package main

import (
	"flag"
	"fmt"
	"math"
	"net"
	"os"
	"os/signal"
	"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 {
	label   string
	short   string
	tx      endpoint
	rx      endpoint
	specs   []*frameSpec
	txStats []*txStats
	rxStats []*rxStats
	streams []lossWindow
	txFDs   []int
	rxFDs   []int

	probeSpec *frameSpec
	probeTxFD int
	probeRxFD int
	cable     *cableStats

	prevConsole sample
	win         *rateWindow
	est         *rateEstimators
	heldFrames  heldValue
	heldSent    heldValue
	drops       uint64
	errBase     sample
	dropBase    uint64
	nicNow      uint64
	nicBase     uint64
}

// Smoothing has to be steady against high-frequency noise yet still chase a
// real change quickly, with bounded state. So the gain is not fixed: the
// innovation is compared against a running estimate of the noise itself (mean
// absolute deviation, as in TCP's rtt/rttvar), and only an innovation that
// stands out above that noise is chased hard.
const (
	estAlphaCalm = 0.015
	estAlphaSnap = 0.45
	estMADBeta   = 0.05
	estNoiseK    = 3.0
)

type rateEstimator struct {
	minStep float64
	relStep float64

	est   float64
	mad   float64
	shown float64
	n     int
}

func (e *rateEstimator) update(x float64) {
	if e.n == 0 {
		e.est, e.shown, e.n = x, x, 1
		return
	}
	err := x - e.est
	abs := math.Abs(err)
	if e.n == 1 {
		e.mad, e.n = abs, 2
	} else {
		e.mad += (abs - e.mad) * estMADBeta
	}

	a := estAlphaCalm
	if e.mad > 0 {
		if excess := abs/(estNoiseK*e.mad) - 1; excess > 0 {
			a = estAlphaCalm + (estAlphaSnap-estAlphaCalm)*math.Min(excess, 1)
		}
	}
	e.est += err * a

	// A deadband on top, so the drawn text only changes when the estimate has
	// actually moved rather than on every frame.
	if math.Abs(e.est-e.shown) > math.Max(e.minStep, e.relStep*math.Abs(e.est)) {
		e.shown = e.est
	}
}

func (e *rateEstimator) value() float64 { return e.shown }

type rateEstimators struct {
	txGbps, rxGbps rateEstimator
	txPPS, rxPPS   rateEstimator
}

func newRateEstimators() *rateEstimators {
	return &rateEstimators{
		txGbps: rateEstimator{minStep: 0.02, relStep: 0.001},
		rxGbps: rateEstimator{minStep: 0.02, relStep: 0.001},
		txPPS:  rateEstimator{minStep: 2000, relStep: 0.002},
		rxPPS:  rateEstimator{minStep: 2000, relStep: 0.002},
	}
}

// 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
}

type rateSample struct {
	t                                    time.Time
	txFrames, txBytes, rxFrames, rxBytes uint64
}

// A sliding window: the counters are sampled every frame and the rate is taken
// across the whole window, so the figure moves every frame while still being
// measured over a long enough span to be steady.
type rateWindow struct {
	samples []rateSample
	idx     int
	filled  bool
}

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

func (w *rateWindow) push(s rateSample) {
	w.samples[w.idx] = s
	w.idx++
	if w.idx == len(w.samples) {
		w.idx = 0
		w.filled = true
	}
}

func (w *rateWindow) span() (oldest, newest rateSample, ok bool) {
	if !w.filled && w.idx < 2 {
		return oldest, newest, false
	}
	n := w.idx - 1
	if n < 0 {
		n = len(w.samples) - 1
	}
	o := 0
	if w.filled {
		o = w.idx
	}
	return w.samples[o], w.samples[n], true
}

type sample struct {
	txFrames, txBytes uint64
	rxFrames, rxBytes uint64
	lost, late        uint64
	crcErr, badMagic  uint64
	badLen            uint64
	txErrs, txShort   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()
		s.txShort += t.short.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.sampleDrops()
	d.errBase = d.snapshot()
	d.dropBase = d.drops
	d.heldFrames = heldValue{}
	d.heldSent = heldValue{}
	d.nicBase = d.nicNow
	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: "UPTIME", width: 9, right: true},
	{title: "DIR", width: 5},
	{title: "TX pps", width: 9, right: true},
	{title: "TX Gb/s", width: 7, right: true},
	{title: "RX pps", width: 9, right: true},
	{title: "RX Gb/s", width: 7, right: true},
	{title: "LOST", width: 11, right: true},
	{title: "LATE", width: 9, right: true},
	{title: "CRC", width: 7, right: true},
	{title: "BADMAG", width: 7, right: true},
	{title: "KDROP", width: 11, right: true},
	{title: "LINK", width: 13, right: true},
	{title: "ERRORS", width: 13, right: true},
	{title: "MIN ns", width: 9, right: true},
	{title: "LEN m", width: 6, right: true},
}

