cabletest/main.go

package main

import (
	"flag"
	"fmt"
	"net"
	"os"
	"os/signal"
	"slices"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
	"syscall"
	"time"

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

const wireOverhead = 24

type endpoint struct {
	name  string
	tag   string
	idx   int
	mac   [6]byte
	mtu   int
	speed float64
}

func (e endpoint) macString() string {
	return fmt.Sprintf("%02x:%02x:%02x:%02x:%02x:%02x",
		e.mac[0], e.mac[1], e.mac[2], e.mac[3], e.mac[4], e.mac[5])
}

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

	probeSpec *frameSpec
	probeTxFD int
	probeRxFD int
	statFD    int
	cable     *cableStats

	// 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
	prevConsole counterSet
	win         *rateWindow
	drops       uint64
	base        counterSet

	heldFrames heldValue
	heldSent   heldValue
	nic        atomic.Uint64
	poller     *nicPoller
}

// Monotonic totals climb by tens of thousands per frame, which is unreadable
// churn at 60Hz, so the drawn value is held and refreshed a few times a second.
type heldValue struct {
	v  uint64
	at time.Time
}

func (h *heldValue) get(now time.Time, cur uint64) uint64 {
	if now.Sub(h.at) >= totalsHold {
		h.v, h.at = cur, now
	}
	return h.v
}

// 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(t time.Time) counterSet {
	d.sampleDrops()
	return counterSet{t: t, s: d.snapshot(), 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) total() uint64 {
	return e.lost + e.corrupt + e.link + e.internal
}

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
	scratch []float64
}

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)]
}

// The median is steady against a bursty sender yet only ever a rate some bucket
// actually measured, so a step change is shown as a step: the old value holds
// until half the ring has turned over and then the new one takes it, passing
// through at most the one bucket the crossing lands on. Averaging the ring
// instead would spend the whole span sliding through rates that never happened.
func (w *rateWindow) median(rate func(prev, cur counterSet, secs float64) float64) float64 {
	n := w.count()
	if n < 2 {
		return 0
	}
	w.scratch = w.scratch[:0]
	prev := w.at(0)
	for i := 1; i < n; i++ {
		cur := w.at(i)
		if secs := cur.t.Sub(prev.t).Seconds(); secs > 0 {
			w.scratch = append(w.scratch, rate(prev, cur, secs))
		}
		prev = cur
	}
	if len(w.scratch) == 0 {
		return 0
	}
	slices.Sort(w.scratch)
	return w.scratch[len(w.scratch)/2]
}

func txRatePPS(p, c counterSet, secs float64) float64 {
	return float64(c.s.txFrames-p.s.txFrames) / secs
}

func rxRatePPS(p, c counterSet, secs float64) float64 {
	return float64(c.s.rxFrames-p.s.rxFrames) / secs
}

func txRateGbps(p, c counterSet, secs float64) float64 {
	return gbps(c.s.txBytes-p.s.txBytes, c.s.txFrames-p.s.txFrames, secs)
}

func rxRateGbps(p, c counterSet, secs float64) float64 {
	return gbps(c.s.rxBytes-p.s.rxBytes, c.s.rxFrames-p.s.rxFrames, secs)
}

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

func lookupEndpoint(name string) (endpoint, error) {
	ifi, err := net.InterfaceByName(name)
	if err != nil {
		return endpoint{}, err
	}
	if len(ifi.HardwareAddr) != 6 {
		return endpoint{}, fmt.Errorf("%s: expected 6-byte MAC, got %q", name, ifi.HardwareAddr)
	}
	var mac [6]byte
	copy(mac[:], ifi.HardwareAddr)
	e := endpoint{name: name, idx: ifi.Index, mac: mac, mtu: ifi.MTU}
	if v, ok := readUint("/sys/class/net/" + name + "/speed"); ok {
		e.speed = float64(v) / 1000
	}
	return e, nil
}

func parseSizes(s string) ([]int, error) {
	var out []int
	for _, f := range strings.Split(s, ",") {
		f = strings.TrimSpace(f)
		if f == "" {
			continue
		}
		v, err := strconv.Atoi(f)
		if err != nil {
			return nil, fmt.Errorf("bad size %q: %w", f, err)
		}
		if v < minFrame {
			return nil, fmt.Errorf("size %d below minimum %d", v, minFrame)
		}
		out = append(out, v)
	}
	if len(out) == 0 {
		return nil, fmt.Errorf("no sizes given")
	}
	return out, nil
}

func (d *direction) snapshot() sample {
	var s sample
	for _, t := range d.txStats {
		s.txFrames += t.frames.Load()
		s.txBytes += t.bytes.Load()
		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.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 are deliberately left running, since they
// are instantaneous and would only blink to zero and back.
func (d *direction) reset() {
	d.mu.Lock()
	d.base = d.capture(time.Now())
	d.mu.Unlock()

