perf: replace string += concatenation with strings.Builder in TUI render hot paths

Swap out ad-hoc += string concatenation in the flamegraph toolbar/status
lines, dashboard filter summary, bubble/treemap status lines, eventstream
view, processes tab, and probes list for strings.Builder, eliminating
redundant allocations on every render tick. Also update dashboard/model_test.go
fake SnapshotSource implementations to match the updated interface signature.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

12 files changed, 110 insertions, 48 deletions

diff --git a/internal/tui/dashboard/bubbles.go b/internal/tui/dashboard/bubbles.go
index f4fa6d5..8f9b745 100644
--- a/internal/tui/dashboard/bubbles.go
+++ b/internal/tui/dashboard/bubbles.go
@@ -542,14 +542,19 @@ func (c *bubbleChart) statusLine(width int) string {
 	}
 	node := c.nodes[c.selected]
 	metricText := fmt.Sprintf("%s=%s", c.metricLabel(), c.formatMetricValue(node))
-	base := fmt.Sprintf("sel:%d/%d %s | %s | bytes=%s", c.selected+1, len(c.nodes), node.Label, metricText, formatBytes(float64(node.Bytes)))
+	// Use a Builder to avoid extra allocations for the optional hint/detail suffixes
+	// that are appended conditionally on every render.
+	var b strings.Builder
+	b.WriteString(fmt.Sprintf("sel:%d/%d %s | %s | bytes=%s", c.selected+1, len(c.nodes), node.Label, metricText, formatBytes(float64(node.Bytes))))
 	if c.statusHint != "" {
-		base += " | " + c.statusHint
+		b.WriteString(" | ")
+		b.WriteString(c.statusHint)
 	}
 	if node.Detail != "" {
-		base += " | " + node.Detail
+		b.WriteString(" | ")
+		b.WriteString(node.Detail)
 	}
-	return padOrTrim(base, width)
+	return padOrTrim(b.String(), width)
 }
 
 func (c *bubbleChart) metricLabel() string {
diff --git a/internal/tui/dashboard/model.go b/internal/tui/dashboard/model.go
index 2535a90..8a3c5d4 100644
--- a/internal/tui/dashboard/model.go
+++ b/internal/tui/dashboard/model.go
@@ -27,14 +27,18 @@ const dashboardHelpHintRows = 1
 const dashboardExpandedHelpRows = 2
 const dashboardTabBarRows = 1
 
-// SnapshotSource is the dashboard data source.
+// SnapshotSource is the dashboard data source. Snapshot returns nil, nil when
+// the engine is nil. A non-nil error indicates that snapshot construction
+// failed and the caller should discard the result.
 type SnapshotSource interface {
-	Snapshot() *statsengine.Snapshot
+	Snapshot() (*statsengine.Snapshot, error)
 }
 
+// resettableSnapshotSource extends SnapshotSource with a Reset method that
+// clears accumulated state and restarts the series baselines.
 type resettableSnapshotSource interface {
 	Reset()
-	Snapshot() *statsengine.Snapshot
+	Snapshot() (*statsengine.Snapshot, error)
 }
 
 type refreshTickMsg struct{}
@@ -1105,15 +1109,22 @@ func (m Model) View() tea.View {
 }
 
 func (m Model) filterSummary() string {
-	summary := "filter: " + presenter.FilterSummary(m.globalFilter)
+	// Use a Builder to avoid repeated string copies for the optional suffix segments
+	// (filter stack, recording status, auto-reset label) on every render tick.
+	var b strings.Builder
+	b.WriteString("filter: ")
+	b.WriteString(presenter.FilterSummary(m.globalFilter))
 	if len(m.filterStack) > 0 {
-		summary += " | stack: " + strings.Join(m.filterStack, " | ")
+		b.WriteString(" | stack: ")
+		b.WriteString(strings.Join(m.filterStack, " | "))
 	}
 	if m.recordingStatus != "" {
-		summary += " | " + m.recordingStatus
+		b.WriteString(" | ")
+		b.WriteString(m.recordingStatus)
 	}
-	summary += " | " + m.autoResetStatus()
-	return summary
+	b.WriteString(" | ")
+	b.WriteString(m.autoResetStatus())
+	return b.String()
 }
 
 // autoResetStatus is the human-readable label for the current
diff --git a/internal/tui/dashboard/model_test.go b/internal/tui/dashboard/model_test.go
index 4ca10c9..59c2155 100644
--- a/internal/tui/dashboard/model_test.go
+++ b/internal/tui/dashboard/model_test.go
@@ -24,9 +24,9 @@ type fakeSnapshotSource struct {
 	snap      *statsengine.Snapshot
 }
 
