MARTe-Integrated-Components/Client/udpscope/FrameDecoder.h

/**
 * @file FrameDecoder.h
 * @brief Per-element timestamp reconstruction for UDPS frames.
 *
 * The C client's udps_frame_element_time() is explicitly an arrival-anchored
 * estimate. It is not sufficient: the kernel frequently delivers several queued
 * datagrams in one burst, so two packets are processed microseconds apart even
 * though each represents ~10 ms of signal, and arrival-time interpolation then
 * crams a packet's samples into that tiny gap — the trace renders as a sawtooth.
 * Source/Applications/StreamHub/UDPSourceSession.cpp documents this failure and
 * solves it; these are the same rules, computed from udps_frame_t's own fields.
 *
 * Three rules deliberately differ, all in the accumulated-scalar case (rule 3).
 *
 * First, the anchor. StreamHub anchors every accumulated-scalar burst on the
 * packet's own hrt, converted with the LOCAL MARTe HighResolutionTimer
 * frequency — correct only because StreamHub runs on the producer's host. A
 * bench scope attaches over the network and has no access to that frequency; it
 * can only regress hrt against arrival time, which is exactly what the bursty
 * delivery above corrupts. So when a SamplingRate is declared this decoder
 * chains bursts instead, using the packet counter to account for loss and
 * arrival time only as a backstop. The consequence is that a declared rate
 * measured against the producer's crystal rather than ours makes the
 * reconstructed timeline drift, and drift that only arrival time can observe
 * must be corrected against arrival time — see rule 3.
 *
 * Second, which end of the burst is anchored. StreamHub converts the packet's
 * hrt into the position of sample 0 and steps forward, so the burst STARTS at
 * the anchor. Here the anchor is arrival time, and the samples were acquired
 * before the packet carrying them landed — so the burst must END there instead.
 * Both branches of rule 3 do this, or two accumulated scalars in one scope, one
 * with a declared rate and one without, would sit a whole burst apart on the
 * shared X axis.
 *
 * Third, the entry condition. UDPSourceSession.cpp:554 routes any update
 * carrying nElems <= 1 to plain arrival time. That is safe for a host-local
 * consumer whose arrival time is the producer's own clock, but wrong here:
 * Accumulate mode flushes on a TIMER, so a short RT cycle legitimately delivers
 * a single sample between two full bursts. Dating that one sample from arrival
 * while its neighbours are chained puts it off the chain, and — worse — leaves
 * lastCounter behind, so the next full burst reads the skipped counter as a lost
 * datagram and reinstates a hole that never existed. So a signal that has
 * already burst keeps every later update on rule 3 regardless of its length; a
 * signal that has never burst is a genuine scalar and is left to rule 5.
 */
#pragma once

#include "TimeBase.h"
#include "Types.h"

#include <vector>

namespace udpscope {

class FrameDecoder {
public:
    /** Installs the signal table. Clears all per-signal timing history. */
    void setSignals(const std::vector<SignalMeta>& signals);

    const std::vector<SignalMeta>& signals() const { return signals_; }

    /** Call once per frame, before any timestamps() call for that frame. */
    void beginFrame(const FrameView& f);

    /**
     * @brief Timestamps for every value of signal @p idx in this frame.
     * @return false when the signal produced nothing usable — an empty slot, or
     *         the first PACKET burst after connect, which has no previous
     *         arrival to span from and would otherwise poison the ring with
     *         wrongly spaced timestamps.
     */
    bool timestamps(const FrameView& f, uint32_t idx, std::vector<double>& tsOut);

    /** Forgets all timing history; call on reconnect. */
    void reset();

private:
    bool packetBurst(uint32_t idx, uint32_t nElems, double wallNow,
                     std::vector<double>& tsOut);

    struct SigState {
        ClockOffset offset;
        double      lastPacketWall  = 0.0;
        bool        lastPacketValid = false;
        /** Raw ticks, not seconds — see the comment in the samplingRate == 0
         *  branch for why a tick difference is the only safe way to measure a
         *  producer-side interval while the rate is still being re-estimated. */
        uint64_t    lastAccHrt      = 0u;
        /** Producer seconds since this signal's first usable packet, built by
         *  SUMMING short tick deltas. Never recomputed from an absolute tick
         *  count; see the samplingRate == 0 branch. */
        double      accProdSec      = 0.0;
        bool        lastAccValid    = false;
        uint32_t    prevAccCount    = 0;
        /** For accumulated scalars (rule 3, either branch): end timestamp of the
         *  most recently emitted burst, and the packet counter it came from. The
         *  next burst is chained onto that end, with the counter gap reinstating
         *  the exact duration of any lost datagrams. */
        double      lastEmittedEnd   = 0.0;
        uint32_t    lastCounter      = 0u;
        /** ARRIVAL time of the packet that produced lastEmittedEnd. Valid
         *  exactly when lastEmittedValid is, so it needs no flag of its own.
         *  Deliberately not lastPacketWall, which belongs to packetBurst() and
         *  is updated on frames rule 3 never emits. This is the only reference
         *  against which a leading timeline can be pulled back: the correction
         *  has to be expressed as a fraction of the wall time that has really
         *  elapsed since this signal's previous burst, because within a single
         *  timestamps() call the wall clock is frozen and every forward step,
         *  however small, increases the lead measured at that instant. */
        double      lastEmittedWall  = 0.0;
        bool        lastEmittedValid = false;
    };

    std::vector<SignalMeta> signals_;
    std::vector<SigState>   state_;
    HrtRateFit              hrtFit_;
};

} /* namespace udpscope */