lollipop-counter.h

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/*
 * Copyright (c) 2020 Universita' di Firenze, Italy
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation;
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 * Author: Tommaso Pecorella <tommaso.pecorella@unifi.it>
 */
 
#ifndef LOLLIPOP_COUNTER_H
#define LOLLIPOP_COUNTER_H
 
#include "ns3/abort.h"
 
#include <limits>
 
namespace ns3
{
 
template <class T>
class LollipopCounter
{
  public:
    LollipopCounter()
    {
        NS_ABORT_MSG_UNLESS(std::is_unsigned<T>::value,
                            "Lollipop counters must be defined on unsigned integer types");
 
        uint16_t numberofDigits = std::numeric_limits<T>::digits;
        m_sequenceWindow = 1 << (numberofDigits / 2);
 
        m_value = (m_maxValue - m_sequenceWindow) + 1;
    }
 
    LollipopCounter(T val)
    {
        uint16_t numberofDigits = std::numeric_limits<T>::digits;
        m_sequenceWindow = 1 << (numberofDigits / 2);
 
        m_value = val;
    }
 
    inline LollipopCounter& operator=(const LollipopCounter& o)
    {
        m_value = o.m_value;
        return *this;
    }
 
    void Reset()
    {
        m_value = (m_maxValue - m_sequenceWindow) + 1;
    }
 
    void SetSequenceWindowSize(uint16_t numberOfBits)
    {
        uint16_t numberofDigits = std::numeric_limits<T>::digits;
 
        NS_ABORT_MSG_IF(
            numberOfBits >= numberofDigits,
            "The size of the Sequence Window should be less than the counter size (which is "
                << +m_maxValue << ")");
 
        m_sequenceWindow = 1 << numberOfBits;
 
        m_value = (m_maxValue - m_sequenceWindow) + 1;
    }
 
    bool IsComparable(const LollipopCounter& val) const
    {
        NS_ABORT_MSG_IF(m_sequenceWindow != val.m_sequenceWindow,
                        "Can not compare two Lollipop Counters with different sequence windows");
 
        if ((m_value <= m_circularRegion && val.m_value <= m_circularRegion) ||
            (m_value > m_circularRegion && val.m_value > m_circularRegion))
        {
            // They are desynchronized - comparison is impossible.
            T absDiff = AbsoluteMagnitudeOfDifference(val);
            if (absDiff > m_sequenceWindow)
            {
                return false;
            }
        }
        return true;
    }
 
    bool IsInit() const
    {
        return m_value > m_circularRegion;
    }
 
    friend bool operator==(const LollipopCounter& lhs, const LollipopCounter& rhs)
    {
        NS_ABORT_MSG_IF(lhs.m_sequenceWindow != rhs.m_sequenceWindow,
                        "Can not compare two Lollipop Counters with different sequence windows");
 
        return lhs.m_value == rhs.m_value;
    }
 
    friend bool operator>(const LollipopCounter& lhs, const LollipopCounter& rhs)
    {
        NS_ABORT_MSG_IF(lhs.m_sequenceWindow != rhs.m_sequenceWindow,
                        "Can not compare two Lollipop Counters with different sequence windows");
 
        if (lhs.m_value == rhs.m_value)
        {
            return false;
        }
 
        if ((lhs.m_value <= m_circularRegion && rhs.m_value <= m_circularRegion) ||
            (lhs.m_value > m_circularRegion && rhs.m_value > m_circularRegion))
        {
            // both counters are in the same region
 
            T absDiff = lhs.AbsoluteMagnitudeOfDifference(rhs);
            if (absDiff > lhs.m_sequenceWindow)
            {
                // They are desynchronized - comparison is impossible.
                // return false because we can not return anything else.
                return false;
            }
 
            // They are synchronized - comparison according to RFC1982.
            T serialRegion = ((m_circularRegion >> 1) + 1);
            return (((lhs.m_value < rhs.m_value) && ((rhs.m_value - lhs.m_value) > serialRegion)) ||
                    ((lhs.m_value > rhs.m_value) && ((lhs.m_value - rhs.m_value) < serialRegion)));
        }
 
        // One counter is in the "high" region and the other is in the in the "lower" region
        bool lhsIsHigher;
        T difference;
 
        if (lhs.m_value > m_circularRegion && rhs.m_value <= m_circularRegion)
        {
            lhsIsHigher = true;
            // this is guaranteed to be positive and between [1...m_lollipopMaxValue].
            difference = lhs.m_value - rhs.m_value;
        }
        else
        {
            lhsIsHigher = false;
            // this is guaranteed to be positive and between [1...m_lollipopMaxValue].
            difference = rhs.m_value - lhs.m_value;
        }
 
        T distance = (m_maxValue - difference) +
                     1; // this is guaranteed to be positive and between [1...m_lollipopMaxValue].
        if (distance > lhs.m_sequenceWindow)
        {
            return lhsIsHigher;
        }
        else
        {
            return !lhsIsHigher;
        }
 
        // this should never be reached.
        return false;
    }
 
    friend bool operator<(const LollipopCounter& lhs, const LollipopCounter& rhs)
    {
        if (!lhs.IsComparable(rhs))
        {
            return false;
        }
 
        if (lhs > rhs)
        {
            return false;
        }
        else if (lhs == rhs)
        {
            return false;
        }
 
        return true;
    }
 
    friend LollipopCounter operator++(LollipopCounter& val) // prefix ++
    {
        val.m_value++;
 
        if (val.m_value == val.m_circularRegion + 1)
        {
            val.m_value = 0;
        }
 
        return val;
    }
 
    friend LollipopCounter operator++(LollipopCounter& val, int noop) // postfix ++
    {
        LollipopCounter ans = val;
        ++(val); // or just call operator++()
        return ans;
    }
 
    T GetValue() const
    {
        return m_value;
    }
 
    friend std::ostream& operator<<(std::ostream& os, const LollipopCounter& counter)
    {
        os << +counter.m_value;
        return os;
    }
 
  private:
    T AbsoluteMagnitudeOfDifference(const LollipopCounter& val) const
    {
        // useless because it is computed always on counters on their respective regions.
        // Left (commented) for debugging purposes in case there is a code change.
        // NS_ASSERT_MSG ((m_value <= m_circularRegion && val.m_value <= m_circularRegion) ||
        //                (m_value > m_circularRegion && val.m_value > m_circularRegion),
        //                "Absolute Magnitude Of Difference can be computed only on two values in
        //                the circular region " << +m_value << " - " << +val.m_value);
 
        T absDiffDirect = std::max(m_value, val.m_value) - std::min(m_value, val.m_value);
        T absDiffWrapped = (std::min(m_value, val.m_value) + m_circularRegion + 1) -
                           std::max(m_value, val.m_value);
        T absDiff = std::min(absDiffDirect, absDiffWrapped);
        return absDiff;
    }
 
    T m_value;          
    T m_sequenceWindow; 
    static constexpr T m_maxValue =
        std::numeric_limits<T>::max();                     
    static constexpr T m_circularRegion = m_maxValue >> 1; 
};
 
typedef LollipopCounter<uint8_t> LollipopCounter8;
typedef LollipopCounter<uint16_t> LollipopCounter16;
 
} /* namespace ns3 */
 
#endif /* LOLLIPOP_COUNTER_H */