Adopt repo standards: scaffold, policies, lint-clean (closes #1)

Standard scaffold: script/ entrypoints with the Makefile as thin shims, a Dockerfile whose lint and test phases gate the build, a Gitea CI workflow running script/cibuild, REPO_POLICIES.md, TODO.md, .editorconfig, .dockerignore, LICENSE, wider .gitignore. .golangci.yml is byte-identical to the canonical copy in the prompts repo.

211 lint findings fixed in code: package globals became functions/fields and a cobra command constructor, magic numbers became named constants, ctx is threaded into the probes, monitor loops split. Tests moved to external _test packages with export_test.go. Behavior unchanged.

Readers will trip over: make lint/test/check now need a Docker daemon; the personal rsync copy/run targets are gone and make run runs locally.
Disclosure: four //nolint:gosec remain on the fixed-argv ping/curl calls and the operator-chosen log file, as the reference repo annotates the same class.
Disclosure: module path left as-is.

Model: opus-4-8 (implementation); fable-5-1 (merge)

internal/cli/export_test.go

@@ -0,0 +1,8 @@
+//go:build linux
+
+package cli
+
+import "github.com/spf13/cobra"
+
+// NewRootCmd exposes newRootCmd for external tests.
+func NewRootCmd() *cobra.Command { return newRootCmd() }

internal/cli/root.go

@@ -1,6 +1,6 @@
 //go:build linux
-// +build linux
 
+// Package cli wires command-line flags to the network monitor and runs it.
 package cli
 
 import (
@@ -15,8 +15,8 @@ import (
 	"git.eeqj.de/sneak/rtnetmon/internal/monitor"
 )
 
-// Config holds the application configuration
-type Config struct {
+// config holds the application configuration.
+type config struct {
 	IfaceA  string
 	LabelA  string
 	IfaceB  string
@@ -25,94 +25,104 @@ type Config struct {
 	LogFile string
 }
 
-var (
-	cfg     Config
-	rootCmd = &cobra.Command{
-		Use:   "rtnetmon",
-		Short: "Real-time network monitoring dashboard",
-		Long: `rtnetmon is a dual-interface network monitoring dashboard that provides
-real-time visibility into network health, packet loss, and latency.`,
-		RunE: runMonitor,
-	}
-)
-
-// Default hosts for monitoring
-var (
-	defaultReachabilityHosts = []string{
+// defaultReachabilityHosts is the default reachability host list.
+func defaultReachabilityHosts() []string {
+	return []string{
 		"8.8.8.8", "8.8.4.4", "google.com", "github.com",
 		"console.aws.amazon.com", "console.cloud.google.com",
 		"fast.com", "datavi.be", "captive.apple.com",
 	}
+}
 
-	defaultPacketLossHosts = []string{
-		"github.com", "google.com", "8.8.8.8", "captive.apple.com", "62.115.190.68",
+// defaultPacketLossHosts is the default packet-loss host list.
+func defaultPacketLossHosts() []string {
+	return []string{
+		"github.com", "google.com", "8.8.8.8",
+		"captive.apple.com", "62.115.190.68",
 	}
+}
 
-	defaultTCPHosts = []string{
+// defaultTCPHosts is the default TCP connect host:port list.
+func defaultTCPHosts() []string {
+	return []string{
 		"datavi.be:443", "fast.com:443",
 		"console.aws.amazon.com:443", "console.cloud.google.com:443",
 		"captive.apple.com:80", "google.com:443",
 	}
-)
-
-func init() {
-	// Define flags
-	rootCmd.Flags().StringVar(&cfg.IfaceA, "ifaceA", "gu0", "primary network interface")
-	rootCmd.Flags().StringVar(&cfg.LabelA, "labelA", "gu LAN - VPN outbound", "label for ifaceA")
-	rootCmd.Flags().StringVar(&cfg.IfaceB, "ifaceB", "backhaul0", "secondary network interface")
-	rootCmd.Flags().StringVar(&cfg.LabelB, "labelB", "Cox cable direct", "label for ifaceB")
-	rootCmd.Flags().StringSliceVar(&cfg.Hosts, "hosts", defaultReachabilityHosts, "comma-separated reachability hosts")
-	rootCmd.Flags().StringVar(&cfg.LogFile, "logfile", "/tmp/rtnetmon.log", "path to log file")
-
-	// Bind flags to viper
-	_ = viper.BindPFlag("ifaceA", rootCmd.Flags().Lookup("ifaceA"))
-	_ = viper.BindPFlag("labelA", rootCmd.Flags().Lookup("labelA"))
-	_ = viper.BindPFlag("ifaceB", rootCmd.Flags().Lookup("ifaceB"))
-	_ = viper.BindPFlag("labelB", rootCmd.Flags().Lookup("labelB"))
-	_ = viper.BindPFlag("hosts", rootCmd.Flags().Lookup("hosts"))
-	_ = viper.BindPFlag("logfile", rootCmd.Flags().Lookup("logfile"))
 }
 
-func runMonitor(cmd *cobra.Command, args []string) error {
+// newRootCmd builds the cobra root command with its flags bound.
+func newRootCmd() *cobra.Command {
+	cfg := &config{}
+	cmd := &cobra.Command{
+		Use:   "rtnetmon",
+		Short: "Real-time network monitoring dashboard",
+		Long: `rtnetmon is a dual-interface network monitoring dashboard that provides
+real-time visibility into network health, packet loss, and latency.`,
+		RunE: func(_ *cobra.Command, _ []string) error {
+			return runMonitor(cfg)
+		},
+	}
+	registerFlags(cmd, cfg)
+
+	return cmd
+}
+
+// registerFlags defines and viper-binds the command's flags.
+func registerFlags(cmd *cobra.Command, cfg *config) {
+	f := cmd.Flags()
+	f.StringVar(&cfg.IfaceA, "ifaceA", "gu0", "primary network interface")
+	f.StringVar(&cfg.LabelA, "labelA", "gu LAN - VPN outbound", "label for ifaceA")
+	f.StringVar(&cfg.IfaceB, "ifaceB", "backhaul0", "secondary network interface")
+	f.StringVar(&cfg.LabelB, "labelB", "Cox cable direct", "label for ifaceB")
+	f.StringSliceVar(&cfg.Hosts, "hosts", defaultReachabilityHosts(),
+		"comma-separated reachability hosts")
+	f.StringVar(&cfg.LogFile, "logfile", "/tmp/rtnetmon.log", "path to log file")
+
+	for _, name := range []string{
+		"ifaceA", "labelA", "ifaceB", "labelB", "hosts", "logfile",
+	} {
+		_ = viper.BindPFlag(name, f.Lookup(name))
+	}
+}
+
+// runMonitor constructs and runs the monitor from cfg.
+func runMonitor(cfg *config) error {
 	monitor.Logf(cfg.LogFile, "Starting rtnetmon")
-	monitor.Logf(cfg.LogFile, "Monitoring interfaces %s and %s", cfg.IfaceA, cfg.IfaceB)
+	monitor.Logf(cfg.LogFile, "Monitoring interfaces %s and %s",
+		cfg.IfaceA, cfg.IfaceB)
 
-	// Create the monitor
-	mon := monitor.NewMonitor(cfg.IfaceA, cfg.LabelA, cfg.IfaceB, cfg.LabelB, cfg.LogFile)
+	mon := monitor.NewMonitor(cfg.IfaceA, cfg.LabelA, cfg.IfaceB, cfg.LabelB,
+		cfg.LogFile)
 
-	// Add reachability hosts
 	for _, host := range cfg.Hosts {
 		mon.AddReachabilityHost(host)
 	}
 
-	// Add packet loss hosts
-	for _, host := range defaultPacketLossHosts {
+	for _, host := range defaultPacketLossHosts() {
 		mon.AddPacketLossHost(host)
 	}
 
-	// Add TCP hosts
-	for _, host := range defaultTCPHosts {
+	for _, host := range defaultTCPHosts() {
 		mon.AddTCPHost(host)
 	}
 
-	// Create context with signal handling
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
-	// Handle signals
 	sig := make(chan os.Signal, 1)
 	signal.Notify(sig, os.Interrupt, syscall.SIGTERM)
+
 	go func() {
 		s := <-sig
 		monitor.Logf(cfg.LogFile, "Signal received: %v", s)
 		cancel()
 	}()
 
-	// Run the monitor
 	return mon.Run(ctx)
 }
 
-// Execute runs the root command
+// Execute builds the root command and runs it.
 func Execute() error {
-	return rootCmd.Execute()
+	return newRootCmd().Execute()
 }

internal/cli/root_test.go

@@ -1,36 +1,25 @@
 //go:build linux
-// +build linux
 
-package cli
+package cli_test
 
 import (
 	"testing"
+
+	"git.eeqj.de/sneak/rtnetmon/internal/cli"
 )
 
-// TestExecute tests that the CLI can be initialized
-func TestExecute(t *testing.T) {
-	// This is a simple compilation test to ensure the CLI package compiles
-	// We can't easily test the full Execute() function as it starts the UI
+func TestNewRootCmd(t *testing.T) {
+	t.Parallel()
 
-	// Test that rootCmd is properly initialized
-	if rootCmd == nil {
-		t.Fatal("rootCmd is nil")
+	cmd := cli.NewRootCmd()
+	if cmd.Use != "rtnetmon" {
+		t.Errorf("Use = %q, want %q", cmd.Use, "rtnetmon")
 	}
 
-	if rootCmd.Use != "rtnetmon" {
-		t.Errorf("Expected rootCmd.Use to be 'rtnetmon', got '%s'", rootCmd.Use)
-	}
-
-	// Test that default configuration is set
-	if len(defaultReachabilityHosts) == 0 {
-		t.Error("defaultReachabilityHosts is empty")
-	}
-
-	if len(defaultPacketLossHosts) == 0 {
-		t.Error("defaultPacketLossHosts is empty")
-	}
-
-	if len(defaultTCPHosts) == 0 {
-		t.Error("defaultTCPHosts is empty")
+	names := []string{"ifaceA", "labelA", "ifaceB", "labelB", "hosts", "logfile"}
+	for _, name := range names {
+		if cmd.Flags().Lookup(name) == nil {
+			t.Errorf("flag %q not registered", name)
+		}
 	}
 }

internal/monitor/export_test.go

@@ -0,0 +1,20 @@
+//go:build linux
+
+package monitor
+
+// Test-only accessors exposing unexported state for white-box assertions.
+
+// ReachabilityHosts returns the configured reachability hosts.
+func (m *Monitor) ReachabilityHosts() []string { return m.reachabilityHosts }
+
+// PacketLossHosts returns the configured packet-loss hosts.
+func (m *Monitor) PacketLossHosts() []string { return m.packetLossHosts }
+
+// TCPHosts returns the configured TCP hosts.
+func (m *Monitor) TCPHosts() []string { return m.tcpHosts }
+
+// InterfaceA returns the first monitored interface.
+func (m *Monitor) InterfaceA() *InterfaceStatus { return m.interfaceA }
+
+// InterfaceB returns the second monitored interface.
+func (m *Monitor) InterfaceB() *InterfaceStatus { return m.interfaceB }

internal/monitor/loops.go

@@ -1,278 +1,319 @@
 //go:build linux
-// +build linux
 
 package monitor
 
 import (
 	"context"
+	crand "crypto/rand"
 	"math"
-	"math/rand"
+	"math/big"
 	"strings"
 	"sync"
 	"time"
 )
 
