Reduce peak client recalculation overhead

internal/dnstt/collector_test.go

@@ -1,6 +1,7 @@
 package dnstt
 
 import (
+	"net/netip"
 	"testing"
 	"time"
 )
@@ -59,3 +60,36 @@ func TestCollectorMatchesSubdomainsToRegisteredTunnel(t *testing.T) {
 		t.Fatalf("active clients = %d, want 1", tunnel.ActiveClients)
 	}
 }
+
+func TestCollectorUpdatesPeakWhenActiveClientChangesGeoKey(t *testing.T) {
+	now := time.Unix(1000, 0)
+	firstResolver := netip.MustParseAddr("192.0.2.53")
+	secondResolver := netip.MustParseAddr("198.51.100.53")
+	c := NewCollector(
+		[]string{"tunnel.example.com"},
+		WithNow(func() time.Time { return now }),
+		WithGeoResolver(fakeGeoResolver{
+			labelNames: []string{"asn"},
+			labels: map[netip.Addr]GeoLabels{
+				firstResolver:  {ASN: "64500"},
+				secondResolver: {ASN: "64501"},
+			},
+		}),
+	)
+
+	c.RecordQueryFrom("tunnel.example.com", "client-a", firstResolver, 120)
+	c.RecordQueryFrom("tunnel.example.com", "client-a", secondResolver, 120)
+
+	foundChangedASN := false
+	for _, series := range c.Snapshot().Tunnels["tunnel.example.com"].Series {
+		if series.ASN == "64501" && series.PeakClients != 1 {
+			t.Fatalf("peak clients for changed ASN = %d, want 1", series.PeakClients)
+		}
+		if series.ASN == "64501" {
+			foundChangedASN = true
+		}
+	}
+	if !foundChangedASN {
+		t.Fatal("series for changed ASN not found")
+	}
+}