//
//  SequenceNumberStats.cpp
//  libraries/networking/src
//
//  Created by Yixin Wang on 6/25/2014
//  Copyright 2014 High Fidelity, Inc.
//
//  Distributed under the Apache License, Version 2.0.
//  See the accompanying file LICENSE or http://www.apache.org/licenses/LICENSE-2.0.html
//

#include "SequenceNumbersStats.h"

#include <limits>

SequenceNumberStats::SequenceNumberStats()
    : _lastReceived(std::numeric_limits<quint16>::max()),
    _missingSet(),
    _numReceived(0),
    _numUnreasonable(0),
    _numEarly(0),
    _numLate(0),
    _numLost(0),
    _numRecovered(0),
    _numDuplicate(0)
{
}

void SequenceNumberStats::sequenceNumberReceived(quint16 incoming, const bool wantExtraDebugging) {

    static const int UINT16_RANGE = std::numeric_limits<uint16_t>::max() + 1;
    static const int MAX_REASONABLE_SEQUENCE_GAP = 1000;  // this must be less than UINT16_RANGE / 2 for rollover handling to work

    _numReceived++;

    // determine our expected sequence number... handle rollover appropriately
    quint16 expected = _numReceived > 0 ? _lastReceived + (quint16)1 : incoming;

    if (incoming == expected) { // on time
        _lastReceived = incoming;
    } else { // out of order

        if (wantExtraDebugging) {
            qDebug() << "out of order... got:" << incoming << "expected:" << expected;
        }

        int incomingInt = (int)incoming;
        int expectedInt = (int)expected;

        // check if the gap between incoming and expected is reasonable, taking possible rollover into consideration
        int absGap = std::abs(incomingInt - expectedInt);
        if (absGap >= UINT16_RANGE - MAX_REASONABLE_SEQUENCE_GAP) {
            // rollover likely occurred between incoming and expected.
            // correct the larger of the two so that it's within [-UINT16_RANGE, -1] while the other remains within [0, UINT16_RANGE-1]
            if (incomingInt > expectedInt) {
                incomingInt -= UINT16_RANGE;
            } else {
                expectedInt -= UINT16_RANGE;
            }
        } else if (absGap > MAX_REASONABLE_SEQUENCE_GAP) {
            // ignore packet if gap is unreasonable
            qDebug() << "ignoring unreasonable packet... sequence:" << incoming
                << "previous:" << _lastReceived;
            _numUnreasonable++;
            return;
        }

        // now that rollover has been corrected for (if it occurred), incoming and expected can be
        // compared to each other directly, though one of them might be negative
        if (incomingInt > expectedInt) { // early
            if (wantExtraDebugging) {
                qDebug() << "this packet is earlier than expected...";
                qDebug() << ">>>>>>>> missing gap=" << (incomingInt - expectedInt);
            }

            _numEarly++;
            _numLost += (incomingInt - expectedInt);

            // add all sequence numbers that were skipped to the missing sequence numbers list
            for (int missingInt = expectedInt; missingInt < incomingInt; missingInt++) {
                _missingSet.insert((quint16)(missingInt < 0 ? missingInt + UINT16_RANGE : missingInt));
            }
            _lastReceived = incoming;
        } else { // late
            if (wantExtraDebugging) {
                qDebug() << "this packet is later than expected...";
            }
            _numLate++;

            // remove this from missing sequence number if it's in there
            if (_missingSet.remove(incoming)) {
                if (wantExtraDebugging) {
                    qDebug() << "found it in _missingSet";
                }
                _numLost--;
                _numRecovered++;
            } else {
                if (wantExtraDebugging) {
                    qDebug() << "sequence:" << incoming << "was NOT found in _missingSet and is probably a duplicate";
                }
                _numDuplicate++;
            }

            // do not update _incomingLastSequence; it shouldn't become smaller
        }
    }

    // prune missing sequence list if it gets too big; sequence numbers that are older than MAX_REASONABLE_SEQUENCE_GAP
    // will be removed.
    if (_missingSet.size() > MAX_REASONABLE_SEQUENCE_GAP) {
        if (wantExtraDebugging) {
            qDebug() << "_missingSet is too large! size:" << _missingSet.size();
        }

        // some older sequence numbers may be from before a rollover point; this must be handled.
        // some sequence numbers in this list may be larger than _incomingLastSequence, indicating that they were received
        // before the most recent rollover.
        int cutoff = (int)_lastReceived - MAX_REASONABLE_SEQUENCE_GAP;
        if (cutoff >= 0) {
            quint16 nonRolloverCutoff = (quint16)cutoff;
            QSet<quint16>::iterator i = _missingSet.begin();
            while (i != _missingSet.end()) {
                quint16 missing = *i;
                if (wantExtraDebugging) {
                    qDebug() << "checking item:" << missing << "is it in need of pruning?";
                    qDebug() << "old age cutoff:" << nonRolloverCutoff;
                }

                if (missing > _lastReceived || missing < nonRolloverCutoff) {
                    i = _missingSet.erase(i);
                    if (wantExtraDebugging) {
                        qDebug() << "pruning really old missing sequence:" << missing;
                    }
                } else {
                    i++;
                }
            }
        } else {
            quint16 rolloverCutoff = (quint16)(cutoff + UINT16_RANGE);
            QSet<quint16>::iterator i = _missingSet.begin();
            while (i != _missingSet.end()) {
                quint16 missing = *i;
                if (wantExtraDebugging) {
                    qDebug() << "checking item:" << missing << "is it in need of pruning?";
                    qDebug() << "old age cutoff:" << rolloverCutoff;
                }

                if (missing > _lastReceived && missing < rolloverCutoff) {
                    i = _missingSet.erase(i);
                    if (wantExtraDebugging) {
                        qDebug() << "pruning really old missing sequence:" << missing;
                    }
                } else {
                    i++;
                }
            }
        }
    }
}