-// UIUpdateChan is used to signal UI updates
-var UIUpdateChan = make(chan struct{}, 100)
+// Probe scheduling and change-detection thresholds.
+const (
+	jitterBaseMillis    = 100
+	jitterRangeMillis   = 800
+	lossChangeThreshold = 0.01
+	tcpChangeThreshold  = 20 // milliseconds
+)
 
-// reachLoop monitors reachability for hosts
+// randomOffset returns a startup jitter to avoid clustering probes at the
+// same instant across loops. crypto/rand is used so no weak PRNG is linked.
+func randomOffset() time.Duration {
+	n, err := crand.Int(crand.Reader, big.NewInt(jitterRangeMillis))
+	if err != nil {
+		return jitterBaseMillis * time.Millisecond
+	}
+
+	return time.Duration(jitterBaseMillis+n.Int64()) * time.Millisecond
+}
+
+// probeAll runs probe for each host concurrently and returns the results
+// keyed by host.
+func probeAll[T any](hosts []string, probe func(host string) T) map[string]T {
+	var (
+		wg  sync.WaitGroup
+		mu  sync.Mutex
+		res = make(map[string]T, len(hosts))
+	)
+
+	for _, h := range hosts {
+		wg.Add(1)
+
+		go func(host string) {
+			defer wg.Done()
+
+			v := probe(host)
+
+			mu.Lock()
+			res[host] = v
+			mu.Unlock()
+		}(h)
+	}
+
+	wg.Wait()
+
+	return res
+}
+
+// reachLoop monitors reachability for hosts on one interface.
 func (m *Monitor) reachLoop(ctx context.Context, st *InterfaceStatus, hosts []string) {
-	m.logf("Starting reachability monitoring for %s with %d hosts", st.Name, len(hosts))
-
-	// Add random offset to avoid clustering at 1-second intervals
-	randomOffset := time.Duration(100+rand.Intn(800)) * time.Millisecond
-	m.logf("Reachability monitoring for %s will start after %v offset", st.Name, randomOffset)
-	time.Sleep(randomOffset)
+	m.logf("Starting reachability monitoring for %s with %d hosts",
+		st.Name, len(hosts))
+	time.Sleep(randomOffset())
 
 	tk := time.NewTicker(time.Second)
 	defer tk.Stop()
+
 	for {
 		select {
 		case <-ctx.Done():
 			m.logf("Stopping reachability monitoring for %s", st.Name)
+
 			return
 		case <-tk.C:
-			var wg sync.WaitGroup
-			res := make(map[string]bool, len(hosts))
-			mu := sync.Mutex{}
-			for _, h := range hosts {
-				wg.Add(1)
-				go func(host string) {
-					defer wg.Done()
-					st.mu.Lock()
-					st.TotalICMPReq++
-					// Increase meter value when a packet is sent
-					st.MeterValue++
-					if st.MeterValue > m.MaxMeterValue {
-						st.MeterValue = m.MaxMeterValue
-					}
-					st.mu.Unlock()
-
-					ok := m.pingOnce(st.Name, host)
-					mu.Lock()
-					res[host] = ok
-					mu.Unlock()
-
-					st.mu.Lock()
-					if ok {
-						st.TotalICMPRep++
-						// Decrease meter value when a packet is successfully received
-						st.MeterValue--
-						if st.MeterValue < 0 {
-							st.MeterValue = 0
-						}
-						// Only update spinner when packets are successfully received
-						st.Spin()
-					} else {
-						st.DroppedCount++
-						st.LastDrop = time.Now()
-
-						// Track lost packets per host
-						st.LostPackets[host]++
-
-						// Trigger UI update on ping failure
-						select {
-						case UIUpdateChan <- struct{}{}:
-						default:
-						}
-					}
-					st.mu.Unlock()
-				}(h)
-			}
-			wg.Wait()
-
-			// Check if reachability status changed
-			statusChanged := false
-			st.mu.Lock()
-			for host, newStatus := range res {
-				if oldStatus, ok := st.Reachable[host]; !ok || oldStatus != newStatus {
-					statusChanged = true
-					break
-				}
-			}
-
-			st.Reachable = res
-			st.LastPing = time.Now()
-
-			// Check if all hosts are reachable
-			allReachable := true
-			for _, ok := range res {
-				if !ok {
-					allReachable = false
-					break
-				}
-			}
-
-			// If all hosts are reachable, gradually decay the meter value
-			if allReachable && st.MeterValue > 0 {
-				st.MeterValue--
-			}
-
-			st.mu.Unlock()
-
-			// Always trigger UI update when reachability status changes
-			if statusChanged {
-				select {
-				case UIUpdateChan <- struct{}{}:
-				default:
-				}
-			}
+			m.reachTick(ctx, st, hosts)
 		}
 	}
 }
 
-// lossLoop monitors packet loss for hosts
-func (m *Monitor) lossLoop(ctx context.Context, st *InterfaceStatus, hosts []string) {
-	m.logf("Starting packet loss monitoring for %s with %d hosts", st.Name, len(hosts))
+// reachTick pings every host concurrently and applies the results.
+func (m *Monitor) reachTick(ctx context.Context, st *InterfaceStatus, hosts []string) {
+	res := probeAll(hosts, func(host string) bool {
+		return m.reachProbe(ctx, st, host)
+	})
+	m.reachApply(st, res)
+}
 
-	// Add random offset to avoid clustering at periodic intervals
-	randomOffset := time.Duration(100+rand.Intn(800)) * time.Millisecond
-	m.logf("Packet loss monitoring for %s will start after %v offset", st.Name, randomOffset)
-	time.Sleep(randomOffset)
+// reachProbe pings a single host and updates per-host counters.
+func (m *Monitor) reachProbe(
+	ctx context.Context, st *InterfaceStatus, host string,
+) bool {
+	st.mu.Lock()
+	st.TotalICMPReq++
+	st.MeterValue = min(st.MeterValue+1, m.MaxMeterValue)
+	st.mu.Unlock()
+
+	ok := m.pingOnce(ctx, st.Name, host)
+
+	st.mu.Lock()
+	defer st.mu.Unlock()
+
+	if ok {
+		st.TotalICMPRep++
+		st.MeterValue = max(st.MeterValue-1, 0)
+		st.Spin()
+
+		return true
+	}
+
+	st.DroppedCount++
+	st.LastDrop = time.Now()
+	st.LostPackets[host]++
+
+	m.notifyUI()
+
+	return false
+}
+
+// reachApply stores the round's results and decays the meter when clean.
+func (m *Monitor) reachApply(st *InterfaceStatus, res map[string]bool) {
+	st.mu.Lock()
+	changed := reachChanged(st.Reachable, res)
+	st.Reachable = res
+	st.LastPing = time.Now()
+
+	if allReachable(res) && st.MeterValue > 0 {
+		st.MeterValue--
+	}
+	st.mu.Unlock()
+
+	if changed {
+		m.notifyUI()
+	}
+}
+
+// reachChanged reports whether any host's reachability differs from before.
+func reachChanged(old, cur map[string]bool) bool {
+	for host, newStatus := range cur {
+		if oldStatus, ok := old[host]; !ok || oldStatus != newStatus {
+			return true
+		}
+	}
+
+	return false
+}
+
+// allReachable reports whether every host in the map is reachable.
+func allReachable(res map[string]bool) bool {
+	for _, ok := range res {
+		if !ok {
+			return false
+		}
+	}
+
+	return true
+}
+
+// lossLoop monitors packet loss for hosts on one interface.
+func (m *Monitor) lossLoop(ctx context.Context, st *InterfaceStatus, hosts []string) {
+	m.logf("Starting packet loss monitoring for %s with %d hosts",
+		st.Name, len(hosts))
+	time.Sleep(randomOffset())
 
 	tk := time.NewTicker(m.PacketLossPeriod)
 	defer tk.Stop()
+
 	for {
 		select {
 		case <-ctx.Done():
 			m.logf("Stopping packet loss monitoring for %s", st.Name)
+
 			return
 		case <-tk.C:
-			var wg sync.WaitGroup
-			res := make(map[string]float64, len(hosts))
-			mu := sync.Mutex{}
-			for _, h := range hosts {
-				wg.Add(1)
-				go func(host string) {
-					defer wg.Done()
-					lp := m.lossPercent(st.Name, host)
-					mu.Lock()
-					res[host] = lp
-					mu.Unlock()
-
-					st.mu.Lock()
-					if lp == 0 {
-						// Only update spinner when there's 0% packet loss
-						st.Spin()
-					} else {
-						// Calculate approximate number of lost packets based on loss percentage
-						lostPackets := int(math.Ceil(float64(m.PacketLossPings) * lp))
-
-						// Update dropped count with the number of lost packets
-						st.DroppedCount += lostPackets
-
-						// Update last drop time if packets were lost
-						if lostPackets > 0 {
-							st.LastDrop = time.Now()
-						}
-
-						// Track lost packets per host
-						st.LostPackets[host] += lostPackets
-
-						// Trigger UI update on packet loss
-						select {
-						case UIUpdateChan <- struct{}{}:
-						default:
-						}
-					}
-					st.mu.Unlock()
-				}(h)
-			}
-			wg.Wait()
-
-			// Check if loss status changed
-			statusChanged := false
-			st.mu.Lock()
-			for host, newLoss := range res {
-				if oldLoss, ok := st.Loss[host]; !ok || math.Abs(oldLoss-newLoss) > 0.01 {
-					statusChanged = true
-					break
-				}
-			}
-
-			for k, v := range res {
-				st.Loss[k] = v
-			}
-			st.mu.Unlock()
-
-			// Always trigger UI update when loss status changes
-			if statusChanged {
-				select {
-				case UIUpdateChan <- struct{}{}:
-				default:
-				}
-			}
+			m.lossTick(ctx, st, hosts)
 		}
 	}
 }
 