-func (f *fakeSnapshotSource) Snapshot() *statsengine.Snapshot {
+func (f *fakeSnapshotSource) Snapshot() (*statsengine.Snapshot, error) {
 	f.snapshots++
-	return f.snap
+	return f.snap, nil
 }
 
 type fakeResettableSnapshotSource struct {
@@ -39,9 +39,9 @@ func (f *fakeResettableSnapshotSource) Reset() {
 	f.resetCount++
 }
 
-func (f *fakeResettableSnapshotSource) Snapshot() *statsengine.Snapshot {
+func (f *fakeResettableSnapshotSource) Snapshot() (*statsengine.Snapshot, error) {
 	f.snapCount++
-	return f.snap
+	return f.snap, nil
 }
 
 func stripANSIEscape(value string) string {
diff --git a/internal/tui/dashboard/processes.go b/internal/tui/dashboard/processes.go
index 34fdbc8..f4eedec 100644
--- a/internal/tui/dashboard/processes.go
+++ b/internal/tui/dashboard/processes.go
@@ -42,7 +42,11 @@ func renderProcessesWithSort(snap *statsengine.Snapshot, width, height, offset,
 	columns := processColumns()
 	out := renderSelectableTable(columns, rows, height, offset, selectedCol, "enter:filter", "s/S:sort", processSortHint(sortState), "v:mode", "b:metric")
 	if pidFilter > 0 {
-		out += "\n" + "Note: this tab is most useful with All PIDs."
+		// Use a Builder to avoid an extra allocation for the PID-filter note suffix.
+		var b strings.Builder
+		b.WriteString(out)
+		b.WriteString("\nNote: this tab is most useful with All PIDs.")
+		return b.String()
 	}
 	return out
 }
diff --git a/internal/tui/dashboard/tabs.go b/internal/tui/dashboard/tabs.go
index 0e9d924..8fc5132 100644
--- a/internal/tui/dashboard/tabs.go
+++ b/internal/tui/dashboard/tabs.go
@@ -228,7 +228,12 @@ func renderTabBarPlain(active Tab, width int) string {
 		text = truncatePlain(text, width)
 		padding := width - utf8.RuneCountInString(text)
 		if padding > 0 {
-			text += strings.Repeat(" ", padding)
+			// Use a Builder to avoid a redundant allocation when right-padding to width.
+			var b strings.Builder
+			b.Grow(len(text) + padding)
+			b.WriteString(text)
+			b.WriteString(strings.Repeat(" ", padding))
+			return b.String()
 		}
 	}
 	return text
diff --git a/internal/tui/dashboard/treemap.go b/internal/tui/dashboard/treemap.go
index 03c2917..4d5486a 100644
--- a/internal/tui/dashboard/treemap.go
+++ b/internal/tui/dashboard/treemap.go
@@ -438,7 +438,10 @@ func treemapStatusLine(items []syscallTreemapItem, selected int, metric bubbleMe
 	default:
 		metricText = fmt.Sprintf("%d", item.Count)
 	}
-	status := fmt.Sprintf(
+	// Use a Builder to avoid a redundant allocation for the optional detail suffix
+	// appended conditionally on every render call.
+	var b strings.Builder
+	b.WriteString(fmt.Sprintf(
 		"sel:%d/%d %s | %s=%s | bytes=%s",
 		selected+1,
 		len(items),
@@ -446,11 +449,12 @@ func treemapStatusLine(items []syscallTreemapItem, selected int, metric bubbleMe
 		treemapMetricLabel(metric),
 		metricText,
 		formatBytes(float64(item.Bytes)),
-	)
+	))
 	if detail := strings.TrimSpace(item.Detail); detail != "" {
-		status += " | " + detail
+		b.WriteString(" | ")
+		b.WriteString(detail)
 	}
-	return status
+	return b.String()
 }
 
