#include "FrameDecoder.h"

#include <cmath>

namespace udpscope {

/** Fallback cycle period before the first inter-packet gap is known. */
static constexpr double kDefaultDt = 1.0e-3;

/**
 * How far a chained burst prediction may sit from where arrival time says it
 * should be before the chain is abandoned and time is re-anchored on arrival.
 *
 * This is a backstop, not the primary mechanism: the packet counter normally
 * accounts for lost datagrams exactly, so the prediction and arrival agree.
 * It catches what the counter cannot describe — a producer restart (the
 * counter returns to zero), a counter that never advances, and a declared
 * sampling rate that does not match the producer's real one. A kernel draining
 * a backlog of queued datagrams can legitimately put the prediction a couple of
 * hundred milliseconds from arrival, so the threshold sits well clear of that.
 * Same value and same reasoning as ClockOffset::kRecalibThresholdS.
 */
static constexpr double kBurstResyncThresholdS = 0.5;

/**
 * Largest counter gap still read as a loss count.
 *
 * A producer restart returns the counter to zero and a reordered datagram makes
 * the unsigned gap wrap to near 2^32; multiplying either by a sample count and
 * calling it elapsed time would fabricate centuries. A million lost updates is
 * already far beyond any outage worth reconstructing.
 */
static constexpr uint32_t kMaxCounterGap = 1000000u;

/**
 * Burst width, as a fraction of nominal, while a leading timeline is being
 * pulled back. See the sole use site for why a leading chain cannot be
 * corrected in one burst and must be bled off instead.
 */
static constexpr double kLeadBleedFactor = 0.9;

void FrameDecoder::setSignals(const std::vector<SignalMeta>& signals) {
    signals_ = signals;
    state_.assign(signals_.size(), SigState{});
    hrtFit_.reset();
}

void FrameDecoder::reset() {
    state_.assign(signals_.size(), SigState{});
    hrtFit_.reset();
}

void FrameDecoder::beginFrame(const FrameView& f) {
    if (f.hrt != 0u) { hrtFit_.add(f.hrt, f.recvTime); }
}

bool FrameDecoder::packetBurst(uint32_t idx, uint32_t nElems, double wallNow,
                               std::vector<double>& tsOut) {
    SigState& st = state_[idx];
    if (!st.lastPacketValid || wallNow <= st.lastPacketWall) {
        /* No previous arrival to span from, or time went backwards. Remember
         * this one and drop the samples rather than store them at made-up
         * spacing. */
        st.lastPacketWall  = wallNow;
        st.lastPacketValid = true;
        return false;
    }

    const double dt = (wallNow - st.lastPacketWall) / static_cast<double>(nElems);
    tsOut.resize(nElems);
    for (uint32_t e = 0; e < nElems; e++) {
        tsOut[e] = st.lastPacketWall + static_cast<double>(e + 1u) * dt;
    }
    st.lastPacketWall = wallNow;
    return true;
}

bool FrameDecoder::timestamps(const FrameView& f, uint32_t idx,
                              std::vector<double>& tsOut) {
    tsOut.clear();
    if (idx >= signals_.size() || idx >= f.numSignals ||
        f.counts == nullptr || f.values == nullptr) {
        return false;
    }

    const SignalMeta& d      = signals_[idx];
    const uint32_t    nElems = f.counts[idx];
    if (nElems == 0u) { return false; }

    const double wallNow = f.recvTime;
    SigState&    st      = state_[idx];

    /* A repeated counter is a duplicated datagram — the same update arriving
     * twice because the host joined the multicast group on two interfaces, say.
     * The C client only de-duplicates fragments, so an unfragmented update
     * reaches us intact both times; emitting it again would double the values
     * and advance the timeline by a burst that never existed. Counter zero is
     * excluded because a producer that never sets one leaves it there. */
    if (st.lastEmittedValid && f.counter != 0u && f.counter == st.lastCounter) {
        return false;
    }

    /* hasTimeSignal() bounds the index against the FRAME's signal count, but
     * the time signal's type code is read from our own table, whose size is
     * independent — a frame carrying more signals than the installed table
     * (briefly possible after a CONFIG change) would otherwise read past it. */
    const bool     hasTimeSig = d.hasTimeSignal(f.numSignals) &&
                                d.timeSignalIdx < signals_.size();
    const uint32_t tIdx       = hasTimeSig ? d.timeSignalIdx : 0u;
    const double   tScale     = hasTimeSig
                                ? TimeSignalScale(signals_[tIdx].typeCode)
                                : 1.0e-6;

