Push the new origin into the sample ring so a reset cannot underflow the totals

main.go

@@ -263,11 +263,13 @@ func (d *direction) snapshot() sample {
 }
 
 // Counters keep climbing in the workers, so resetting just moves the origin
-// everything is measured from. Rates are deliberately left running, since they
+// everything is measured from. Rates and the rolling error window are about now
-// are instantaneous and would only blink to zero and back.
+// 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(time.Now())
+	d.win.push(d.base)
 	d.mu.Unlock()
 
 	d.heldFrames = heldValue{}

main_test.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+// A reset re-bases from a fresh capture while the ring still holds buckets from
+// just before it, so the newest bucket must not be left behind the new origin.
+func TestResetDoesNotUnderflowTotals(t *testing.T) {
+	d := &direction{
+		win:     newRateWindow(8),
+		cable:   newCableStats(),
+		rxStats: []*rxStats{{}},
+	}
+
+	d.rxStats[0].frames.Store(100)
+	d.rxStats[0].bytes.Store(6400)
+	d.sample(time.Now())
+
+	d.rxStats[0].frames.Store(150)
+	d.rxStats[0].bytes.Store(9600)
+	d.reset()
+
+	v := d.displayView(time.Now())
+	if v.rxFrames != 0 {
+		t.Errorf("rxFrames = %d, want 0", v.rxFrames)
+	}
+	if v.rxGot != 0 {
+		t.Errorf("rxGot = %d, want 0", v.rxGot)
+	}
+
+	// The rolling window and the rates are about now rather than since the
+	// reset, so the buckets from before it have to survive.
+	if got := d.win.count(); got < 2 {
+		t.Errorf("ring holds %d buckets after reset, want the pre-reset history kept", got)
+	}
+}