-// tcpLoop monitors TCP connectivity for hosts
+// lossTick measures loss for every host concurrently and applies results.
+func (m *Monitor) lossTick(ctx context.Context, st *InterfaceStatus, hosts []string) {
+	res := probeAll(hosts, func(host string) float64 {
+		return m.lossProbe(ctx, st, host)
+	})
+	m.lossApply(st, res)
+}
+
+// lossProbe measures loss for a host and updates per-host counters.
+func (m *Monitor) lossProbe(
+	ctx context.Context, st *InterfaceStatus, host string,
+) float64 {
+	lp := m.lossPercent(ctx, st.Name, host)
+
+	st.mu.Lock()
+	defer st.mu.Unlock()
+
+	if lp == 0 {
+		st.Spin()
+
+		return lp
+	}
+
+	lostPackets := int(math.Ceil(float64(m.PacketLossPings) * lp))
+	st.DroppedCount += lostPackets
+
+	if lostPackets > 0 {
+		st.LastDrop = time.Now()
+	}
+
+	st.LostPackets[host] += lostPackets
+
+	m.notifyUI()
+
+	return lp
+}
+
+// lossApply stores the round's loss results.
+func (m *Monitor) lossApply(st *InterfaceStatus, res map[string]float64) {
+	st.mu.Lock()
+	changed := lossChanged(st.Loss, res)
+
+	for k, v := range res {
+		st.Loss[k] = v
+	}
+	st.mu.Unlock()
+
+	if changed {
+		m.notifyUI()
+	}
+}
+
+// lossChanged reports whether any host's loss moved beyond the threshold.
+func lossChanged(old, cur map[string]float64) bool {
+	for host, newLoss := range cur {
+		if oldLoss, ok := old[host]; !ok || math.Abs(oldLoss-newLoss) > lossChangeThreshold {
+			return true
+		}
+	}
+
+	return false
+}
+
+// tcpLoop monitors TCP connectivity for hosts on one interface.
 func (m *Monitor) tcpLoop(ctx context.Context, st *InterfaceStatus, hosts []string) {
 	m.logf("Starting TCP monitoring for %s with %d hosts", st.Name, len(hosts))
-
-	// Add random offset to avoid clustering at 1-second intervals
-	randomOffset := time.Duration(100+rand.Intn(800)) * time.Millisecond
-	m.logf("TCP monitoring for %s will start after %v offset", st.Name, randomOffset)
-	time.Sleep(randomOffset)
+	time.Sleep(randomOffset())
 
 	tk := time.NewTicker(time.Second)
 	defer tk.Stop()
+
 	for {
 		select {
 		case <-ctx.Done():
 			m.logf("Stopping TCP monitoring for %s", st.Name)
+
 			return
 		case <-tk.C:
-			statusChanged := false
-
-			for _, hp := range hosts {
-				ms := float64(m.tcpDuration(st.Name, hp).Milliseconds())
-				st.mu.Lock()
-
-				// Check if TCP latency significantly changed
-				hist := st.TCP[hp]
-				if len(hist) > 0 {
-					lastMs := hist[len(hist)-1]
-					if math.Abs(lastMs-ms) > 20 { // 20ms threshold for significant change
-						statusChanged = true
-					}
-				} else {
-					// First measurement
-					statusChanged = true
-				}
-
-				if ms < float64(m.TCPTimeout.Milliseconds()) {
-					// Only update spinner on successful TCP connections
-					st.Spin()
-				} else {
-					// Trigger UI update on TCP timeout
-					select {
-					case UIUpdateChan <- struct{}{}:
-					default:
-					}
-				}
-
-				if len(hist) >= m.StatsHistory {
-					hist = hist[1:]
-				}
-				st.TCP[hp] = append(hist, ms)
-
-				// Update lost packets for the host (without port)
-				hostName := strings.Split(hp, ":")[0]
-				if ms >= float64(m.TCPTimeout.Milliseconds()) {
-					st.LostPackets[hostName]++
-				}
-
-				st.mu.Unlock()
-			}
-
-			// Always trigger UI update when TCP status changes significantly
-			if statusChanged {
-				select {
-				case UIUpdateChan <- struct{}{}:
-				default:
-				}
-			}
+			m.tcpTick(st, hosts)
 		}
 	}
 }
+
+// tcpTick measures TCP latency for each host and redraws on change.
+func (m *Monitor) tcpTick(st *InterfaceStatus, hosts []string) {
+	changed := false
+
+	for _, hp := range hosts {
+		if m.tcpProbe(st, hp) {
+			changed = true
+		}
+	}
+
+	if changed {
+		m.notifyUI()
+	}
+}
+
+// tcpProbe measures one host's latency, records it, and reports whether the
+// latency changed significantly.
+func (m *Monitor) tcpProbe(st *InterfaceStatus, hp string) bool {
+	ms := float64(m.tcpDuration(st.Name, hp).Milliseconds())
+
+	st.mu.Lock()
+	defer st.mu.Unlock()
+
+	hist := st.TCP[hp]
+	changed := tcpSignificant(hist, ms)
+
+	if ms < float64(m.TCPTimeout.Milliseconds()) {
+		st.Spin()
+	} else {
+		m.notifyUI()
+	}
+
+	if len(hist) >= m.StatsHistory {
+		hist = hist[1:]
+	}
+
+	st.TCP[hp] = append(hist, ms)
+
+	host := strings.Split(hp, ":")[0]
+	if ms >= float64(m.TCPTimeout.Milliseconds()) {
+		st.LostPackets[host]++
+	}
+
+	return changed
+}
+
+// tcpSignificant reports whether ms differs meaningfully from the last
+// sample (or there is no prior sample).
+func tcpSignificant(hist []float64, ms float64) bool {
+	if len(hist) == 0 {
+		return true
+	}
+
+	return math.Abs(hist[len(hist)-1]-ms) > tcpChangeThreshold
+}

internal/monitor/monitor.go

@@ -1,11 +1,14 @@
 //go:build linux
-// +build linux
 
+// Package monitor implements the dual-interface real-time network
+// monitoring dashboard: ICMP reachability, packet loss, TCP latency, and
+// the terminal UI that renders them.
 package monitor
 
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"math"
 	"net"
@@ -19,16 +22,48 @@ import (
 	tcell "github.com/gdamore/tcell/v2"
 )
 
-// Non-configurable constants (meter characters)
+// Meter glyphs used to render the ASCII loss meter.
 const (
-	// ASCII characters for the meter
 	MeterStart = '['
 	MeterEnd   = ']'
 	MeterFill  = '='
 	MeterEmpty = ' '
 )
 
-// Monitor represents the network monitoring system
+// Default monitor timing configuration.
+const (
+	defaultICMPTimeout      = 500 * time.Millisecond
+	defaultTCPTimeout       = 500 * time.Millisecond
+	defaultPacketLossPings  = 20
+	defaultPacketLossPeriod = 5 * time.Second
+	defaultStatsHistory     = 300
+	defaultScreenRefresh    = 500 * time.Millisecond
+)
+
+// Default display geometry (column widths and meter sizing).
+const (
+	defaultMeterWidth     = 7
+	defaultMeterFillWidth = 5
+	defaultMaxMeterValue  = 10
+	defaultHostWidth      = 30
+	defaultNumWidth       = 7
+	defaultStdWidth       = 8
+	defaultNWidth         = 6
+	defaultLostWidth      = 7
+)
+
+// Miscellaneous timing constants.
+const (
+	ipInfoTimeout    = 2 * time.Second
+	lossQueryTimeout = 3 * time.Second
+	uiUpdateBuffer   = 100
+	logFileMode      = 0o644
+)
+
+// errNoIPv4 is returned when an interface has no usable IPv4 address.
+var errNoIPv4 = errors.New("no IPv4 address on interface")
+
+// Monitor represents the network monitoring system.
 type Monitor struct {
 	// Configuration
 	ICMPTimeout      time.Duration
@@ -63,95 +98,95 @@ type Monitor struct {
 	// Runtime state
 	screen    tcell.Screen
 	startTime time.Time
+	uiUpdate  chan struct{}
 
 	// Synchronization
 	mu sync.RWMutex
 }
 
-// NewMonitor creates a new Monitor instance with default settings
+// NewMonitor creates a new Monitor instance with default settings.
 func NewMonitor(ifaceA, labelA, ifaceB, labelB, logFile string) *Monitor {
 	return &Monitor{
-		// Default timing configuration
-		ICMPTimeout:      500 * time.Millisecond,
-		TCPTimeout:       500 * time.Millisecond,
-		PacketLossPings:  20,
-		PacketLossPeriod: 5 * time.Second,
-		StatsHistory:     300,
-		ScreenRefresh:    500 * time.Millisecond,
+		ICMPTimeout:      defaultICMPTimeout,
+		TCPTimeout:       defaultTCPTimeout,
+		PacketLossPings:  defaultPacketLossPings,
+		PacketLossPeriod: defaultPacketLossPeriod,
+		StatsHistory:     defaultStatsHistory,
+		ScreenRefresh:    defaultScreenRefresh,
 
-		// Default display configuration
-		MeterWidth:     7,
-		MeterFillWidth: 5,
-		MaxMeterValue:  10,
-		HostWidth:      30,
-		NumWidth:       7,
-		StdWidth:       8,
-		NWidth:         6,
-		LostWidth:      7,
+		MeterWidth:     defaultMeterWidth,
+		MeterFillWidth: defaultMeterFillWidth,
+		MaxMeterValue:  defaultMaxMeterValue,
+		HostWidth:      defaultHostWidth,
+		NumWidth:       defaultNumWidth,
+		StdWidth:       defaultStdWidth,
+		NWidth:         defaultNWidth,
+		LostWidth:      defaultLostWidth,
 
-		// Initialize host lists
 		reachabilityHosts: []string{},
 		packetLossHosts:   []string{},
 		tcpHosts:          []string{},
 
-		// Initialize interfaces
 		interfaceA: NewInterfaceStatus(ifaceA, labelA),
 		interfaceB: NewInterfaceStatus(ifaceB, labelB),
 
-		// Logging
-		logFile: logFile,
-
+		logFile:   logFile,
 		startTime: time.Now(),
+		uiUpdate:  make(chan struct{}, uiUpdateBuffer),
 	}
 }
 
-// AddReachabilityHost adds a host for reachability monitoring
+// AddReachabilityHost adds a host for reachability monitoring.
 func (m *Monitor) AddReachabilityHost(host string) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
+
 	m.reachabilityHosts = append(m.reachabilityHosts, host)
 }
 
-// AddPacketLossHost adds a host for packet loss monitoring
+// AddPacketLossHost adds a host for packet loss monitoring.
 func (m *Monitor) AddPacketLossHost(host string) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
+
 	m.packetLossHosts = append(m.packetLossHosts, host)
 }
 
-// AddTCPHost adds a host:port for TCP connectivity monitoring
+// AddTCPHost adds a host:port for TCP connectivity monitoring.
 func (m *Monitor) AddTCPHost(hostPort string) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
+
 	m.tcpHosts = append(m.tcpHosts, hostPort)
 }
 
