_VIDEO_CODING_SEQUENCE_NUMBER_UTIL_H_
+
+#include <limits>
+#include <type_traits>
+
+#include "webrtc/base/mod_ops.h"
+
+namespace webrtc {
+
+// Test if the sequence number |a| is ahead or at sequence number |b|.
+//
+// If |M| is an even number and the two sequence numbers are at max distance
+// from each other, then the sequence number with the highest value is
+// considered to be ahead.
+template <typename T, T M>
+inline bool AheadOrAt(T a, T b) {
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
+  const T maxDist = M / 2;
+  if (!(M & 1) && MinDiff<T, M>(a, b) == maxDist)
+    return b < a;
+  return ForwardDiff<T, M>(b, a) <= maxDist;
+}
+
+template <typename T>
+inline bool AheadOrAt(T a, T b) {
+  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 the sequence number |a| is ahead of sequence number |b|.
+//
+// If |M| is an even number and the two sequence numbers are at max distance
+// from each other, then the sequence number with the highest value is
+// considered to be ahead.
+template <typename T, T M>
+inline bool AheadOf(T a, T b) {
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
+  return a != b && AheadOrAt<T, M>(a, b);
+}
+
+template <typename T>
+inline bool AheadOf(T a, T b) {
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
+  return a != b && AheadOrAt(a, b);
+}
+
+namespace internal {
+
+template <typename T, typename M>
+struct SeqNumComp;
+
+template <typename T, T M>
+struct SeqNumComp<T, std::integral_constant<T, M>> {
+  bool operator()(T a, T b) const { return AheadOf<T, M>(a, b); }
+};
+
+template <typename T>
+struct SeqNumComp<T, std::integral_constant<T, T(0)>> {
+  bool operator()(T a, T b) const { return AheadOf<T>(a, b); }
+};
+
+}  // namespace internal
+
+// Comparator used to compare sequence numbers in a continuous fashion.
+//
+// WARNING! If used to sort sequence numbers of length M then the interval
+//          covered by the sequence numbers may not be larger than floor(M/2).
+template <typename T, T M = 0>
+struct AscendingSeqNumComp
+    : private internal::SeqNumComp<T, std::integral_constant<T, M>> {
+  bool operator()(T a, T b) const {
+    return internal::SeqNumComp<T, std::integral_constant<T, M>>::operator()(a,
+                                                                             b);
+  }
+};
+
+// Comparator used to compare sequence numbers in a continuous fashion.
+//
+// WARNING! If used to sort sequence numbers of length M then the interval
+//          covered by the sequence numbers may not be larger than floor(M/2).
+template <typename T, T M = 0>
+struct DescendingSeqNumComp
+    : private internal::SeqNumComp<T, std::integral_constant<T, M>> {
+  bool operator()(T a, T b) const {
+    return internal::SeqNumComp<T, std::integral_constant<T, M>>::operator()(b,
+                                                                             a);
+  }
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_VIDEO_CODING_SEQUENCE_NUMBER_UTIL_H_