	d.heldFrames = heldValue{}
	d.heldSent = heldValue{}
	d.cable.reset()
}

// Returns the new start time, so the uptime shown alongside the totals counts
// from the reset rather than from launch.
func resetAll(dirs []*direction, stats *streamTable) time.Time {
	for _, d := range dirs {
		d.reset()
	}
	stats.sinceHeader = 0
	fmt.Println(stats.rule("counters reset"))
	return time.Now()
}

func (d *direction) sampleDrops() {
	for _, fd := range d.rxFDs {
		d.drops += packetDrops(fd)
	}
}

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

var intervalCols = []colSpec{
	{title: "ELAPSED", width: 9, right: true},
	{title: "DIR", width: 5},
	{title: "TX packets/s", width: 12, right: true},
	{title: "TX bits/s", width: 10, right: true},
	{title: "RX packets/s", width: 12, right: true},
	{title: "RX bits/s", width: 10, right: true},
	{title: "LOST", width: 9, right: true},
	{title: "CORRUPT", width: 9, right: true},
	{title: "LINK", width: 9, right: true},
	{title: "INTERNAL", width: 9, right: true},
	{title: "ERRORS", width: 9, right: true},
	{title: "MIN ns", width: 9, right: true},
	{title: "LEN m", width: 6, right: true},
}

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

func errsBetween(b, n counterSet) errs {
	return errs{
		lost: n.s.lost - b.s.lost,
		// Three ways of noticing one thing: a payload that does not match its
		// checksum, a header that is not ours, and a length that cannot be.
		corrupt: (n.s.crcErr - b.s.crcErr) + (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,
		rxGot:    now.s.rxBytes - d.base.s.rxBytes,
		cable:    d.cable.view(),
		since:    errsBetween(d.base, now),
	}
}

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

func (d *direction) view(t time.Time) view {
	d.mu.Lock()
	defer d.mu.Unlock()

	now := d.capture(t)
	p := d.prevConsole
	d.prevConsole = now

	v := d.counters(now)
	secs := now.t.Sub(p.t).Seconds()
	if secs <= 0 {
		return v
	}
	v.txPPS = txRatePPS(p, now, secs)
	v.rxPPS = rxRatePPS(p, now, secs)
	v.txGbps = txRateGbps(p, now, secs)
	v.rxGbps = rxRateGbps(p, now, secs)
	return v
}

func (d *direction) sample(t time.Time) {
	d.mu.Lock()
	d.win.push(d.capture(t))
	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(t time.Time) view {
	d.mu.Lock()
	n := d.win.count()
	if n == 0 {
		d.mu.Unlock()
		return view{cable: d.cable.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.txPPS = d.win.median(txRatePPS)
	v.rxPPS = d.win.median(rxRatePPS)
	v.txGbps = d.win.median(txRateGbps)
	v.rxGbps = d.win.median(rxRateGbps)
	d.mu.Unlock()

	v.rxFrames = d.heldFrames.get(t, v.rxFrames)
	v.rxGot = d.heldSent.get(t, v.rxGot)
	return v
}

func (d *direction) row(elapsed time.Duration, v view, target float64, length string) []string {
	return []string{
		scaleTime(elapsed),
		paint(d.short, cCyan),
		scaleSI(v.txPPS),
		rateCell(v.txGbps*1e9, target*1e9),
		scaleSI(v.rxPPS),
		rateCell(v.rxGbps*1e9, target*1e9),
		statusCell(v.since.lost),
		statusCell(v.since.corrupt),
		statusCell(v.since.link),
		statusCell(v.since.internal),
		statusCell(v.since.total()),
		paint(v.cable.minText(), cCyan),
		paint(length, cCyan),
	}
}

// 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()
	d.prevConsole = d.base
}

func buildDirection(label string, tx, rx endpoint, sizes []int, cfg config) (*direction, error) {
	d := &direction{
		short:   tx.tag + "→" + rx.tag,
		streams: newLossWindows(cfg.streams),
		cable:   newCableStats(),
	}
	// 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)
	if err != nil {
		return nil, fmt.Errorf("%s: %w", label, err)
	}
	d.win = newRateWindow(int(rateWindowSpan/sampleInterval) + 1)