-// Run starts the monitoring system
+// Run starts the monitoring system.
 func (m *Monitor) Run(ctx context.Context) error {
 	m.logf("Starting monitor run")
-
-	// Initialize screen
 	m.logf("Initializing screen")
+
 	scr, err := tcell.NewScreen()
 	if err != nil {
 		m.logf("Error creating screen: %v", err)
+
 		return fmt.Errorf("error creating screen: %w", err)
 	}
-	if err = scr.Init(); err != nil {
+
+	err = scr.Init()
+	if err != nil {
 		m.logf("Error initializing screen: %v", err)
+
 		return fmt.Errorf("error initializing screen: %w", err)
 	}
+
 	m.screen = scr
 	m.logf("Screen initialized")
 
-	// Create cancellable context
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
-	// Handle keyboard events
 	go m.keyboardEventLoop(ctx, cancel)
 
-	// Start monitoring goroutines
 	m.logf("Starting monitoring goroutines")
 	m.mu.RLock()
 	reachHosts := append([]string{}, m.reachabilityHosts...)
@@ -166,7 +201,6 @@ func (m *Monitor) Run(ctx context.Context) error {
 	go m.tcpLoop(ctx, m.interfaceA, tcpHosts)
 	go m.tcpLoop(ctx, m.interfaceB, tcpHosts)
 
-	// Run UI loop
 	m.logf("Starting UI loop")
 	m.uiLoop(ctx)
 	m.logf("UI loop exited, monitor ending")
@@ -174,36 +208,49 @@ func (m *Monitor) Run(ctx context.Context) error {
 	return nil
 }
 
-// keyboardEventLoop handles keyboard input
-func (m *Monitor) keyboardEventLoop(ctx context.Context, cancel context.CancelFunc) {
-	m.logf("Starting keyboard event loop")
-	for {
-		if ev := m.screen.PollEvent(); ev != nil {
-			m.logf("Event received: %T", ev)
-			if ke, ok := ev.(*tcell.EventKey); ok {
-				m.logf("Key event: %v, rune: %c", ke.Key(), ke.Rune())
-				if ke.Key() == tcell.KeyCtrlC || ke.Rune() == 'q' {
-					m.logf("Quit key detected")
-					cancel()
-					return
-				}
-			}
-
-			// Trigger UI update on any event
-			select {
-			case UIUpdateChan <- struct{}{}:
-			default:
-			}
-		}
+// notifyUI requests a screen redraw without blocking if one is pending.
+func (m *Monitor) notifyUI() {
+	select {
+	case m.uiUpdate <- struct{}{}:
+	default:
 	}
 }
 
-// logf logs a formatted message
-func (m *Monitor) logf(format string, v ...interface{}) {
+// keyboardEventLoop handles keyboard input. The context is accepted for a
+// uniform loop signature; shutdown is driven by cancel on the quit key and
+// by the screen being finalized (PollEvent then returns nil).
+func (m *Monitor) keyboardEventLoop(_ context.Context, cancel context.CancelFunc) {
+	m.logf("Starting keyboard event loop")
+
+	for {
+		ev := m.screen.PollEvent()
+		if ev == nil {
+			continue
+		}
+
+		m.logf("Event received: %T", ev)
+
+		if ke, ok := ev.(*tcell.EventKey); ok {
+			m.logf("Key event: %v, rune: %c", ke.Key(), ke.Rune())
+
+			if ke.Key() == tcell.KeyCtrlC || ke.Rune() == 'q' {
+				m.logf("Quit key detected")
+				cancel()
+
+				return
+			}
+		}
+
+		m.notifyUI()
+	}
+}
+
+// logf logs a formatted message.
+func (m *Monitor) logf(format string, v ...any) {
 	Logf(m.logFile, format, v...)
 }
 
-// InterfaceStatus holds the runtime status for a network interface
+// InterfaceStatus holds the runtime status for a network interface.
 type InterfaceStatus struct {
 	Name, Label, IPInfo        string
 	Reachable                  map[string]bool
@@ -218,7 +265,7 @@ type InterfaceStatus struct {
 	mu                         sync.RWMutex
 }
 
-// NewInterfaceStatus creates a new interface status
+// NewInterfaceStatus creates a new interface status.
 func NewInterfaceStatus(name, label string) *InterfaceStatus {
 	return &InterfaceStatus{
 		Name:        name,
@@ -232,137 +279,164 @@ func NewInterfaceStatus(name, label string) *InterfaceStatus {
 	}
 }
 
-// ipInfoResp holds the IP info response
+// ipInfoResp holds the ipinfo.io response.
 type ipInfoResp struct {
 	IP       string `json:"ip"`
 	Hostname string `json:"hostname"`
 	Org      string `json:"org"`
 }
 
-// fetchIPInfo fetches IP information for an interface
+// fetchIPInfo fetches public IP information as seen from an interface.
 func fetchIPInfo(iface string) string {
-	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	ctx, cancel := context.WithTimeout(context.Background(), ipInfoTimeout)
 	defer cancel()
-	out, _ := exec.CommandContext(ctx, "curl", "-s", "--interface", iface, "--max-time", "2", "ipinfo.io").Output()
+
+	cmd := exec.CommandContext( //nolint:gosec // G204: fixed argv, operator CLI input
+		ctx, "curl", "-s", "--interface", iface, "--max-time", "2", "ipinfo.io")
+
+	out, _ := cmd.Output()
+
 	var r ipInfoResp
+
 	_ = json.Unmarshal(out, &r)
 	if r.IP == "" {
 		return "(ipinfo error)"
 	}
+
 	return fmt.Sprintf("%s [%s] %s", r.IP, r.Hostname, r.Org)
 }
 
-// pingOnce performs a single ping
-func (m *Monitor) pingOnce(iface, host string) bool {
-	ctx, cancel := context.WithTimeout(context.Background(), m.ICMPTimeout)
+// pingOnce performs a single ping over the named interface.
+func (m *Monitor) pingOnce(ctx context.Context, iface, host string) bool {
+	ctx, cancel := context.WithTimeout(ctx, m.ICMPTimeout)
 	defer cancel()
-	return exec.CommandContext(ctx, "ping", "-I", iface, "-c1", "-W1", host).Run() == nil
+
+	cmd := exec.CommandContext( //nolint:gosec // G204: fixed argv, operator CLI input
+		ctx, "ping", "-I", iface, "-c1", "-W1", host)
+
+	return cmd.Run() == nil
 }
 
-// lossPercent calculates packet loss percentage
-func (m *Monitor) lossPercent(iface, host string) float64 {
-	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+// lossPercent measures the packet loss fraction (0..1) for a host.
+func (m *Monitor) lossPercent(ctx context.Context, iface, host string) float64 {
+	ctx, cancel := context.WithTimeout(ctx, lossQueryTimeout)
 	defer cancel()
-	out, err := exec.CommandContext(ctx, "ping", "-q", "-i", "0.05",
-		"-c", fmt.Sprint(m.PacketLossPings), "-W1", "-I", iface, host).CombinedOutput()
+
+	cmd := exec.CommandContext( //nolint:gosec // G204: fixed argv, operator CLI input
+		ctx, "ping", "-q", "-i", "0.05",
+		"-c", strconv.Itoa(m.PacketLossPings), "-W1", "-I", iface, host)
+
+	out, err := cmd.CombinedOutput()
 	if err != nil {
 		return 1.0
 	}
+
 	for _, ln := range strings.Split(string(out), "\n") {
-		if strings.Contains(ln, "packet loss") {
-			for _, f := range strings.Fields(ln) {
-				if strings.HasSuffix(f, "%") {
-					p, _ := strconv.ParseFloat(strings.TrimSuffix(f, "%"), 64)
-					return p / 100.0
-				}
+		if !strings.Contains(ln, "packet loss") {
+			continue
+		}
+
+		for _, f := range strings.Fields(ln) {
+			before, ok := strings.CutSuffix(f, "%")
+			if !ok {
+				continue
 			}
+
+			p, _ := strconv.ParseFloat(before, 64)
+
+			return p / percentFull
 		}
 	}
+
 	return 1.0
 }
 
-// localAddr gets the local address for an interface
+// localAddr returns the first IPv4 address bound to an interface.
 func localAddr(iface string) (net.Addr, error) {
 	ifi, err := net.InterfaceByName(iface)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("interface %s: %w", iface, err)
 	}
+
 	add, _ := ifi.Addrs()
 	for _, a := range add {
 		if ipnet, ok := a.(*net.IPNet); ok && ipnet.IP.To4() != nil {
 			return &net.TCPAddr{IP: ipnet.IP}, nil
 		}
 	}
-	return nil, fmt.Errorf("no IPv4 on %s", iface)
+
+	return nil, fmt.Errorf("%w: %s", errNoIPv4, iface)
 }
 
-// tcpDuration measures TCP connection duration
+// tcpDuration measures how long a TCP connection to hp takes over iface.
 func (m *Monitor) tcpDuration(iface, hp string) time.Duration {
 	la, err := localAddr(iface)
 	if err != nil {
 		return m.TCPTimeout
 	}
+
 	d := net.Dialer{Timeout: m.TCPTimeout, LocalAddr: la}
 	st := time.Now()
+
 	c, err := d.Dial("tcp", hp)
 	if err != nil {
 		return m.TCPTimeout
 	}
-	c.Close()
+
+	_ = c.Close()
+
 	return time.Since(st)
 }
 
-// MinMaxAvgStd calculates min, max, average and standard deviation
-func MinMaxAvgStd(xs []float64) (min, max, avg, std float64) {
+// MinMaxAvgStd returns the minimum, maximum, mean and standard deviation.
+func MinMaxAvgStd(xs []float64) (float64, float64, float64, float64) {
 	if len(xs) == 0 {
-		return
+		return 0, 0, 0, 0
 	}
-	min, max = xs[0], xs[0]
+
+	mn, mx := xs[0], xs[0]
+
 	var sum float64
+
 	for _, v := range xs {
-		if v < min {
-			min = v
-		}
-		if v > max {
-			max = v
-		}
+		mn = min(mn, v)
+		mx = max(mx, v)
 		sum += v
 	}
-	avg = sum / float64(len(xs))
-	var vs float64
+
+	avg := sum / float64(len(xs))
+
+	var variance float64
+
 	for _, v := range xs {
-		d := v - avg
-		vs += d * d
+		variance += (v - avg) * (v - avg)
 	}
-	std = math.Sqrt(vs / float64(len(xs)))
-	return
+
+	return mn, mx, avg, math.Sqrt(variance / float64(len(xs)))
 }
 
-// Spin advances the spinner frame
+// Spin advances the spinner frame.
 func (st *InterfaceStatus) Spin() {
-	st.SpinFrame = (st.SpinFrame + 1) % len(Spins)
+	st.SpinFrame = (st.SpinFrame + 1) % len(spins())
 }
 
-// IsHealthy checks if the interface is healthy
+// IsHealthy reports whether the interface currently looks healthy.
 func (st *InterfaceStatus) IsHealthy(tcpTimeout time.Duration) bool {
 	st.mu.RLock()
 	defer st.mu.RUnlock()
 
-	// Check if there are any currently unreachable hosts
 	for _, ok := range st.Reachable {
 		if !ok {
 			return false
 		}
 	}
 
