net/dcsctp/common/sequence_numbers.h

/*
 *  Copyright (c) 2021 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 NET_DCSCTP_COMMON_SEQUENCE_NUMBERS_H_
#define NET_DCSCTP_COMMON_SEQUENCE_NUMBERS_H_

#include <cstdint>
#include <limits>
#include <utility>

#include "net/dcsctp/common/internal_types.h"
#include "rtc_base/numerics/sequence_number_unwrapper.h"

namespace dcsctp {

// UnwrappedSequenceNumber handles wrapping sequence numbers and unwraps them to
// an int64_t value space, to allow wrapped sequence numbers to be easily
// compared for ordering.
//
// Sequence numbers are expected to be monotonically increasing, but they do not
// need to be unwrapped in order, as long as the difference to the previous one
// is not larger than half the range of the wrapped sequence number.
//
// The WrappedType must be a webrtc::StrongAlias type.
template <typename WrappedType>
class UnwrappedSequenceNumber {
 public:
  static_assert(
      !std::numeric_limits<typename WrappedType::UnderlyingType>::is_signed,
      "The wrapped type must be unsigned");
  static_assert(
      std::numeric_limits<typename WrappedType::UnderlyingType>::max() <
          std::numeric_limits<int64_t>::max(),
      "The wrapped type must be less than the int64_t value space");

  // The unwrapper is a sort of factory and converts wrapped sequence numbers to
  // unwrapped ones.
  class Unwrapper {
   public:
    Unwrapper() = default;
    Unwrapper(const Unwrapper&) = default;
    Unwrapper& operator=(const Unwrapper&) = default;

    // Given a wrapped `value`, and with knowledge of its current last seen
    // largest number, will return a value that can be compared using normal
    // operators, such as less-than, greater-than etc.
    //
    // This will also update the Unwrapper's state, to track the last seen
    // largest value.
    UnwrappedSequenceNumber<WrappedType> Unwrap(WrappedType value) {
      return UnwrappedSequenceNumber<WrappedType>(unwrapper_.Unwrap(*value));
    }

    // Similar to `Unwrap`, but will not update the Unwrappers's internal state.
    UnwrappedSequenceNumber<WrappedType> PeekUnwrap(WrappedType value) const {
      return UnwrappedSequenceNumber<WrappedType>(
          unwrapper_.PeekUnwrap(*value));
    }

    // Resets the Unwrapper to its pristine state. Used when a sequence number
    // is to be reset to zero.
    void Reset() { unwrapper_.Reset(); }

   private:
    webrtc::SeqNumUnwrapper<typename WrappedType::UnderlyingType> unwrapper_;
  };

  // Returns the wrapped value this type represents.
  WrappedType Wrap() const {
    return static_cast<WrappedType>(value_ % kValueLimit);
  }

  template <typename H>
  friend H AbslHashValue(H state,
                         const UnwrappedSequenceNumber<WrappedType>& hash) {
    return H::combine(std::move(state), hash.value_);
  }

  bool operator==(const UnwrappedSequenceNumber<WrappedType>& other) const {
    return value_ == other.value_;
  }
  bool operator!=(const UnwrappedSequenceNumber<WrappedType>& other) const {
    return value_ != other.value_;
  }
  bool operator<(const UnwrappedSequenceNumber<WrappedType>& other) const {
    return value_ < other.value_;
  }
  bool operator>(const UnwrappedSequenceNumber<WrappedType>& other) const {
    return value_ > other.value_;
  }
  bool operator>=(const UnwrappedSequenceNumber<WrappedType>& other) const {
    return value_ >= other.value_;
  }
  bool operator<=(const UnwrappedSequenceNumber<WrappedType>& other) const {
    return value_ <= other.value_;
  }

  // Const accessors for underlying value.
  constexpr const int64_t* operator->() const { return &value_; }
  constexpr const int64_t& operator*() const& { return value_; }
  constexpr const int64_t&& operator*() const&& { return std::move(value_); }
  constexpr const int64_t& value() const& { return value_; }
  constexpr const int64_t&& value() const&& { return std::move(value_); }
  constexpr explicit operator const int64_t&() const& { return value_; }

  // Increments the value.
  void Increment() { ++value_; }

  // Returns the next value relative to this sequence number.
  UnwrappedSequenceNumber<WrappedType> next_value() const {
    return UnwrappedSequenceNumber<WrappedType>(value_ + 1);
  }

  // Returns a new sequence number based on `value`, and adding `delta` (which
  // may be negative).
  static UnwrappedSequenceNumber<WrappedType> AddTo(
      UnwrappedSequenceNumber<WrappedType> value,
      int delta) {
    return UnwrappedSequenceNumber<WrappedType>(value.value_ + delta);
  }

  // Returns the absolute difference between `lhs` and `rhs`.
  static typename WrappedType::UnderlyingType Difference(
      UnwrappedSequenceNumber<WrappedType> lhs,
      UnwrappedSequenceNumber<WrappedType> rhs) {
    return (lhs.value_ > rhs.value_) ? (lhs.value_ - rhs.value_)
                                     : (rhs.value_ - lhs.value_);
  }

 private:
  explicit UnwrappedSequenceNumber(int64_t value) : value_(value) {}
  static constexpr int64_t kValueLimit =
      static_cast<int64_t>(1)
      << std::numeric_limits<typename WrappedType::UnderlyingType>::digits;

  int64_t value_;
};

// Unwrapped Transmission Sequence Numbers (TSN)
using UnwrappedTSN = UnwrappedSequenceNumber<TSN>;

// Unwrapped Stream Sequence Numbers (SSN)
using UnwrappedSSN = UnwrappedSequenceNumber<SSN>;

// Unwrapped Message Identifier (MID)
using UnwrappedMID = UnwrappedSequenceNumber<MID>;

}  // namespace dcsctp

#endif  // NET_DCSCTP_COMMON_SEQUENCE_NUMBERS_H_