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

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

		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{})
	}

	// Deliberately given no flow rule: a few frames a second does not need a
	// queue of its own, and the stamps are taken at the wire either way.
	d.probeSpec = newFrameSpec(rx.mac, tx.mac, cfg.probeEther, []int{probeSize})
	fd, err := openTxSocket(tx.idx)
	if err != nil {
		return nil, fmt.Errorf("%s probe tx socket: %w", label, err)
	}
	if err := enableTxTimestamps(fd); err != nil {
		return nil, fmt.Errorf("%s probe tx timestamps: %w", label, err)
	}
	d.probeTxFD = fd
	fd, err = openRxSocket(rx.idx, cfg.probeEther)
	if err != nil {
		return nil, fmt.Errorf("%s probe rx socket: %w", label, err)
	}
	if err := enableRxTimestamps(fd); err != nil {
		return nil, fmt.Errorf("%s probe rx timestamps: %w", label, err)
	}
	d.probeRxFD = fd

	return d, nil
}

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

	sender := &probeSender{fd: d.probeTxFD, spec: d.probeSpec, stats: d.cable}
	wg.Add(1)
	go func() {
		defer wg.Done()
		sender.run(doneTx, startTx)
	}()

	receiver := &probeReceiver{fd: d.probeRxFD, stats: d.cable, ready: rxReady}
	wg.Add(1)
	go func() {
		defer wg.Done()
		receiver.run(doneRx)
	}()

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

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

type config struct {
	streams    int
	batch      int
	probeEther uint16
	nsPerM     float64
}

func main() {
	var (
		// The names the kernel gives the only two ports built into it, since as
		// PID 1 there is no udev to rename them and no command line to pass.
		aName    = flag.String("a", "eth0", "first interface")
		bName    = flag.String("b", "eth1", "second interface")
		sizesArg = flag.String("sizes", "64,128,256,512,1024,1280,1514", "frame sizes in bytes, excluding FCS, cycled per packet")
		streams  = flag.Int("streams", 7, "independent streams per direction, capped by rx rings; each gets its own ethertype, steered by a flow rule to its own rx queue")
		batch    = flag.Int("batch", 64, "frames per sendmmsg/recvmmsg call")

		nsPerM = flag.Float64("ns-per-m", 4.85, "mean of both directions, per metre of cable")
	)
	flag.Parse()

	// Nothing here is recoverable by the time it reaches this point, and as PID 1
	// a plain exit would panic the kernel anyway with less to show for it.
	if err := run(*aName, *bName, *sizesArg,
		*streams, *batch, *nsPerM); err != nil {
		panic(err)
	}
}

const (
	reportInterval = time.Second
	// Deliberately not tied to the refresh: letting a slow or blocked draw set
	// the sampling clock would stretch the window it reports.
	sampleInterval = 16 * time.Millisecond
	// How far back the shown errors reach and how many buckets the median runs
	// over, so a step in the rate lands half this late.
	rateWindowSpan = time.Second
	totalsHold     = 50 * time.Millisecond
)

// One sampler for both directions, so their buckets share an instant and the
// cable length, which needs a figure from each, never mixes two moments.
type sampler struct {
	dirs []*direction
}

func (s *sampler) run(done *atomic.Bool, startTx <-chan struct{}) {
	<-startTx

	tick := time.NewTicker(sampleInterval)
	defer tick.Stop()

	for !done.Load() {
		now := <-tick.C
		for _, d := range s.dirs {
			d.sample(now)
		}
	}
}

func run(aName, bName, sizesArg string,
	nStreams, batch int, nsPerM float64) error {

	sizes, err := parseSizes(sizesArg)
	if err != nil {
		return err
	}

	if err := reportChecks("BOOT", bootstrap()); err != nil {
		return err
	}

	a, err := lookupEndpoint(aName)
	if err != nil {
		return err
	}
	b, err := lookupEndpoint(bName)
	if err != nil {
		return err
	}
	for _, e := range []endpoint{a, b} {
		for _, s := range sizes {
			if s > e.mtu+ethHdrLen {
				return fmt.Errorf("size %d exceeds %s MTU %d (max frame %d)", s, e.name, e.mtu, e.mtu+ethHdrLen)
			}
		}
	}

	a.tag, b.tag = "A", "B"
	ifnames := []string{a.name, b.name}

	if nStreams < 1 {
		return fmt.Errorf("need at least one stream")
	}
	ethertypes := make([]uint16, nStreams)
	for i := range ethertypes {
		ethertypes[i] = uint16(etherBase + i)
	}

	if err := reportChecks("HOST SETTINGS", configureSystem(ifnames, ethertypes)); err != nil {
		return err
	}

	cfg := config{
		streams:    nStreams,
		batch:      batch,
		probeEther: uint16(etherBase + nStreams),
		nsPerM:     nsPerM,
	}

	var dirs []*direction
	for _, p := range [][2]endpoint{{a, b}, {b, a}} {
		d, err := buildDirection(p[0].name+"->"+p[1].name, p[0], p[1], sizes, cfg)
		if err != nil {
			return err
		}
		dirs = append(dirs, d)
	}
	defer func() {
		for _, d := range dirs {
			d.close()
		}
	}()