-	// Check if there's any current packet loss
 	for _, lp := range st.Loss {
 		if lp > 0 {
 			return false
 		}
 	}
 
-	// Check if there are any TCP timeouts
 	for _, hist := range st.TCP {
 		if len(hist) == 0 || hist[len(hist)-1] >= float64(tcpTimeout.Milliseconds()) {
 			return false
@@ -372,23 +446,29 @@ func (st *InterfaceStatus) IsHealthy(tcpTimeout time.Duration) bool {
 	return true
 }
 
-// Spinners and braille characters
-var (
-	Spins        = []rune{'|', '/', '-', '\\'}
-	BrailleSpins = []rune{
-		'⠉', '⠘', '⠰', '⠠', '⠄', '⠆', '⠇', '⠋',
-	}
-)
+// spins returns the ASCII spinner frames.
+func spins() []rune {
+	return []rune{'|', '/', '-', '\\'}
+}
 
-// Logf is a simple logging function
-func Logf(logFile string, format string, v ...interface{}) {
+// brailleSpins returns the braille clock spinner frames.
+func brailleSpins() []rune {
+	return []rune{'⠉', '⠘', '⠰', '⠠', '⠄', '⠆', '⠇', '⠋'}
+}
+
+// Logf appends a timestamped formatted message to logFile, if set.
+func Logf(logFile, format string, v ...any) {
 	if logFile == "" {
 		return
 	}
-	f, err := os.OpenFile(logFile, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+
+	f, err := os.OpenFile( //nolint:gosec // G304: operator --logfile path
+		logFile, os.O_APPEND|os.O_CREATE|os.O_WRONLY, logFileMode)
 	if err != nil {
 		return
 	}
-	defer f.Close()
-	fmt.Fprintf(f, time.Now().Format("2006-01-02 15:04:05.000 ")+format+"\n", v...)
+	defer func() { _ = f.Close() }()
+
+	_, _ = fmt.Fprintf(f,
+		time.Now().Format("2006-01-02 15:04:05.000 ")+format+"\n", v...)
 }

internal/monitor/monitor_test.go

@@ -1,125 +1,110 @@
 //go:build linux
-// +build linux
 
-package monitor
+package monitor_test
 
 import (
 	"testing"
+
+	"git.eeqj.de/sneak/rtnetmon/internal/monitor"
 )
 
-// TestNewMonitor tests the creation of a new Monitor
 func TestNewMonitor(t *testing.T) {
-	// Test that we can create a monitor without errors
-	mon := NewMonitor("test0", "Test Interface A", "test1", "Test Interface B", "/tmp/test.log")
+	t.Parallel()
 
-	if mon == nil {
-		t.Fatal("NewMonitor returned nil")
-	}
+	mon := monitor.NewMonitor("test0", "Test Interface A", "test1",
+		"Test Interface B", "/tmp/test.log")
 
-	// Check interfaces
-	if mon.interfaceA == nil {
+	if mon.InterfaceA() == nil {
 		t.Fatal("interfaceA is nil")
 	}
 
-	if mon.interfaceB == nil {
+	if mon.InterfaceB() == nil {
 		t.Fatal("interfaceB is nil")
 	}
 
-	if mon.interfaceA.Name != "test0" {
-		t.Errorf("Expected interfaceA.Name to be 'test0', got '%s'", mon.interfaceA.Name)
+	if mon.InterfaceA().Name != "test0" {
+		t.Errorf("interfaceA.Name = %q, want %q", mon.InterfaceA().Name, "test0")
 	}
 
-	if mon.interfaceB.Name != "test1" {
-		t.Errorf("Expected interfaceB.Name to be 'test1', got '%s'", mon.interfaceB.Name)
+	if mon.InterfaceB().Name != "test1" {
+		t.Errorf("interfaceB.Name = %q, want %q", mon.InterfaceB().Name, "test1")
 	}
 
-	// Check default configuration
 	if mon.ICMPTimeout.Milliseconds() != 500 {
-		t.Errorf("Expected ICMPTimeout to be 500ms, got %dms", mon.ICMPTimeout.Milliseconds())
+		t.Errorf("ICMPTimeout = %dms, want 500ms", mon.ICMPTimeout.Milliseconds())
 	}
 
 	if mon.PacketLossPings != 20 {
-		t.Errorf("Expected PacketLossPings to be 20, got %d", mon.PacketLossPings)
+		t.Errorf("PacketLossPings = %d, want 20", mon.PacketLossPings)
 	}
 
-	// Check that host lists are initialized
-	if mon.reachabilityHosts == nil {
+	if mon.ReachabilityHosts() == nil {
 		t.Error("reachabilityHosts is nil")
 	}
 
-	if mon.packetLossHosts == nil {
+	if mon.PacketLossHosts() == nil {
 		t.Error("packetLossHosts is nil")
 	}
 
-	if mon.tcpHosts == nil {
+	if mon.TCPHosts() == nil {
 		t.Error("tcpHosts is nil")
 	}
 }
 
-// TestAddHosts tests adding hosts to the monitor
 func TestAddHosts(t *testing.T) {
-	mon := NewMonitor("test0", "Test A", "test1", "Test B", "")
+	t.Parallel()
+
+	mon := monitor.NewMonitor("test0", "Test A", "test1", "Test B", "")
 
-	// Test adding reachability hosts
 	mon.AddReachabilityHost("8.8.8.8")
 	mon.AddReachabilityHost("google.com")
 
-	if len(mon.reachabilityHosts) != 2 {
-		t.Errorf("Expected 2 reachability hosts, got %d", len(mon.reachabilityHosts))
+	if len(mon.ReachabilityHosts()) != 2 {
+		t.Errorf("reachability hosts = %d, want 2", len(mon.ReachabilityHosts()))
 	}
 
-	// Test adding packet loss hosts
 	mon.AddPacketLossHost("github.com")
 
-	if len(mon.packetLossHosts) != 1 {
-		t.Errorf("Expected 1 packet loss host, got %d", len(mon.packetLossHosts))
+	if len(mon.PacketLossHosts()) != 1 {
+		t.Errorf("packet loss hosts = %d, want 1", len(mon.PacketLossHosts()))
 	}
 
-	// Test adding TCP hosts
 	mon.AddTCPHost("google.com:443")
 	mon.AddTCPHost("github.com:443")
 
-	if len(mon.tcpHosts) != 2 {
-		t.Errorf("Expected 2 TCP hosts, got %d", len(mon.tcpHosts))
+	if len(mon.TCPHosts()) != 2 {
+		t.Errorf("tcp hosts = %d, want 2", len(mon.TCPHosts()))
 	}
 }
 
-// TestMinMaxAvgStd tests the statistics calculation function
 func TestMinMaxAvgStd(t *testing.T) {
+	t.Parallel()
+
 	tests := []struct {
 		name                      string
 		data                      []float64
 		wantMin, wantMax, wantAvg float64
 	}{
-		{
-			name:    "empty slice",
-			data:    []float64{},
-			wantMin: 0, wantMax: 0, wantAvg: 0,
-		},
-		{
-			name:    "single value",
-			data:    []float64{5.0},
-			wantMin: 5.0, wantMax: 5.0, wantAvg: 5.0,
-		},
-		{
-			name:    "multiple values",
-			data:    []float64{1.0, 2.0, 3.0, 4.0, 5.0},
-			wantMin: 1.0, wantMax: 5.0, wantAvg: 3.0,
-		},
+		{"empty slice", []float64{}, 0, 0, 0},
+		{"single value", []float64{5.0}, 5.0, 5.0, 5.0},
+		{"multiple values", []float64{1.0, 2.0, 3.0, 4.0, 5.0}, 1.0, 5.0, 3.0},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			min, max, avg, _ := MinMaxAvgStd(tt.data)
+			t.Parallel()
 
-			if min != tt.wantMin {
-				t.Errorf("MinMaxAvgStd() min = %v, want %v", min, tt.wantMin)
+			mn, mx, avg, _ := monitor.MinMaxAvgStd(tt.data)
+			if mn != tt.wantMin {
+				t.Errorf("min = %v, want %v", mn, tt.wantMin)
 			}
-			if max != tt.wantMax {
-				t.Errorf("MinMaxAvgStd() max = %v, want %v", max, tt.wantMax)
+
+			if mx != tt.wantMax {
+				t.Errorf("max = %v, want %v", mx, tt.wantMax)
 			}
+
 			if avg != tt.wantAvg {
-				t.Errorf("MinMaxAvgStd() avg = %v, want %v", avg, tt.wantAvg)
+				t.Errorf("avg = %v, want %v", avg, tt.wantAvg)
 			}
 		})
 	}

internal/monitor/styles.go

@@ -1,23 +1,62 @@
 //go:build linux
-// +build linux
 
 package monitor
 
 import tcell "github.com/gdamore/tcell/v2"
 
-// Color styles
-var (
-	CBrightGreen = tcell.StyleDefault.Foreground(tcell.ColorGreen).Bold(true)
-	CGreen       = tcell.StyleDefault.Foreground(tcell.ColorGreen)
-	CDimGreen    = tcell.StyleDefault.Foreground(tcell.ColorGreen).Dim(true)
-	CYellow      = tcell.StyleDefault.Foreground(tcell.ColorYellow)
-	COrange      = tcell.StyleDefault.Foreground(tcell.ColorOrange)
-	CRed         = tcell.StyleDefault.Foreground(tcell.ColorRed)
-	CBrightRed   = tcell.StyleDefault.Foreground(tcell.ColorRed).Bold(true)
-	CDefault     = tcell.StyleDefault
+// percentFull is the top of the 0..100 percentage scale.
+const percentFull = 100.0
 
-	// Rainbow colors for the spinner
-	CRainbow = []tcell.Style{
+// Meter fill thresholds, expressed as percent-good (100 = best, 0 = worst).
+const (
+	meterExcellent = 90
+	meterGood      = 75
+	meterFair      = 60
+	meterMediocre  = 40
+	meterPoor      = 20
+)
+
+// TCP latency thresholds, in milliseconds.
+const (
+	latencyGood     = 50
+	latencyModerate = 100
+	latencyHigh     = 200
+)
+
+// lossWarnPercent is the packet-loss level above which the display warns.
+const lossWarnPercent = 5
+
+func styleBrightGreen() tcell.Style {
+	return tcell.StyleDefault.Foreground(tcell.ColorGreen).Bold(true)
+}
+
+func styleGreen() tcell.Style {
+	return tcell.StyleDefault.Foreground(tcell.ColorGreen)
+}
+
+func styleDimGreen() tcell.Style {
+	return tcell.StyleDefault.Foreground(tcell.ColorGreen).Dim(true)
+}
+
+func styleYellow() tcell.Style {
+	return tcell.StyleDefault.Foreground(tcell.ColorYellow)
+}
+
+func styleOrange() tcell.Style {
+	return tcell.StyleDefault.Foreground(tcell.ColorOrange)
+}
+
+func styleRed() tcell.Style {
+	return tcell.StyleDefault.Foreground(tcell.ColorRed)
+}
+
+func styleBrightRed() tcell.Style {
+	return tcell.StyleDefault.Foreground(tcell.ColorRed).Bold(true)
+}
+
+// rainbow returns the color cycle used for spinners and the clock.
+func rainbow() []tcell.Style {
+	return []tcell.Style{
 		tcell.StyleDefault.Foreground(tcell.ColorRed),
 		tcell.StyleDefault.Foreground(tcell.ColorOrange),
 		tcell.StyleDefault.Foreground(tcell.ColorYellow),
@@ -25,63 +64,55 @@ var (
 		tcell.StyleDefault.Foreground(tcell.ColorBlue),
 		tcell.StyleDefault.Foreground(tcell.ColorPurple),
 	}
-)
+}
 
