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>

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.
template <typename WrappedType>
class UnwrappedSequenceNumber {
 public:
  static_assert(!std::numeric_limits<WrappedType>::is_signed,
                "The wrapped type must be unsigned");
  static_assert(std::numeric_limits<WrappedType>::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() : largest_(kValueLimit) {}
    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) {
      WrappedType wrapped_largest =
          static_cast<WrappedType>(largest_ % kValueLimit);
      int64_t result = largest_ + Delta(value, wrapped_largest);
      if (largest_ < result) {
        largest_ = result;
      }
      return UnwrappedSequenceNumber<WrappedType>(result);
    }

    // Similar to `Unwrap`, but will not update the Unwrappers's internal state.
    UnwrappedSequenceNumber<WrappedType> PeekUnwrap(WrappedType value) const {
      WrappedType uint32_largest =
          static_cast<WrappedType>(largest_ % kValueLimit);
      int64_t result = largest_ + Delta(value, uint32_largest);
      return UnwrappedSequenceNumber<WrappedType>(result);
    }

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

   private:
    static int64_t Delta(WrappedType value, WrappedType prev_value) {
      static constexpr WrappedType kBreakpoint = kValueLimit / 2;
      WrappedType diff = value - prev_value;
      diff %= kValueLimit;
      if (diff < kBreakpoint) {
        return static_cast<int64_t>(diff);
      }
      return static_cast<int64_t>(diff) - kValueLimit;
    }

    int64_t largest_;
  };

  // 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_;
  }

  // Increments the value.
  void Increment() { ++value_; }
  UnwrappedSequenceNumber<WrappedType> next_value() const {
    return UnwrappedSequenceNumber<WrappedType>(value_ + 1);
  }

  // Adds a delta to the current value.
  UnwrappedSequenceNumber<WrappedType> AddTo(int delta) const {
    return UnwrappedSequenceNumber<WrappedType>(value_ + delta);
  }

  // Compares the difference between two sequence numbers.
  WrappedType Difference(UnwrappedSequenceNumber<WrappedType> other) const {
    return value_ - other.value_;
  }

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

  int64_t value_;
};

// Unwrapped Transmission Sequence Numbers (TSN)
using UnwrappedTSN = UnwrappedSequenceNumber<uint32_t>;

// Unwrapped Stream Sequence Numbers (SSN)
using UnwrappedSSN = UnwrappedSequenceNumber<uint16_t>;

// Unwrapped Message Identifier (MID)
using UnwrappedMID = UnwrappedSequenceNumber<uint32_t>;

}  // namespace dcsctp

#endif  // NET_DCSCTP_COMMON_SEQUENCE_NUMBERS_H_