	var linkRows [][]string
	for _, e := range []endpoint{a, b} {
		linkRows = append(linkRows, []string{
			paint(e.tag, cCyan), e.name, e.macString(),
			fmt.Sprintf("%.0f Gb/s", e.speed), fmt.Sprintf("%d", e.mtu),
		})
	}
	fmt.Println(renderBox("LINKS",
		[]string{"TAG", "INTERFACE", "MAC", "SPEED", "MTU"},
		[]bool{false, false, false, true, true}, linkRows))

	target := a.speed
	if target <= 0 {
		target = 10
	}
	sizeStrs := make([]string, len(sizes))
	for i, s := range sizes {
		sizeStrs[i] = fmt.Sprintf("%d", s)
	}
	fmt.Println(renderBox("CONFIG",
		[]string{"SETTING", "VALUE"},
		[]bool{false, false}, [][]string{
			{"frame sizes", strings.Join(sizeStrs, " ")},
			{"streams", fmt.Sprintf("%d per direction, ethertypes 0x%04x-0x%04x",
				nStreams, ethertypes[0], ethertypes[len(ethertypes)-1])},
			{"probe", fmt.Sprintf("ethertype 0x%04x every %s", cfg.probeEther, probeInterval)},
			{"batch", fmt.Sprintf("%d frames per syscall", batch)},
			{"calibration", fmt.Sprintf("%g ns/m, zero taken from the shortest delay seen so far", nsPerM)},
			{"buffers", fmt.Sprintf("sndbuf %s, rcvbuf %s",
				humanBytes(uint64(sockBufSize(dirs[0].txFDs[0], unix.SO_SNDBUF))),
				humanBytes(uint64(sockBufSize(dirs[0].rxFDs[0], unix.SO_RCVBUF))))},
		}))
	fmt.Println()

	var doneTx, doneRx atomic.Bool
	var wg sync.WaitGroup
	var rxReady sync.WaitGroup
	startTx := make(chan struct{})
	for _, d := range dirs {
		rxReady.Add(len(d.rxFDs) + 1)
	}
	for _, d := range dirs {
		d.start(&wg, &doneTx, &doneRx, cfg, &rxReady, startTx)
	}
	samp := &sampler{dirs: dirs}
	wg.Add(1)
	go func() {
		defer wg.Done()
		samp.run(&doneRx, startTx)
	}()
	rxReady.Wait()

	sig := make(chan os.Signal, 1)
	signal.Notify(sig, syscall.SIGINT, syscall.SIGTERM)

	space, restoreTerm := watchSpace()
	defer restoreTerm()

	disp, err := newDisplay()
	if err != nil {
		return fmt.Errorf("display: %w", err)
	}
	defer disp.close()

	touch, err := watchTouch(disp.fb.pw, disp.fb.ph)
	if err != nil {
		return fmt.Errorf("touchscreen: %w", err)
	}

	for _, d := range dirs {
		d.primeCounters()
	}

	start := time.Now()
	close(startTx)
	tick := time.NewTicker(reportInterval)
	defer tick.Stop()

	views := make([]view, len(dirs))
	rows := make([]view, len(dirs))
	stats := &streamTable{cols: intervalCols, headerEvery: 20}
	for {
		select {
		case <-sig:
			doneTx.Store(true)
			doneRx.Store(true)
			wg.Wait()
			return nil
		case <-space:
			start = resetAll(dirs, stats)
		case <-disp.fb.flips:
			now := time.Now()
			px, py, down := touch.get()
			x, y := disp.fb.fromPanel(px, py)
			if disp.holdReset(x, y, down, now) {
				start = resetAll(dirs, stats)
			}
			for i, d := range dirs {
				views[i] = d.displayView(now)
			}
			// Empty until the probe has a stamp from each direction, so the
			// panel shows nothing there rather than a placeholder.
			cable := ""
			if m, ok := cfg.cableMetres(views); ok {
				cable = fmt.Sprintf("%.1f", m)
			}
			if err := disp.render(totalView(views), now.Sub(start), cable); err != nil {
				return err
			}
		case now := <-tick.C:
			elapsed := now.Sub(start)
			// Length needs both directions, so every row is sampled before any of
			// them is printed.
			for i, d := range dirs {
				rows[i] = d.view(now)
			}
			length := "-"
			if m, ok := cfg.cableMetres(rows); ok {
				length = fmt.Sprintf("%.1f", m)
			}
			for i, d := range dirs {
				for _, line := range stats.emit(d.row(elapsed, rows[i], target, length)) {
					fmt.Println(line)
				}
			}
		}
	}
}