-// MeterColorForValue returns the appropriate color style for a meter value
-// value: current value of the meter
-// maxValue: maximum value of the meter
-// reverse: if true, low values are good (green) and high values are bad (red)
-//
-//	if false, high values are good (green) and low values are bad (red)
+// MeterColorForValue returns the color style for a meter value. When reverse
+// is true, low values are good (green) and high values bad (red); otherwise
+// high values are good.
 func MeterColorForValue(value, maxValue int, reverse bool) tcell.Style {
-	// Calculate percentage
-	percentage := float64(value) / float64(maxValue) * 100
-
-	// For reverse mode (low is good), invert the percentage
+	percentage := float64(value) / float64(maxValue) * percentFull
 	if reverse {
-		percentage = 100 - percentage
+		percentage = percentFull - percentage
 	}
 
-	// Return color based on percentage (after any reversal)
-	// Now high percentage always means "good" and low percentage means "bad"
 	switch {
-	case percentage >= 90:
-		return CBrightGreen
-	case percentage >= 75:
-		return CGreen
-	case percentage >= 60:
-		return CDimGreen
-	case percentage >= 40:
-		return CYellow
-	case percentage >= 20:
-		return COrange
+	case percentage >= meterExcellent:
+		return styleBrightGreen()
+	case percentage >= meterGood:
+		return styleGreen()
+	case percentage >= meterFair:
+		return styleDimGreen()
+	case percentage >= meterMediocre:
+		return styleYellow()
+	case percentage >= meterPoor:
+		return styleOrange()
 	default:
-		return CRed
+		return styleRed()
 	}
 }
 
-// StyleLatency returns the appropriate style for latency values
+// StyleLatency returns the style for a TCP latency in milliseconds.
 func StyleLatency(ms float64) tcell.Style {
 	switch {
-	case ms < 50:
-		return CBrightGreen
-	case ms < 100:
-		return CGreen
-	case ms < 200:
-		return CYellow
+	case ms < latencyGood:
+		return styleBrightGreen()
+	case ms < latencyModerate:
+		return styleGreen()
+	case ms < latencyHigh:
+		return styleYellow()
 	default:
-		return CRed
+		return styleRed()
 	}
 }
 
-// StyleLoss returns the appropriate style for packet loss percentages
+// StyleLoss returns the style for a packet-loss percentage.
 func StyleLoss(p float64) tcell.Style {
 	switch {
 	case p == 0:
-		return CBrightGreen
-	case p < 5:
-		return CYellow
+		return styleBrightGreen()
+	case p < lossWarnPercent:
+		return styleYellow()
 	default:
-		return CBrightRed
+		return styleBrightRed()
 	}
 }

internal/monitor/styles_test.go

@@ -1,104 +1,48 @@
 //go:build linux
-// +build linux
 
-package monitor
+package monitor_test
 
 import (
 	"testing"
 
 	tcell "github.com/gdamore/tcell/v2"
+
+	"git.eeqj.de/sneak/rtnetmon/internal/monitor"
 )
 
-// TestMeterColorForValue tests the MeterColorForValue function
 func TestMeterColorForValue(t *testing.T) {
-	tests := []struct {
-		name      string
-		value     int
-		maxValue  int
-		reverse   bool
-		wantColor tcell.Style
-	}{
-		// Reverse mode tests (low is good, high is bad)
-		{
-			name:      "reverse mode: 0% (best)",
-			value:     0,
-			maxValue:  10,
-			reverse:   true,
-			wantColor: CBrightGreen,
-		},
-		{
-			name:      "reverse mode: 10% (good)",
-			value:     1,
-			maxValue:  10,
-			reverse:   true,
-			wantColor: CBrightGreen,
-		},
-		{
-			name:      "reverse mode: 50% (medium)",
-			value:     5,
-			maxValue:  10,
-			reverse:   true,
-			wantColor: CYellow,
-		},
-		{
-			name:      "reverse mode: 90% (bad)",
-			value:     9,
-			maxValue:  10,
-			reverse:   true,
-			wantColor: CRed,
-		},
-		{
-			name:      "reverse mode: 100% (worst)",
-			value:     10,
-			maxValue:  10,
-			reverse:   true,
-			wantColor: CRed,
-		},
+	t.Parallel()
 
-		// Normal mode tests (high is good, low is bad)
-		{
-			name:      "normal mode: 0% (worst)",
-			value:     0,
-			maxValue:  10,
-			reverse:   false,
-			wantColor: CRed,
-		},
-		{
-			name:      "normal mode: 10% (bad)",
-			value:     1,
-			maxValue:  10,
-			reverse:   false,
-			wantColor: CRed,
-		},
-		{
-			name:      "normal mode: 50% (medium)",
-			value:     5,
-			maxValue:  10,
-			reverse:   false,
-			wantColor: CYellow,
-		},
-		{
-			name:      "normal mode: 90% (good)",
-			value:     9,
-			maxValue:  10,
-			reverse:   false,
-			wantColor: CBrightGreen,
-		},
-		{
-			name:      "normal mode: 100% (best)",
-			value:     10,
-			maxValue:  10,
-			reverse:   false,
-			wantColor: CBrightGreen,
-		},
+	green := tcell.StyleDefault.Foreground(tcell.ColorGreen).Bold(true)
+	yellow := tcell.StyleDefault.Foreground(tcell.ColorYellow)
+	red := tcell.StyleDefault.Foreground(tcell.ColorRed)
+
+	tests := []struct {
+		name            string
+		value, maxValue int
+		reverse         bool
+		want            tcell.Style
+	}{
+		{"reverse 0% best", 0, 10, true, green},
+		{"reverse 10% good", 1, 10, true, green},
+		{"reverse 50% medium", 5, 10, true, yellow},
+		{"reverse 90% bad", 9, 10, true, red},
+		{"reverse 100% worst", 10, 10, true, red},
+		{"normal 0% worst", 0, 10, false, red},
+		{"normal 10% bad", 1, 10, false, red},
+		{"normal 50% medium", 5, 10, false, yellow},
+		{"normal 90% good", 9, 10, false, green},
+		{"normal 100% best", 10, 10, false, green},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			got := MeterColorForValue(tt.value, tt.maxValue, tt.reverse)
-			if got != tt.wantColor {
+			t.Parallel()
+
+			got := monitor.MeterColorForValue(tt.value, tt.maxValue, tt.reverse)
+			if got != tt.want {
 				t.Errorf("MeterColorForValue(%d, %d, %v) = %v, want %v",
-					tt.value, tt.maxValue, tt.reverse, got, tt.wantColor)
+					tt.value, tt.maxValue, tt.reverse, got, tt.want)
 			}
 		})
 	}

internal/monitor/ui.go

@@ -1,5 +1,4 @@
 //go:build linux
-// +build linux
 
 package monitor
 
@@ -13,283 +12,331 @@ import (
 	tcell "github.com/gdamore/tcell/v2"
 )
 
-// Frame counter for the timestamp spinner (ticks once per second)
-var (
-	timestampSpinFrame      = 0
-	lastTimestampSpinUpdate = time.Now()
+// Fixed rows at the top of the display.
+const (
+	rowTopRule    = 0
+	rowClock      = 1
+	rowBottomRule = 2
+	rowRuntime    = 3
+	rowFirstIface = 5
 )
 
-// Put writes text to the screen at the specified position with style
+// Column offsets and spacing within the drawn output.
+const (
+	colLeft       = 0
+	colSpinner    = 3 // after the "== " prefix
+	colMeter      = 5 // after the spinner
+	colGap        = 1 // single-space gap between fields
+	clockGap      = 5 // space between the two clocks
+	lineStep      = 1
+	blockGap      = 2 // blank line plus the following line
+	headerAdvance = 4 // rule + content + rule + blank line
+)
+
+// ICMP summary table geometry.
+const (
+	icmpLabelWidth = 8
+	icmpValWidth   = 9
+	icmpColGap     = 4
+)
+
+// Put writes text to the screen at the specified position with style.
 func Put(scr tcell.Screen, x, y int, txt string, st tcell.Style) {
 	for i, r := range txt {
 		scr.SetContent(x+i, y, r, nil, st)
 	}
 }
 
-// HLine creates a horizontal line of the specified width
+// HLine returns a horizontal line of the specified width.
 func HLine(w int) string { return strings.Repeat("=", w) }
 
-// DrawRainbowText draws text with rainbow colors
+// DrawRainbowText draws text cycling through the given color styles.
 func DrawRainbowText(scr tcell.Screen, x, y int, text string, colors []tcell.Style) {
 	for i, char := range text {
-		// Cycle through the rainbow colors
-		colorIndex := i % len(colors)
-		style := colors[colorIndex]
-		// Draw the character with the current rainbow color
+		style := colors[i%len(colors)]
 		scr.SetContent(x+i, y, char, nil, style)
 	}
 }
 
-// CreateMeter creates a visual meter using ASCII characters
+// msStr formats a millisecond value as a compact "%.0fms" string.
+func msStr(v float64) string {
+	return fmt.Sprintf("%.0fms", v)
+}
+
+// CreateMeter creates a visual meter using ASCII characters.
 func (m *Monitor) CreateMeter(value int) (string, tcell.Style) {
-	if value > m.MaxMeterValue {
-		value = m.MaxMeterValue
-	}
-	if value < 0 {
-		value = 0
-	}
+	value = max(min(value, m.MaxMeterValue), 0)
 
-	// Calculate the number of fill characters to show
-	fillCount := value * m.MeterFillWidth / m.MaxMeterValue
-	if fillCount > m.MeterFillWidth {
-		fillCount = m.MeterFillWidth
-	}
+	fillCount := min(value*m.MeterFillWidth/m.MaxMeterValue, m.MeterFillWidth)
 
-	// Build the meter string
 	var b strings.Builder
+
 	b.WriteRune(MeterStart)
 
-	// Add fill characters
-	for i := 0; i < fillCount; i++ {
+	for range fillCount {
 		b.WriteRune(MeterFill)
 	}
 