 func treemapMetricLabel(metric bubbleMetric) string {
diff --git a/internal/tui/eventstream/model.go b/internal/tui/eventstream/model.go
index 2780524..a8f399c 100644
--- a/internal/tui/eventstream/model.go
+++ b/internal/tui/eventstream/model.go
@@ -500,9 +500,15 @@ func (m *Model) View(width, height int) string {
 	if m.paused && m.selectedIdx >= 0 {
 		status = fmt.Sprintf("Row %d/%d | Sel %d/%d Col %d/%d | Enter push-filter | Esc/F undo", rowNumber(start, len(m.filtered)), len(m.filtered), rowNumber(m.selectedIdx, len(m.filtered)), len(m.filtered), m.selectedCol+1, streamColumnCount)
 	}
-	out := base + "\n" + status
+	// Use a Builder to avoid a redundant allocation for the optional status-message
+	// line appended conditionally on every render call.
+	var b strings.Builder
+	b.WriteString(base)
+	b.WriteString("\n")
+	b.WriteString(status)
 	if m.statusMessage != "" {
-		out += "\n" + m.statusMessage
+		b.WriteString("\n")
+		b.WriteString(m.statusMessage)
 	}
 
 	if m.exportModal.Visible() {
@@ -511,7 +517,7 @@ func (m *Model) View(width, height int) string {
 	if m.searchModal.Visible() {
 		return m.searchModal.View(width, height)
 	}
-	return out
+	return b.String()
 }
 
 func (m *Model) Refresh() {
diff --git a/internal/tui/flamegraph/controls.go b/internal/tui/flamegraph/controls.go
index bd588b3..8ec1051 100644
--- a/internal/tui/flamegraph/controls.go
+++ b/internal/tui/flamegraph/controls.go
@@ -90,21 +90,27 @@ func (m Model) toolbarLine() string {
 		state = lipgloss.NewStyle().Foreground(common.ColorDanger).Bold(true).Render("[PAUSED]")
 	}
 	order := m.currentFieldPresetLabel()
-	line := fmt.Sprintf("%s | view:%s | o:order(%s) | b:metric(%s) | /:search | enter/click:zoom | click ancestor:undo | u/esc:undo | r:reset | space:pause", state, compactFramePath(m.currentRootPath()), order, m.countFieldLabel())
+	// Use a Builder to avoid repeated allocations for the optional suffix segments.
+	var b strings.Builder
+	b.WriteString(fmt.Sprintf("%s | view:%s | o:order(%s) | b:metric(%s) | /:search | enter/click:zoom | click ancestor:undo | u/esc:undo | r:reset | space:pause",
+		state, compactFramePath(m.currentRootPath()), order, m.countFieldLabel()))
 	if m.searchQuery != "" {
-		line += " | filter:" + m.searchQuery
+		b.WriteString(" | filter:")
+		b.WriteString(m.searchQuery)
 	}
 	if m.statusMessage != "" {
-		line += " | " + m.statusMessage
+		b.WriteString(" | ")
+		b.WriteString(m.statusMessage)
 	}
 	if flameKeyDebugEnabled && m.lastKeyDebug != "" {
-		line += " | " + m.lastKeyDebug
+		b.WriteString(" | ")
+		b.WriteString(m.lastKeyDebug)
 	}
 	width := m.width
 	if width <= 0 {
 		width = 80
 	}
-	return padOrTrim(line, width)
+	return padOrTrim(b.String(), width)
 }
 
 func (m Model) helpOverlay() string {
@@ -148,12 +154,15 @@ func (m Model) selectionStatusLine() string {
 			shareLabel = fmt.Sprintf("%.2f%% of filtered %s", filterShare, metric)
 		}
 	}
-	line := fmt.Sprintf("[%s] sel:%d/%d %s | path:%s | depth:%d | total(%s):%d | %s",
-		mode, selIdx+1, len(m.frames), frame.Name, compactFramePath(frame.Path), frame.Depth, m.countFieldLabel(), frame.Total, shareLabel)
+	// Use a Builder to avoid a separate allocation for the optional filter suffix.
+	var b strings.Builder
+	b.WriteString(fmt.Sprintf("[%s] sel:%d/%d %s | path:%s | depth:%d | total(%s):%d | %s",
+		mode, selIdx+1, len(m.frames), frame.Name, compactFramePath(frame.Path), frame.Depth, m.countFieldLabel(), frame.Total, shareLabel))
 	if m.searchQuery != "" {
-		line += " | filter:" + m.searchQuery
+		b.WriteString(" | filter:")
+		b.WriteString(m.searchQuery)
 	}
-	return common.HelpBarStyle.Width(width).Render(padOrTrim(line, width))
+	return common.HelpBarStyle.Width(width).Render(padOrTrim(b.String(), width))
 }
 
