package main

import (
	"fmt"
	"math"
	"sync"
	"sync/atomic"
	"time"

	"golang.org/x/sys/unix"
)

const wireOverhead = 24

type direction struct {
	specs   []*frameSpec
	txStats []*txStats
	rxStats []*rxStats
	streams []lossWindow
	txFDs   []int
	rxFDs   []int
	statFD  int

	// While a cable measure runs, every failure counter in this direction is
	// suppressed at its source rather than counted, hidden and reverted.
	measuring *atomic.Bool

	// Guards everything the sampler touches. The counters are read on their own
	// clock and drawn on another, and the two must not read them at once:
	// sampleDrops consumes what it reads, so a second caller would see a gap.
	mu    sync.Mutex
	win   *rateWindow
	drops uint64
	base  counterSet

	// The smeared receive bucket the display draws its rate from, refreshed by
	// the sampler because the buckets are keyed by the shared read clock and
	// staleness has to be judged against the wall.
	rateFrames uint64
	rateBytes  uint64
	epoch      int64
	epochAt    time.Time

	// The settled window: each completed bucket enters once, donates its
	// excess backward once, and leaves for display once no later bucket can
	// still refill it. Recomputing the smear from the raw ring every sample
	// instead would show each excess twice — once as the donation and again,
	// unspent, when its bucket reaches the display slot of a later window.
	smFrames [smearWindow]uint64
	smBytes  [smearWindow]uint64
	smLen    int
	smNext   int64

	nic    atomic.Uint64
	poller *nicPoller
}

// Everything the display reads, taken at one instant, so a pair of these
// describes both the rates and the errors over the span between them.
type counterSet struct {
	t     time.Time
	s     sample
	drops uint64
	nic   uint64
}

func (d *direction) capture() counterSet {
	d.sampleDrops()
	s := d.snapshot()
	return counterSet{t: time.Now(), s: s, drops: d.drops, nic: d.nic.Load()}
}

// What someone testing a cable is asking, rather than how each failure happened
// to be noticed.
type errs struct {
	lost     uint64
	corrupt  uint64
	link     uint64
	internal uint64
}

func (e errs) add(o errs) errs {
	return errs{
		lost: e.lost + o.lost, corrupt: e.corrupt + o.corrupt,
		link: e.link + o.link, internal: e.internal + o.internal,
	}
}

// A ring of one bucket per drawn frame, spanning rateWindowSpan. Rates come
// from the gap between adjacent buckets and errors from the ends of the ring,
// so both slide forward every frame instead of stepping once a second.
type rateWindow struct {
	buf    []counterSet
	idx    int
	filled bool
}

func newRateWindow(n int) *rateWindow {
	return &rateWindow{buf: make([]counterSet, n)}
}

func (w *rateWindow) push(c counterSet) {
	w.buf[w.idx] = c
	w.idx++
	if w.idx == len(w.buf) {
		w.idx = 0
		w.filled = true
	}
}

func (w *rateWindow) count() int {
	if w.filled {
		return len(w.buf)
	}
	return w.idx
}

// Indexed oldest first, so a partly filled ring reads the same as a full one.
func (w *rateWindow) at(i int) counterSet {
	if w.filled {
		i += w.idx
	}
	return w.buf[i%len(w.buf)]
}

// How long the frontier may sit still before the wire is taken to have gone
// quiet. It only advances when frames arrive on every stream, so a frozen
// frontier means a stream has stopped delivering rather than an unchanged rate.
const rateStale = 100 * time.Millisecond

// An epoch is only reachable once a worker drains frames into it, so each
// stream's newest epoch is a frontier: everything that queue has been read
// through. Epochs behind the lowest frontier — the leader's would claim
// buckets the stragglers are still filling — are closed to further commits,
// so each is settled into the window exactly once. The display takes the
// window's oldest bucket, the one no later bucket can still refill, so the
// headline runs one window behind real time.
func (d *direction) readRateBucket(now time.Time) {
	newest := int64(math.MaxInt64)
	for _, r := range d.rxStats {
		if e := r.newest.Load(); e < newest {
			newest = e
		}
	}
	if newest > d.epoch {
		d.epoch, d.epochAt = newest, now
	}
	if d.smNext == 0 && d.epoch > 1 {
		d.smNext = d.epoch - 1
	}
	for e := d.smNext; e > 0 && e < d.epoch; e++ {
		d.settle(e)
		d.smNext = e + 1
	}
	d.rateFrames, d.rateBytes = 0, 0
	if d.smLen == 0 || now.Sub(d.epochAt) > rateStale {
		return
	}
	d.rateFrames, d.rateBytes = d.smFrames[0], d.smBytes[0]
}