// One interval's numbers, shared by the console table and the framebuffer so
// both always show the same figures.
type view struct {
	txPPS, rxPPS     float64
	txGbps, rxGbps   float64
	txFrames, txSent uint64
	rxFrames, rxGot  uint64
	lost, late       uint64
	crc, badMagic    uint64
	kdrop, link      uint64
	errors           uint64
	cable            cableView
}

// Cumulative fields, which need no rate window and are identical for both the
// console and the display.
func (d *direction) counters(now sample) view {
	b := d.errBase
	v := view{
		txFrames: now.txFrames - b.txFrames,
		txSent:   now.txBytes - b.txBytes,
		rxFrames: now.rxFrames - b.rxFrames,
		rxGot:    now.rxBytes - b.rxBytes,
		lost:     now.lost - b.lost,
		late:     now.late - b.late,
		crc:      now.crcErr - b.crcErr,
		badMagic: now.badMagic - b.badMagic,
		kdrop:    d.drops - d.dropBase,
		cable:    d.cable.view(),
	}
	// A frame the stack refused and a frame the driver dropped are the same
	// failure seen from either side of the ring, and never the same frame twice:
	// a send that fails never reaches the driver to be dropped.
	v.link = d.nicNow - d.nicBase +
		(now.txErrs - b.txErrs) + (now.rxErrs - b.rxErrs)
	v.errors = v.lost + v.crc + v.badMagic + (now.badLen - b.badLen) + v.kdrop + v.link
	return v
}

func (d *direction) view(prev *sample, secs float64) view {
	now := d.snapshot()
	p := *prev
	*prev = now
	d.sampleDrops()

	v := d.counters(now)
	txF := now.txFrames - p.txFrames
	rxF := now.rxFrames - p.rxFrames
	v.txPPS = float64(txF) / secs
	v.rxPPS = float64(rxF) / secs
	v.txGbps = gbps(now.txBytes-p.txBytes, txF, secs)
	v.rxGbps = gbps(now.rxBytes-p.rxBytes, rxF, secs)
	return v
}

func (d *direction) displayView(t time.Time) view {
	now := d.snapshot()
	d.sampleDrops()
	d.win.push(rateSample{t, now.txFrames, now.txBytes, now.rxFrames, now.rxBytes})

	v := d.counters(now)
	v.txFrames = d.heldFrames.get(t, v.txFrames)
	v.txSent = d.heldSent.get(t, v.txSent)

	o, n, ok := d.win.span()
	if !ok {
		return v
	}
	secs := n.t.Sub(o.t).Seconds()
	if secs <= 0 {
		return v
	}
	txF := n.txFrames - o.txFrames
	rxF := n.rxFrames - o.rxFrames
	d.est.txPPS.update(float64(txF) / secs)
	d.est.rxPPS.update(float64(rxF) / secs)
	d.est.txGbps.update(gbps(n.txBytes-o.txBytes, txF, secs))
	d.est.rxGbps.update(gbps(n.rxBytes-o.rxBytes, rxF, secs))

	v.txPPS = d.est.txPPS.value()
	v.rxPPS = d.est.rxPPS.value()
	v.txGbps = d.est.txGbps.value()
	v.rxGbps = d.est.rxGbps.value()
	return v
}

func (d *direction) row(elapsed time.Duration, v view, target float64, length string) []string {
	return []string{
		uptime(elapsed),
		paint(d.short, cCyan),
		commas(uint64(v.txPPS)),
		rateCell(v.txGbps, target),
		commas(uint64(v.rxPPS)),
		rateCell(v.rxGbps, target),
		statusCell(v.lost),
		statusCell(v.late),
		statusCell(v.crc),
		statusCell(v.badMagic),
		statusCell(v.kdrop),
		statusCell(v.link),
		statusCell(v.errors),
		paint(v.cable.minText(), cCyan),
		paint(length, cCyan),
	}
}

// Read once a second rather than per frame, since these are sysfs files; the
// display uses whatever the last sample left behind.
func (d *direction) sampleNIC() {
	tx := readNIC(d.tx.name)
	rx := readNIC(d.rx.name)
	d.nicNow = tx.tx + rx.rx + tx.carrierDown
}

func buildDirection(label string, tx, rx endpoint, patIdx int, sizes []int, cfg config) (*direction, error) {
	d := &direction{
		label:   label,
		short:   tx.tag + "→" + rx.tag,
		tx:      tx,
		rx:      rx,
		streams: newLossWindows(cfg.streams),
		cable:   newCableStats(),
	}

	for i := 0; i < cfg.streams; i++ {
		et := uint16(etherBase + i)
		d.specs = append(d.specs, newFrameSpec(patIdx, 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{})
	}

	// The probe carries its own ethertype so it lands on a socket of its own, but
	// it is left unsteered: it is a few frames a second and does not need a queue
	// to itself, and the stamps are taken at the wire either way.
	d.probeSpec = newFrameSpec(patIdx, 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

	// Whatever the interfaces have counted before now is not ours.
	d.sampleNIC()
	d.nicBase = d.nicNow
	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)
	}()
}