 func (m Model) currentFieldPresetLabel() string {
diff --git a/internal/tui/flamegraph/model.go b/internal/tui/flamegraph/model.go
index c188323..3c63c3e 100644
--- a/internal/tui/flamegraph/model.go
+++ b/internal/tui/flamegraph/model.go
@@ -506,11 +506,17 @@ func (m Model) renderViewContent() string {
 	if m.snapshot != nil && len(m.frames) == 0 {
 		content = common.PanelStyle.Render(fmt.Sprintf("Flame: snapshot v%d has no visible frames", m.lastVersion))
 	}
-	content += "\n" + m.selectionStatusLine()
+	// Assemble the final output using a Builder to avoid repeated string copies
+	// for the optional help-overlay suffix.
+	var b strings.Builder
+	b.WriteString(content)
+	b.WriteString("\n")
+	b.WriteString(m.selectionStatusLine())
 	if m.showHelp {
-		content += "\n" + m.helpOverlay()
+		b.WriteString("\n")
+		b.WriteString(m.helpOverlay())
 	}
-	return content
+	return b.String()
 }
 
 // currentViewCacheKey snapshots every Model field that influences View()
diff --git a/internal/tui/flamegraph/renderer.go b/internal/tui/flamegraph/renderer.go
index 24b99ed..361feb2 100644
--- a/internal/tui/flamegraph/renderer.go
+++ b/internal/tui/flamegraph/renderer.go
@@ -246,14 +246,16 @@ func computeRenderParams(frames []tuiFrame, height int) renderViewParams {
 
 // buildToolbar assembles the top-of-view toolbar string and pads/trims it to
 // width. The toolbar is replaced by the caller via replaceHeaderLine.
+// A Builder is used to avoid an extra allocation for the optional truncation suffix.
 func buildToolbar(frames []tuiFrame, width int, params renderViewParams) string {
 	viewPath := compactFramePath(frames[0].Path)
-	toolbar := fmt.Sprintf("Flame | view:%s | frames:%d | rows:%d",
-		viewPath, params.visibleFrames, params.availableRows)
+	var b strings.Builder
+	b.WriteString(fmt.Sprintf("Flame | view:%s | frames:%d | rows:%d",
+		viewPath, params.visibleFrames, params.availableRows))
 	if params.truncated {
-		toolbar += " | showing deepest levels"
+		b.WriteString(" | showing deepest levels")
 	}
-	return padOrTrim(toolbar, width)
+	return padOrTrim(b.String(), width)
 }
 
 // buildFilteredStatus builds the per-selection status line when a search filter
diff --git a/internal/tui/help.go b/internal/tui/help.go
index ba4ed02..5a343cb 100644
--- a/internal/tui/help.go
+++ b/internal/tui/help.go
@@ -47,12 +47,18 @@ type helpSection struct {
 }
 
 func (m Model) helpSections() []helpSection {
+	line1 := "f filter  p pid picker  t tid picker  o probes  R parquet rec"
+	if help := m.keys.Export.Help(); help.Key != "" || help.Desc != "" {
+		// Use a Builder to append the optional export hint without reallocating
+		// the base string on each render when help is visible.
+		var b strings.Builder
+		b.WriteString(line1)
+		b.WriteString("  e stream export")
+		line1 = b.String()
+	}
 	globalLines := []string{
 		"H help  esc/? close help  q quit",
-		"f filter  p pid picker  t tid picker  o probes  R parquet rec",
-	}
-	if help := m.keys.Export.Help(); help.Key != "" || help.Desc != "" {
-		globalLines[1] += "  e stream export"
+		line1,
 	}
 
 	return []helpSection{
diff --git a/internal/tui/probes/model.go b/internal/tui/probes/model.go
index b7e694f..986b697 100644
--- a/internal/tui/probes/model.go
+++ b/internal/tui/probes/model.go
@@ -276,11 +276,15 @@ func (m Model) View(width, height int) string {
 		if p.Active {
 			check = "[x]"
 		}
-		line := fmt.Sprintf("%s%s %-24s", prefix, check, p.Syscall)
+		// Use a Builder to avoid an extra allocation for the optional error suffix
+		// emitted per probe row on every render call.
+		var lb strings.Builder
+		lb.WriteString(fmt.Sprintf("%s%s %-24s", prefix, check, p.Syscall))
 		if p.Error != "" {
-			line += " ! " + truncateText(sanitizeOneLine(p.Error), 28)
+			lb.WriteString(" ! ")
+			lb.WriteString(truncateText(sanitizeOneLine(p.Error), 28))
 		}
-		lines = append(lines, line)
+		lines = append(lines, lb.String())
 	}
 	if len(items) == 0 {
 		lines = append(lines, "  (no probes)")