    /* Rule 1: one stamp per element, straight from the time signal. */
    if (d.timeMode == kTimeFullArray && hasTimeSig &&
        f.counts[tIdx] >= nElems && f.values[tIdx] != nullptr) {
        const double* tv = f.values[tIdx];
        const double  t0 = tv[0] * tScale;
        (void) st.offset.map(t0, wallNow);
        const double base = st.offset.offset();
        tsOut.resize(nElems);
        for (uint32_t e = 0; e < nElems; e++) {
            tsOut[e] = base + tv[e] * tScale;
        }
        return true;
    }

    /* Rule 2: anchor from the time signal, spread by the sampling rate. */
    if ((d.timeMode == kTimeFirstSample || d.timeMode == kTimeLastSample) &&
        hasTimeSig && f.counts[tIdx] >= 1u && f.values[tIdx] != nullptr) {
        const double anchor = st.offset.map(f.values[tIdx][0] * tScale, wallNow);
        const double dt     = (d.samplingRate > 0.0) ? (1.0 / d.samplingRate) : 0.0;
        tsOut.resize(nElems);
        for (uint32_t e = 0; e < nElems; e++) {
            tsOut[e] = (d.timeMode == kTimeFirstSample)
                     ? (anchor + static_cast<double>(e) * dt)
                     : (anchor - static_cast<double>(nElems - 1u - e) * dt);
        }
        return true;
    }

    /* Rule 3: accumulated scalar, based on declared sampling rate or hrt.
     *
     * When samplingRate is declared the inter-element step is exact and we
     * anchor from the end of the previous burst rather than from arrival time
     * or hrt.  This makes the output immune to arrival jitter: even when the
     * kernel delivers two packets microseconds apart each burst starts exactly
     * one sample period after the previous burst ended.
     *
     * When samplingRate is absent we must derive dt from the hrt gap, which
     * requires the HrtRateFit to be ready. Until then we fall back to
     * packetBurst (arrival-time spanning), which is accurate during the normal
     * pre-burst delivery phase that precedes the fit becoming ready. */
    if (d.numElements() == 1u && nElems > 1u) {
        const double dt = (d.samplingRate > 0.0)
                          ? (1.0 / d.samplingRate)
                          : 0.0;

        if (d.samplingRate > 0.0) {
            /* Where arrival time says this burst begins: its last element was
             * acquired just before the packet landed. */
            const double arrivalAnchor =
                wallNow - static_cast<double>(nElems - 1u) * dt;

            /* Chaining onto the end of the previous burst is immune to arrival
             * jitter — a kernel draining several queued datagrams microseconds
             * apart still yields contiguous timestamps. What a bare chain gets
             * wrong is loss: it closes the hole a dropped datagram left, and
             * every later sample is then dated early for the rest of the run.
             *
             * The wire says exactly how much is missing. counter increments
             * once per update, so a gap of g means g-1 lost packets, each
             * carrying (as far as we can tell) as many samples as the last one
             * we saw. Reinstating that duration keeps the chain honest without
             * consulting arrival time at all. */
            double base = arrivalAnchor;
            double step = dt;
            if (st.lastEmittedValid) {
                /* Unsigned subtraction wraps, so this stays right across the
                 * counter's own 2^32 rollover. A gap far larger than any real
                 * outage is a restart or a reordered datagram rather than a
                 * loss count; claim nothing and let the backstop below decide. */
                const uint32_t gap  = f.counter - st.lastCounter;
                const double   lost = (gap > 1u && gap <= kMaxCounterGap)
                    ? static_cast<double>(gap - 1u) *
                      static_cast<double>(st.prevAccCount)
                    : 0.0;
                const double predicted = st.lastEmittedEnd + dt * (1.0 + lost);