func (d *direction) settle(e int64) {
	if d.smLen == smearWindow {
		copy(d.smFrames[:], d.smFrames[1:])
		copy(d.smBytes[:], d.smBytes[1:])
		d.smLen--
	}
	var f, b uint64
	for _, r := range d.rxStats {
		rf, rb := r.bucket(e)
		f += rf
		b += rb
	}
	d.smFrames[d.smLen], d.smBytes[d.smLen] = f, b
	d.smLen++
	fillBack(d.smFrames[:d.smLen], d.smBytes[:d.smLen])
}

type sample struct {
	rxFrames, rxBytes uint64
	lost, late        uint64
	crcErr, badMagic  uint64
	badHdr            uint64
	badLen            uint64
	txErrs            uint64
	rxErrs            uint64
}

func (d *direction) snapshot() sample {
	var s sample
	for _, t := range d.txStats {
		s.txErrs += t.errs.Load()
	}
	for _, r := range d.rxStats {
		s.rxFrames += r.frames.Load()
		s.rxBytes += r.bytes.Load()
		s.crcErr += r.crcErr.Load()
		s.badMagic += r.badMagic.Load()
		s.badHdr += r.badHdr.Load()
		s.badLen += r.badLen.Load()
		s.rxErrs += r.rxErrs.Load()
	}
	for i := range d.streams {
		s.lost += d.streams[i].lost.Load()
		s.late += d.streams[i].late.Load()
	}
	return s
}

// Counters keep climbing in the workers, so resetting just moves the origin
// everything is measured from. Rates and the rolling error window are about now
// rather than since the reset, so they keep running; the origin goes into the
// ring so the newest bucket never sits behind it.
func (d *direction) reset() {
	d.mu.Lock()
	d.base = d.capture()
	d.win.push(d.base)
	d.mu.Unlock()
}

// The socket statistic is read-and-clear, so it is always consumed; a drop
// during a cable measure is the blip's and is discarded at this source.
func (d *direction) sampleDrops() {
	for _, fd := range d.rxFDs {
		n := packetDrops(fd)
		if !d.measuring.Load() {
			d.drops += n
		}
	}
}

func gbps(bytes, frames uint64, secs float64) float64 {
	return float64((bytes+frames*wireOverhead)*8) / secs / 1e9
}

// Shared by the console table and the framebuffer so both show the same
// figures.
type view struct {
	rxPPS             float64
	rxGbps            float64
	rxFrames, rxBytes uint64
	since             errs
	window            errs
}

func errsBetween(b, n counterSet) errs {
	return errs{
		lost: n.s.lost - b.s.lost,
		// Four ways of noticing one thing: a payload that does not match its
		// checksum, a header that does not match its own, a header that is not
		// ours, and a length that cannot be.
		corrupt: (n.s.crcErr - b.s.crcErr) + (n.s.badHdr - b.s.badHdr) +
			(n.s.badMagic - b.s.badMagic) + (n.s.badLen - b.s.badLen),
		// What the hardware reported. Nothing the host declined to send is here,
		// so this one going red means the cable.
		link: (n.nic - b.nic) + (n.s.rxErrs - b.s.rxErrs),
		// Ours rather than the cable's. A late frame is unreachable while each
		// stream has a flow rule to its own queue, which is exactly why it is
		// worth counting.
		internal: (n.drops - b.drops) + (n.s.late - b.s.late) +
			(n.s.txErrs - b.s.txErrs),
	}
}

func (d *direction) counters(now counterSet) view {
	return view{
		rxFrames: now.s.rxFrames - d.base.s.rxFrames,
		rxBytes:  now.s.rxBytes - d.base.s.rxBytes,
		since:    errsBetween(d.base, now),
	}
}

func totalView(views []view) view {
	var t view
	for _, v := range views {
		t.rxPPS += v.rxPPS
		t.rxGbps += v.rxGbps
		t.rxFrames += v.rxFrames
		t.rxBytes += v.rxBytes
		t.since = t.since.add(v.since)
		t.window = t.window.add(v.window)
	}
	return t
}

func (d *direction) sample() {
	d.mu.Lock()
	d.win.push(d.capture())
	d.readRateBucket(time.Now())
	d.mu.Unlock()
}