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)
}

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

func main() {
	var (
		aName    = flag.String("a", "", "first interface")
		bName    = flag.String("b", "", "second interface")
		sizesArg = flag.String("sizes", "64,128,256,512,1024,1280,1514", "frame sizes in bytes, excluding FCS, cycled per packet")
		patArg   = flag.String("pattern", "prbs", "payload pattern")
		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")
		duplex   = flag.Bool("duplex", true, "run both directions simultaneously")
		zeroNS   = flag.Float64("zero-ns", 4327.5, "both directions summed at zero cable length; belongs to the media adapters, recalibrate when they change")
		nsPerM   = flag.Float64("ns-per-m", 10.909, "both directions summed, per metre of cable")
	)
	flag.Parse()

	if err := run(*aName, *bName, *sizesArg, *patArg,
		*streams, *batch, *duplex, *zeroNS, *nsPerM); err != nil {
		fmt.Fprintln(os.Stderr, "error:", err)
		os.Exit(1)
	}
}

const (
	reportInterval = time.Second
	// Redraw fast so the panel feels live, but measure rates over a much longer
	// window than a frame, since a frame's worth of a bursty sender is noise.
	displayInterval = 16 * time.Millisecond
	// Only long enough to take the edge off one frame's sample; the estimator
	// does the real smoothing, so this stays small and bounded.
	rateWindowSpan = 250 * time.Millisecond
	totalsHold     = 50 * time.Millisecond
)

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

	if aName == "" || bName == "" {
		return fmt.Errorf("both -a and -b are required")
	}
	sizes, err := parseSizes(sizesArg)
	if err != nil {
		return err
	}
	patIdx, err := patternIndex(patArg)
	if 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)
	}

	var fatal []string
	var tuneRows [][]string
	for _, r := range configureSystem(ifnames, ethertypes) {
		tuneRows = append(tuneRows, []string{r.item, r.status(), r.detail()})
		if r.fatal {
			fatal = append(fatal, r.item)
		}
	}
	fmt.Println(renderBox("HOST SETTINGS",
		[]string{"CHECK", "STATUS", "DETAIL"},
		[]bool{false, false, false}, tuneRows))
	if len(fatal) > 0 {
		return fmt.Errorf("cannot test with %s in this state", strings.Join(fatal, ", "))
	}

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

	var dirs []*direction
	d0, err := buildDirection(a.name+"->"+b.name, a, b, patIdx, sizes, cfg)
	if err != nil {
		return err
	}
	dirs = append(dirs, d0)
	if duplex {
		d1, err := buildDirection(b.name+"->"+a.name, b, a, patIdx, sizes, cfg)
		if err != nil {
			return err
		}
		dirs = append(dirs, d1)
	}
	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("TEST",
		[]string{"SETTING", "VALUE"},
		[]bool{false, false}, [][]string{
			{"pattern", patterns[patIdx].name},
			{"frame sizes", strings.Join(sizeStrs, " ")},
			{"streams", fmt.Sprintf("%d per direction, ethertypes 0x%04x-0x%04x, one rx queue each",
				nStreams, ethertypes[0], ethertypes[len(ethertypes)-1])},
			{"batch", fmt.Sprintf("%d frames per syscall", batch)},
			{"payload verify", "crc32c on every frame"},
			{"cable probe", fmt.Sprintf("%d-byte frame on ethertype 0x%04x every %s, hardware stamped at both macs",
				probeSize, cfg.probeEther, probeInterval)},
			{"duplex", fmt.Sprintf("%v", duplex)},
			{"socket buffers", fmt.Sprintf("sndbuf %s, rcvbuf %s (granted)",
				humanBytes(uint64(sockBufSize(dirs[0].txFDs[0], unix.SO_SNDBUF))),
				humanBytes(uint64(sockBufSize(dirs[0].rxFDs[0], unix.SO_RCVBUF))))},
		}))
	fmt.Println(paint("rates are per interval; error counts are cumulative, press space to reset them", cDim))
	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)
	}
	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.w, disp.fb.h)
	if err != nil {
		return fmt.Errorf("touchscreen: %w", err)
	}

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

	last := time.Now()
	views := make([]view, len(dirs))
	rows := make([]view, len(dirs))
	for _, d := range dirs {
		d.win = newRateWindow(int(rateWindowSpan/displayInterval) + 1)
		d.est = newRateEstimators()
	}
	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 now := <-frame.C:
			if x, y, down := touch.get(); disp.holdReset(x, y, down, now) {
				start = resetAll(dirs, stats)
			}
			for i, d := range dirs {
				views[i] = d.displayView(now)
			}
			disp.render(dirs, views, now.Sub(start), target, cfg.cableText(views))
		case now := <-tick.C:
			secs := now.Sub(last).Seconds()
			last = now
			elapsed := now.Sub(start)
			// Length needs both directions, so every row is sampled before any of
			// them is printed.
			for i, d := range dirs {
				d.sampleNIC()
				rows[i] = d.view(&d.prevConsole, secs)
			}
			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)
				}
			}
		}
	}
}