rtc_base/numerics/sequence_number_util.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 RTC_BASE_NUMERICS_SEQUENCE_NUMBER_UTIL_H_
#define RTC_BASE_NUMERICS_SEQUENCE_NUMBER_UTIL_H_

#include <limits>
#include <type_traits>

#include "rtc_base/numerics/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 typename std::enable_if<(M > 0), bool>::type 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, T M>
inline typename std::enable_if<(M == 0), bool>::type 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;
}

template <typename T>
inline bool AheadOrAt(T a, T b) {
  return AheadOrAt<T, 0>(a, b);
}

// 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 = 0>
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);
}

// 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 {
  bool operator()(T a, T b) const { return AheadOf<T, M>(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 {
  bool operator()(T a, T b) const { return AheadOf<T, M>(b, a); }
};

}  // namespace webrtc

#endif  // RTC_BASE_NUMERICS_SEQUENCE_NUMBER_UTIL_H_