// Draws what the sampler last put in the ring rather than reading the counters
// again, so the display never participates in the measurement.
func (d *direction) displayView() view {
	d.mu.Lock()
	n := d.win.count()
	if n == 0 {
		d.mu.Unlock()
		return view{}
	}
	v := d.counters(d.win.at(n - 1))
	if n >= 2 {
		v.window = errsBetween(d.win.at(0), d.win.at(n-1))
	}
	v.rxPPS = float64(d.rateFrames) / rateBucketSecs
	v.rxGbps = gbps(d.rateBytes, d.rateFrames, rateBucketSecs)
	d.mu.Unlock()
	return v
}

// Whatever the interfaces counted before now is not ours, and no interval has
// elapsed yet, so every baseline starts here and nothing is reported until the
// first one completes.
func (d *direction) primeCounters() {
	d.poller.prime()
	d.reset()
}

func buildDirection(label string, tx, rx endpoint, measuring *atomic.Bool) (*direction, error) {
	// Built before the windows, since each window judges sequence numbers against
	// the frontier its own sender publishes.
	txs := make([]*txStats, numStreams)
	for i := range txs {
		txs[i] = &txStats{}
	}
	d := &direction{
		txStats:   txs,
		streams:   newLossWindows(txs),
		measuring: measuring,
	}
	// Held open for the life of the run: the stats ioctl is issued five times a
	// second and reopening a socket for each one is pure overhead.
	statFD, err := unix.Socket(unix.AF_INET, unix.SOCK_DGRAM, 0)
	if err != nil {
		return nil, fmt.Errorf("%s stats socket: %w", label, err)
	}
	d.statFD = statFD

	d.poller, err = newNICPoller(statFD, tx.name, rx.name, &d.nic, measuring)
	if err != nil {
		return nil, fmt.Errorf("%s: %w", label, err)
	}
	d.win = newRateWindow(int(rateWindowSpan/sampleInterval) + 1)

	for i := 0; i < numStreams; i++ {
		et := uint16(etherBase + i)
		d.specs = append(d.specs, newFrameSpec(rx.mac, tx.mac, et, frameSizes))

		fd, err := openTxSocket(tx.idx)
		if err != nil {
			return nil, fmt.Errorf("%s tx socket: %w", label, err)
		}
		d.txFDs = append(d.txFDs, fd)

		fd, err = openRxSocket(rx.idx, et)
		if err != nil {
			return nil, fmt.Errorf("%s rx socket for 0x%04x: %w", label, et, err)
		}
		d.rxFDs = append(d.rxFDs, fd)
		d.rxStats = append(d.rxStats, &rxStats{})
	}

	return d, nil
}

func (d *direction) start(wg *sync.WaitGroup, done *atomic.Bool, rxReady *sync.WaitGroup, startTx <-chan struct{}) {
	for i, fd := range d.txFDs {
		w := &txWorker{
			fd:        fd,
			stream:    uint16(i),
			spec:      d.specs[i],
			batch:     batchSize,
			stats:     d.txStats[i],
			measuring: d.measuring,
			startTx:   startTx,
		}
		wg.Add(1)
		go func() {
			defer wg.Done()
			defer holdPanic()
			w.run(done)
		}()
	}
	for i, fd := range d.rxFDs {
		w := &rxWorker{
			fd:        fd,
			batch:     batchSize,
			stream:    uint16(i),
			spec:      d.specs[i],
			stats:     d.rxStats[i],
			loss:      &d.streams[i],
			measuring: d.measuring,
			ready:     rxReady,
		}
		wg.Add(1)
		go func() {
			defer wg.Done()
			defer holdPanic()
			w.run(done)
		}()
	}

	wg.Add(1)
	go func() {
		defer wg.Done()
		defer holdPanic()
		d.poller.run(done, startTx)
	}()
}

func (d *direction) close() {
	for _, fd := range d.txFDs {
		unix.Close(fd)
	}
	for _, fd := range d.rxFDs {
		unix.Close(fd)
	}
	unix.Close(d.statFD)
}