Rate from the newest bucket, stamped when the counters are actually read

main.go

@@ -6,7 +6,6 @@ import (
 	"net"
 	"os"
 	"os/signal"
-	"slices"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -85,9 +84,10 @@ type counterSet struct {
 	nic   uint64
 }
 
-func (d *direction) capture(t time.Time) counterSet {
+func (d *direction) capture() counterSet {
 	d.sampleDrops()
-	return counterSet{t: t, s: d.snapshot(), drops: d.drops, nic: d.nic.Load()}
+	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
@@ -114,10 +114,9 @@ func (e errs) add(o errs) errs {
 // 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
+	buf    []counterSet
+	idx    int
+	filled bool
 }
 
 func newRateWindow(n int) *rateWindow {
@@ -148,30 +147,17 @@ func (w *rateWindow) at(i int) counterSet {
 	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 {
+func (w *rateWindow) latest(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 {
+	prev, cur := w.at(n-2), w.at(n-1)
+	secs := cur.t.Sub(prev.t).Seconds()
+	if secs <= 0 {
 		return 0
 	}
-	slices.Sort(w.scratch)
-	return w.scratch[len(w.scratch)/2]
+	return rate(prev, cur, secs)
 }
 
 func txRatePPS(p, c counterSet, secs float64) float64 {
@@ -248,7 +234,7 @@ func (d *direction) snapshot() sample {
 // ring so the newest bucket never sits behind it.
 func (d *direction) reset() {
 	d.mu.Lock()
-	d.base = d.capture(time.Now())
+	d.base = d.capture()
 	d.win.push(d.base)
 	d.mu.Unlock()
 
@@ -347,11 +333,11 @@ func totalView(views []view) view {
 	return t
 }
 
-func (d *direction) view(t time.Time) view {
+func (d *direction) view() view {
 	d.mu.Lock()
 	defer d.mu.Unlock()
 
-	now := d.capture(t)
+	now := d.capture()
 	p := d.prevConsole
 	d.prevConsole = now
 
@@ -367,9 +353,9 @@ func (d *direction) view(t time.Time) view {
 	return v
 }
 
-func (d *direction) sample(t time.Time) {
+func (d *direction) sample() {
 	d.mu.Lock()
-	d.win.push(d.capture(t))
+	d.win.push(d.capture())
 	d.mu.Unlock()
 }
 
@@ -386,10 +372,10 @@ func (d *direction) displayView(t time.Time) view {
 	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)
+	v.txPPS = d.win.latest(txRatePPS)
+	v.rxPPS = d.win.latest(rxRatePPS)
+	v.txGbps = d.win.latest(txRateGbps)
+	v.rxGbps = d.win.latest(rxRateGbps)
 	d.mu.Unlock()
 
 	v.rxFrames = d.heldFrames.get(t, v.rxFrames)
@@ -586,8 +572,8 @@ const (
 	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.
+// One sampler for both directions, so they are read back to back on one clock
+// rather than drifting apart on two.
 type sampler struct {
 	dirs []*direction
 }
@@ -599,9 +585,9 @@ func (s *sampler) run(done *atomic.Bool, startTx <-chan struct{}) {
 	defer tick.Stop()
 
 	for !done.Load() {
-		now := <-tick.C
+		<-tick.C
 		for _, d := range s.dirs {
-			d.sample(now)
+			d.sample()
 		}
 	}
 }
@@ -764,7 +750,7 @@ func run(aName, bName string, nsPerM float64) error {
 			// 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)
+				rows[i] = d.view()
 			}
 			length := "-"
 			if m, ok := cableMetres(rows, nsPerM); ok {

main_test.go

@@ -16,7 +16,7 @@ func TestResetDoesNotUnderflowTotals(t *testing.T) {
 
 	d.rxStats[0].frames.Store(100)
 	d.rxStats[0].bytes.Store(6400)
-	d.sample(time.Now())
+	d.sample()
 
 	d.rxStats[0].frames.Store(150)
 	d.rxStats[0].bytes.Store(9600)