-	// Add empty spaces
-	for i := 0; i < m.MeterFillWidth-fillCount; i++ {
+	for range m.MeterFillWidth - fillCount {
 		b.WriteRune(MeterEmpty)
 	}
 
 	b.WriteRune(MeterEnd)
 
-	// Get the appropriate style for this meter value
-	// Using reverse=true because for packet loss, low values are good
+	// reverse=true: for packet loss, low values are good.
 	style := MeterColorForValue(value, m.MaxMeterValue, true)
 
 	return b.String(), style
 }
 
-// DrawInterface draws the interface status on the screen
+// DrawInterface draws the interface status on the screen, returning the next
+// free row.
 func (m *Monitor) DrawInterface(scr tcell.Screen, y, w int, st *InterfaceStatus) int {
-	Put(scr, 0, y, HLine(w), CDefault)
+	Put(scr, colLeft, y, HLine(w), tcell.StyleDefault)
 
-	// header line
 	healthy := st.IsHealthy(m.TCPTimeout)
-	style := CBrightGreen
+	y = m.drawHeader(scr, y, w, st, healthy)
+
+	m.mu.RLock()
+	tcpHosts := append([]string{}, m.tcpHosts...)
+	m.mu.RUnlock()
+
+	st.mu.RLock()
+	defer st.mu.RUnlock()
+
+	y = drawReachability(scr, y, st)
+	y = drawPacketLoss(scr, y, st)
+	y = m.drawTCPTable(scr, y, st, tcpHosts)
+	y = drawICMPStats(scr, y, st)
+
+	return y
+}
+
+// drawHeader renders the interface title, spinner and meter line.
+func (m *Monitor) drawHeader(
+	scr tcell.Screen, y, w int, st *InterfaceStatus, healthy bool,
+) int {
+	style := styleBrightGreen()
 	if !healthy {
-		style = CBrightRed
+		style = styleBrightRed()
 	}
+
 	st.mu.RLock()
 	header := fmt.Sprintf("%s: %s —   %s", st.Name, st.Label, st.IPInfo)
-	spinChar := Spins[st.SpinFrame]
+	spinChar := spins()[st.SpinFrame]
 	spinnerFrame := st.SpinFrame
 	meterValue := st.MeterValue
 	st.mu.RUnlock()
 
-	// Create the meter with appropriate style
 	meter, meterStyle := m.CreateMeter(meterValue)
+	colors := rainbow()
+	spinnerStyle := colors[spinnerFrame%len(colors)]
 
-	// Get rainbow color for spinner (cycle through colors)
-	spinnerStyle := CRainbow[spinnerFrame%len(CRainbow)]
+	Put(scr, colLeft, y+lineStep, "== ", tcell.StyleDefault)
+	Put(scr, colSpinner, y+lineStep, string(spinChar)+" ", spinnerStyle)
+	Put(scr, colMeter, y+lineStep, meter+" ", meterStyle)
+	Put(scr, colMeter+m.MeterWidth+colGap, y+lineStep, header, style)
+	Put(scr, colLeft, y+blockGap, HLine(w), tcell.StyleDefault)
 
-	Put(scr, 0, y+1, "== ", CDefault)
-	Put(scr, 3, y+1, string(spinChar)+" ", spinnerStyle) // Colorful spinner
-	Put(scr, 5, y+1, meter+" ", meterStyle)              // Colored meter
-	Put(scr, 5+m.MeterWidth+1, y+1, header, style)       // Move the header after the meter
-	Put(scr, 0, y+2, HLine(w), CDefault)
-	y += 4
+	return y + headerAdvance
+}
 
-	/* reachability */
-	st.mu.RLock()
+// drawReachability renders the reachability summary. The caller holds
+// st.mu.RLock.
+func drawReachability(scr tcell.Screen, y int, st *InterfaceStatus) int {
 	total := len(st.Reachable)
 	good := 0
+
 	for _, ok := range st.Reachable {
 		if ok {
 			good++
 		}
 	}
+
 	age := time.Since(st.LastPing).Round(time.Second)
-	reachStyle := CBrightGreen
+
+	reachStyle := styleBrightGreen()
 	if good != total {
-		reachStyle = CBrightRed
+		reachStyle = styleBrightRed()
 	}
 
-	// Get last drop time and age
 	dropTimeStr := "never"
 	dropAgeStr := "N/A"
+
 	if !st.LastDrop.IsZero() {
 		dropTimeStr = st.LastDrop.Format("15:04:05")
 		dropAgeStr = time.Since(st.LastDrop).Round(time.Second).String()
 	}
 
-	// Simplified reachability line without drop info
-	Put(scr, 0, y, fmt.Sprintf("Reachable: %d/%d (at %s, age %s)",
+	Put(scr, colLeft, y, fmt.Sprintf("Reachable: %d/%d (at %s, age %s)",
 		good, total, st.LastPing.Format("15:04:05"), age), reachStyle)
-	y++
+	y += lineStep
 
 	if good == total {
-		Put(scr, 0, y, "Unreachable: none", CDefault)
+		Put(scr, colLeft, y, "Unreachable: none", tcell.StyleDefault)
 	} else {
-		var down []string
+		down := make([]string, 0, len(st.Reachable))
+
 		for h, ok := range st.Reachable {
 			if !ok {
 				down = append(down, h)
 			}
 		}
-		// Sort for consistent display
+
 		sort.Strings(down)
-		// Add drop info to the end of the unreachable line
-		Put(scr, 0, y, fmt.Sprintf("Unreachable: %s (last drop at %s, age %s)",
-			strings.Join(down, ", "), dropTimeStr, dropAgeStr), CBrightRed)
+		Put(scr, colLeft, y, fmt.Sprintf("Unreachable: %s (last drop at %s, age %s)",
+			strings.Join(down, ", "), dropTimeStr, dropAgeStr), styleBrightRed())
 	}
-	y += 2
 
-	/* packet loss */
-	Put(scr, 0, y, "Packet Loss:", CDefault)
-	y++
+	return y + blockGap
+}
 
-	// Get all hosts and sort them for consistent display order
-	var lossHosts []string
+// drawPacketLoss renders the per-host packet-loss list. The caller holds
+// st.mu.RLock.
+func drawPacketLoss(scr tcell.Screen, y int, st *InterfaceStatus) int {
+	Put(scr, colLeft, y, "Packet Loss:", tcell.StyleDefault)
+	y += lineStep
+
+	lossHosts := make([]string, 0, len(st.Loss))
 	for host := range st.Loss {
 		lossHosts = append(lossHosts, host)
 	}
+
 	sort.Strings(lossHosts)
 
-	// Find the maximum length of host names for alignment
 	maxHostLen := 0
 	for _, host := range lossHosts {
-		if len(host) > maxHostLen {
-			maxHostLen = len(host)
-		}
+		maxHostLen = max(maxHostLen, len(host))
 	}
 
-	// Add 1 for the colon
-	maxHostLen += 1
+	maxHostLen++ // room for the colon
 
 	for _, host := range lossHosts {
-		p := st.Loss[host] * 100
-		// Use the maxHostLen for consistent alignment
-		Put(scr, 0, y, fmt.Sprintf("%-*s %5.0f%%", maxHostLen, host+":", p), StyleLoss(p))
-		y++
+		p := st.Loss[host] * percentFull
+		Put(scr, colLeft, y, fmt.Sprintf("%-*s %5.0f%%", maxHostLen, host+":", p),
+			StyleLoss(p))
+		y += lineStep
 	}
+
 	dAge := "N/A"
 	if !st.LastDrop.IsZero() {
 		dAge = time.Since(st.LastDrop).Round(time.Second).String()
 	}
-	Put(scr, 0, y, fmt.Sprintf("Dropped: %d (last at %s, age %s)",
-		st.DroppedCount, st.LastDrop.Format("15:04:05"), dAge), CDefault)
-	y += 2
 
-	/* TCP table */
-	Put(scr, 0, y, "TCP Connect Stats:", CDefault)
-	y++
-	// header row
+	Put(scr, colLeft, y, fmt.Sprintf("Dropped: %d (last at %s, age %s)",
+		st.DroppedCount, st.LastDrop.Format("15:04:05"), dAge), tcell.StyleDefault)
+
+	return y + blockGap
+}
+
+// drawTCPTable renders the TCP connect-stats table. The caller holds
+// st.mu.RLock.
+func (m *Monitor) drawTCPTable(
+	scr tcell.Screen, y int, st *InterfaceStatus, tcpHosts []string,
+) int {
+	Put(scr, colLeft, y, "TCP Connect Stats:", tcell.StyleDefault)
+	y += lineStep
+
 	headerRow := fmt.Sprintf("%-*s %*s %*s %*s %*s %*s %*s %*s",
 		m.HostWidth, "Host", m.NumWidth, "last", m.NumWidth, "min", m.NumWidth, "avg",
 		m.NumWidth, "max", m.StdWidth, "stddev", m.NWidth, "n", m.LostWidth, "lost")
-	Put(scr, 0, y, headerRow, CDefault)
-	y++
-
-	m.mu.RLock()
-	tcpHosts := append([]string{}, m.tcpHosts...)
-	m.mu.RUnlock()
+	Put(scr, colLeft, y, headerRow, tcell.StyleDefault)
+	y += lineStep
 
 	for _, hp := range tcpHosts {
 		hist := st.TCP[hp]
 		if len(hist) == 0 {
 			continue
 		}
-		last := hist[len(hist)-1]
-		mi, ma, av, sd := MinMaxAvgStd(hist)
 
-		// Get host name without port for lost packets lookup
-		hostName := strings.Split(hp, ":")[0]
-		lost := st.LostPackets[hostName]
-
-		// Add lost column
-		row := fmt.Sprintf("%-*s %*s %*s %*s %*s %*s %*d %*d",
-			m.HostWidth, hp,
-			m.NumWidth, fmt.Sprintf("%.0fms", last),
-			m.NumWidth, fmt.Sprintf("%.0fms", mi),
-			m.NumWidth, fmt.Sprintf("%.0fms", av),
-			m.NumWidth, fmt.Sprintf("%.0fms", ma),
-			m.StdWidth, fmt.Sprintf("%.0fms", sd),
-			m.NWidth, len(hist),
-			m.LostWidth, lost,
-		)
-		Put(scr, 0, y, row, CDefault)
-		// colourise individual numbers
-		Put(scr, m.HostWidth+1, y, fmt.Sprintf("%*s", m.NumWidth, fmt.Sprintf("%.0fms", last)), StyleLatency(last))
-		Put(scr, m.HostWidth+1+m.NumWidth+1, y, fmt.Sprintf("%*s", m.NumWidth, fmt.Sprintf("%.0fms", mi)), StyleLatency(mi))
-		Put(scr, m.HostWidth+1+m.NumWidth*2+2, y, fmt.Sprintf("%*s", m.NumWidth, fmt.Sprintf("%.0fms", av)), StyleLatency(av))
-		Put(scr, m.HostWidth+1+m.NumWidth*3+3, y, fmt.Sprintf("%*s", m.NumWidth, fmt.Sprintf("%.0fms", ma)), StyleLatency(ma))
-		Put(scr, m.HostWidth+1+m.NumWidth*4+4, y, fmt.Sprintf("%*s", m.StdWidth, fmt.Sprintf("%.0fms", sd)), CDefault)
-		// Colorize the lost packets column
-		lostStyle := CDefault
-		if lost > 0 {
-			lostStyle = CRed
-		}
-		Put(scr, m.HostWidth+1+m.NumWidth*4+4+m.StdWidth+1+m.NWidth+1, y, fmt.Sprintf("%*d", m.LostWidth, lost), lostStyle)
-		y++
-	}
-	y++
-
-	/* ICMP stats - combined into a single line with headers */
-	// Calculate total lost packets
-	lost := st.TotalICMPReq - st.TotalICMPRep
-	if lost < 0 {
-		lost = 0
+		m.drawTCPRow(scr, y, st, hp, hist)
+		y += lineStep
 	}
 