                /* Backstop for what the counter cannot express: a producer
                 * restart, a counter stuck at zero, or a declared rate that is
                 * simply wrong. Beyond this the chain is not recoverable and
                 * arrival time is the better of two bad answers. */
                if (std::fabs(predicted - arrivalAnchor) <= kBurstResyncThresholdS) {
                    base = predicted;
                }

                if (base <= st.lastEmittedEnd) {
                    /* Re-anchoring here would step backwards, and the ring, the
                     * trigger and the exporter all require a signal's stamps to
                     * increase. Rejecting the correction outright is not an
                     * option either: `predicted` is never less than
                     * lastEmittedEnd + dt, so rejection would make the backstop
                     * one-directional and let a timeline that runs FAST — two
                     * hosts' crystals differ by tens of ppm, so this is certain
                     * on a long session, not hypothetical — drift ahead of the
                     * wall clock without bound.
                     *
                     * So compress instead of stepping back: start immediately
                     * after the previous burst and spread this one out to
                     * arrival. A single packet is drawn narrower than its true
                     * width, and in exchange the timeline is back in step. */
                    if (wallNow > st.lastEmittedEnd) {
                        step = (wallNow - st.lastEmittedEnd) /
                               static_cast<double>(nElems);
                        base = st.lastEmittedEnd + step;
                    } else {
                        /* The timeline has run PAST arrival: our last burst is
                         * dated later than the moment this packet landed. There
                         * is no room to spread into, and no single burst can
                         * remove the excess without stepping back. So bleed it
                         * off — draw each burst a fixed fraction narrower than
                         * nominal until the timeline is back inside the
                         * threshold, then normal chaining resumes. The factor
                         * only has to shrink a burst faster than the clock
                         * mismatch grows it, and a 10 % squeeze outruns the
                         * tens-of-ppm crystal error that causes this by orders
                         * of magnitude. */
                        step = dt * kLeadBleedFactor;
                        base = st.lastEmittedEnd + step;
                    }
                }
            }
            tsOut.resize(nElems);
            for (uint32_t e = 0; e < nElems; e++) {
                tsOut[e] = base + static_cast<double>(e) * step;
            }
            st.lastEmittedEnd   = tsOut[nElems - 1u];
            st.lastCounter      = f.counter;
            st.prevAccCount     = nElems;
            st.lastEmittedValid = true;
            return true;
        }

        /* No declared rate: need hrt-derived dt. */
        if (!hrtFit_.ready() || f.hrt == 0u) {
            return packetBurst(idx, nElems, wallNow, tsOut);
        }
        const double rate = hrtFit_.ticksPerSecond();

        /* Difference raw TICKS, never two toSeconds() results.
         *
         * hrt counts from the producer's boot, so it is already ~1e11 ticks when
         * the scope attaches, while the fit is re-estimated on every packet and
         * wobbles by a few parts in 1e4. toSeconds() multiplies that relative
         * wobble by the whole elapsed epoch: tens of milliseconds of jitter on a
         * value whose consecutive difference is a few milliseconds. Subtracting
         * two such results measures the wobble, not the interval.
         *
         * Anchoring on the first usable packet keeps the wobble on the elapsed
         * interval since attach, which is short, and ClockOffset absorbs the
         * arbitrary epoch that anchoring leaves behind exactly as it would
         * absorb the producer's boot epoch. */
        if (!st.hrtRefValid) {
            st.hrtRef      = f.hrt;
            st.hrtRefValid = true;
        }
        const double sinceRef = (f.hrt >= st.hrtRef)
            ?  static_cast<double>(f.hrt - st.hrtRef) / rate
            : -static_cast<double>(st.hrtRef - f.hrt) / rate;
        const double base = st.offset.map(sinceRef, wallNow);

        double hrtDt = kDefaultDt;
        if (st.lastAccValid && st.prevAccCount > 0u && f.hrt > st.lastAccHrt) {
            /* The flushes carry contiguous RT cycles, so the gap divided by the
             * previous packet's sample count is exactly one cycle period. */
            hrtDt = (static_cast<double>(f.hrt - st.lastAccHrt) / rate) /
                    static_cast<double>(st.prevAccCount);
        }

        tsOut.resize(nElems);
        for (uint32_t e = 0; e < nElems; e++) {
            tsOut[e] = base + static_cast<double>(e) * hrtDt;
        }
        st.lastAccHrt   = f.hrt;
        st.lastAccValid = true;
        st.prevAccCount = nElems;
        return true;
    }

    /* Rule 4: PACKET burst with no time reference at all. */
    if (nElems > 1u) {
        return packetBurst(idx, nElems, wallNow, tsOut);
    }

    /* Rule 5: plain scalar. */
    tsOut.assign(1, wallNow);
    return true;
}

} /* namespace udpscope */