-	// Create styles based on values
-	lostStyle := CDefault
-	if lost > 0 {
-		lostStyle = CRed
-	}
-
-	// Define column widths and positions
-	const valWidth = 9
-
-	// Format the values with right alignment
-	reqVal := fmt.Sprintf("%9d", st.TotalICMPReq)
-	repVal := fmt.Sprintf("%9d", st.TotalICMPRep)
-	lostVal := fmt.Sprintf("%9d", lost)
-
-	// Header texts with the same width as values for right alignment
-	reqHeader := fmt.Sprintf("%9s", "Requests")
-	repHeader := fmt.Sprintf("%9s", "Replies")
-	lostHeader := fmt.Sprintf("%9s", "Lost")
-
-	// Draw the header line
-	Put(scr, 0, y, "        ", CDefault)
-	Put(scr, 8, y, reqHeader, CDefault)
-	Put(scr, 8+valWidth+4, y, repHeader, CDefault)
-	Put(scr, 8+2*(valWidth+4), y, lostHeader, CDefault)
-	y++
-
-	// Draw the values line
-	Put(scr, 0, y, "ICMP:   ", CDefault)
-	Put(scr, 8, y, reqVal, CDefault)
-	Put(scr, 8+valWidth+4, y, repVal, CDefault)
-	Put(scr, 8+2*(valWidth+4), y, lostVal, lostStyle)
-
-	y += 2
-	st.mu.RUnlock()
-	return y
+	return y + lineStep
 }
 
-// uiLoop runs the UI event loop
+// drawTCPRow renders one TCP host's statistics row. The caller holds
+// st.mu.RLock.
+func (m *Monitor) drawTCPRow(
+	scr tcell.Screen, y int, st *InterfaceStatus, hp string, hist []float64,
+) {
+	last := hist[len(hist)-1]
+	mi, ma, av, sd := MinMaxAvgStd(hist)
+
+	host := strings.Split(hp, ":")[0]
+	lost := st.LostPackets[host]
+
+	row := fmt.Sprintf("%-*s %*s %*s %*s %*s %*s %*d %*d",
+		m.HostWidth, hp,
+		m.NumWidth, msStr(last),
+		m.NumWidth, msStr(mi),
+		m.NumWidth, msStr(av),
+		m.NumWidth, msStr(ma),
+		m.StdWidth, msStr(sd),
+		m.NWidth, len(hist),
+		m.LostWidth, lost,
+	)
+	Put(scr, colLeft, y, row, tcell.StyleDefault)
+
+	// Overlay the numeric columns colored by value, at the same offsets the
+	// base row above laid them out.
+	x := m.HostWidth + colGap
+	Put(scr, x, y, fmt.Sprintf("%*s", m.NumWidth, msStr(last)), StyleLatency(last))
+	x += m.NumWidth + colGap
+	Put(scr, x, y, fmt.Sprintf("%*s", m.NumWidth, msStr(mi)), StyleLatency(mi))
+	x += m.NumWidth + colGap
+	Put(scr, x, y, fmt.Sprintf("%*s", m.NumWidth, msStr(av)), StyleLatency(av))
+	x += m.NumWidth + colGap
+	Put(scr, x, y, fmt.Sprintf("%*s", m.NumWidth, msStr(ma)), StyleLatency(ma))
+	x += m.NumWidth + colGap
+	Put(scr, x, y, fmt.Sprintf("%*s", m.StdWidth, msStr(sd)), tcell.StyleDefault)
+	x += m.StdWidth + colGap
+	x += m.NWidth + colGap // n column already drawn by the base row
+
+	lostStyle := tcell.StyleDefault
+	if lost > 0 {
+		lostStyle = styleRed()
+	}
+
+	Put(scr, x, y, fmt.Sprintf("%*d", m.LostWidth, lost), lostStyle)
+}
+
+// drawICMPStats renders the ICMP request/reply/lost summary. The caller
+// holds st.mu.RLock.
+func drawICMPStats(scr tcell.Screen, y int, st *InterfaceStatus) int {
+	lost := max(st.TotalICMPReq-st.TotalICMPRep, 0)
+
+	lostStyle := tcell.StyleDefault
+	if lost > 0 {
+		lostStyle = styleRed()
+	}
+
+	reqCol := icmpLabelWidth
+	repCol := reqCol + icmpValWidth + icmpColGap
+	lostCol := repCol + icmpValWidth + icmpColGap
+
+	Put(scr, reqCol, y, fmt.Sprintf("%*s", icmpValWidth, "Requests"), tcell.StyleDefault)
+	Put(scr, repCol, y, fmt.Sprintf("%*s", icmpValWidth, "Replies"), tcell.StyleDefault)
+	Put(scr, lostCol, y, fmt.Sprintf("%*s", icmpValWidth, "Lost"), tcell.StyleDefault)
+	y += lineStep
+
+	reqStr := fmt.Sprintf("%*d", icmpValWidth, st.TotalICMPReq)
+	repStr := fmt.Sprintf("%*d", icmpValWidth, st.TotalICMPRep)
+	lostStr := fmt.Sprintf("%*d", icmpValWidth, lost)
+
+	Put(scr, colLeft, y, "ICMP:   ", tcell.StyleDefault)
+	Put(scr, reqCol, y, reqStr, tcell.StyleDefault)
+	Put(scr, repCol, y, repStr, tcell.StyleDefault)
+	Put(scr, lostCol, y, lostStr, lostStyle)
+
+	return y + blockGap
+}
+
+// uiLoop runs the UI event loop.
 func (m *Monitor) uiLoop(ctx context.Context) {
 	m.logf("UI loop started")
+
 	defer func() {
 		m.logf("UI loop cleanup")
 		m.screen.Clear()
@@ -298,67 +345,53 @@ func (m *Monitor) uiLoop(ctx context.Context) {
 		m.logf("Screen finalized")
 	}()
 
-	// Load PST location
 	pstLoc, err := time.LoadLocation("America/Los_Angeles")
 	if err != nil {
 		m.logf("Error loading PST location: %v", err)
+
 		pstLoc = time.UTC
 	}
 
-	// Function to draw the screen
+	timestampSpinFrame := 0
+	lastTimestampSpinUpdate := time.Now()
+
 	drawScreen := func() {
 		w, _ := m.screen.Size()
 		m.screen.Clear()
 
-		// Draw the top horizontal line
-		Put(m.screen, 0, 0, HLine(w), CDefault)
+		Put(m.screen, colLeft, rowTopRule, HLine(w), tcell.StyleDefault)
 
-		// Get current time and format it
 		now := time.Now()
 		timeStr := now.Format(time.RFC1123Z)
-
-		// Format time in PST
 		pstTimeStr := now.In(pstLoc).Format(time.RFC1123Z)
 
-		// Update the timestamp spinner once per second
 		if time.Since(lastTimestampSpinUpdate) >= time.Second {
-			timestampSpinFrame = (timestampSpinFrame + 1) % len(BrailleSpins)
+			timestampSpinFrame = (timestampSpinFrame + 1) % len(brailleSpins())
 			lastTimestampSpinUpdate = now
 		}
 
-		// Get braille spinner character
-		brailleChar := BrailleSpins[timestampSpinFrame]
+		brailleChar := brailleSpins()[timestampSpinFrame]
+		colors := rainbow()
 
-		// Draw the header with timestamp and spinner
-		Put(m.screen, 0, 1, "== ", CDefault)
-		Put(m.screen, 3, 1, string(brailleChar)+" ", CDefault)
+		Put(m.screen, colLeft, rowClock, "== ", tcell.StyleDefault)
+		Put(m.screen, colSpinner, rowClock, string(brailleChar)+" ", tcell.StyleDefault)
+		DrawRainbowText(m.screen, colMeter, rowClock, timeStr, colors)
+		DrawRainbowText(m.screen, colMeter+len(timeStr)+clockGap, rowClock,
+			pstTimeStr, colors)
+		Put(m.screen, colLeft, rowBottomRule, HLine(w), tcell.StyleDefault)
 
-		// Draw the timestamp with rainbow colors
-		DrawRainbowText(m.screen, 5, 1, timeStr, CRainbow)
+		runtime := time.Since(m.startTime).Round(time.Second).String()
+		Put(m.screen, colLeft, rowRuntime, "Runtime: "+runtime, tcell.StyleDefault)
 
-		// Add 5 space gap and PST time with rainbow colors
-		DrawRainbowText(m.screen, 5+len(timeStr)+5, 1, pstTimeStr, CRainbow)
-
-		// Draw the bottom horizontal line
-		Put(m.screen, 0, 2, HLine(w), CDefault)
-
-		// Draw the runtime
-		Put(m.screen, 0, 3, "Runtime: "+time.Since(m.startTime).Round(time.Second).String(), CDefault)
-
-		// Draw interfaces
-		y := 5
+		y := rowFirstIface
 		y = m.DrawInterface(m.screen, y, w, m.interfaceA)
 		_ = m.DrawInterface(m.screen, y, w, m.interfaceB)
 
-		// Show the screen
 		m.screen.Show()
 	}
 
-	// Initial draw
 	drawScreen()
 
-	// Even without a ticker, ensure we update at least every second
-	// This is a backup in case there are no spinner updates
 	backupTicker := time.NewTicker(time.Second)
 	defer backupTicker.Stop()
 
@@ -366,12 +399,11 @@ func (m *Monitor) uiLoop(ctx context.Context) {
 		select {
 		case <-ctx.Done():
 			m.logf("Context cancelled, exiting UI loop")
+
 			return
-		case <-UIUpdateChan:
-			// Update on spinner ticks (no rate limiting)
+		case <-m.uiUpdate:
 			drawScreen()
 		case <-backupTicker.C:
-			// Fallback to ensure we update at least once per second
 			drawScreen()
 		}
 	}