fix(compare): gap deltas beyond reference laps

Reference-null laps now emit null deltas for all drivers and exclude that
lap window from aligned cumulative totals so later comparisons stay sane.
Challenger-null behavior is unchanged. Compare tab meta documents the policy.

Closes #46

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/components/CompareView.tsx

@@ -229,7 +229,9 @@ export function CompareView({ sessionKey, results, drivers }: Props) {
         <div>
           <div className="compare-section-title">Race pace</div>
           <div className="compare-section-meta">
-            Cumulative lap-time delta vs {referenceLabel ?? 'reference'}
+            Cumulative lap-time delta vs {referenceLabel ?? 'reference'}. Deltas are plotted
+            only where the reference lap is valid; gaps appear when the reference has no lap
+            time.
           </div>
         </div>
         <SectionState

frontend/src/lib/delta.ts

@@ -21,15 +21,31 @@ export function formatDeltaSeconds(delta: number): string {
   return `${sign}${delta.toFixed(1)}s`
 }
 
-function buildCumulative(lapTimes: ReadonlyArray<number | null>): number[] {
+/**
+ * Cumulative lap time aligned to reference-valid laps only.
+ * Laps where the reference is null are skipped for every series so later deltas
+ * do not compare against a frozen baseline while challengers keep accumulating.
+ */
+function buildAlignedCumulative(
+  lapTimes: ReadonlyArray<number | null>,
+  referenceLapTimes: ReadonlyArray<number | null>,
+): number[] {
   const cumulative: number[] = []
   let running = 0
-  for (const lap of lapTimes) {
+  const length = Math.max(lapTimes.length, referenceLapTimes.length)
-    if (lap !== null) {
+
+  for (let i = 0; i < length; i++) {
+    if (referenceLapTimes[i] == null) {
+      cumulative.push(running)
+      continue
+    }
+    const lap = lapTimes[i]
+    if (lap != null) {
       running += lap
     }
     cumulative.push(running)
   }
+
   return cumulative
 }
 
@@ -47,7 +63,8 @@ function resolveReference(
 /**
  * Compute per-lap cumulative time delta for each non-reference driver.
  * Positive = behind reference; negative = ahead.
- * Null laps carry cumulative forward but emit null in deltas (skip when plotting).
+ * Deltas are only emitted where the reference lap is valid; reference-null laps
+ * gap every series. Challenger-null laps gap only that driver's line.
  */
 export function computeCumulativeDeltas(
   series: ReadonlyArray<DeltaSeries>,
@@ -56,24 +73,22 @@ export function computeCumulativeDeltas(
   const reference = resolveReference(series, referenceLabel)
   if (!reference) return []
 
-  const refCumulative = buildCumulative(reference.lapTimes)
+  const refLapTimes = reference.lapTimes
+  const refCumulative = buildAlignedCumulative(refLapTimes, refLapTimes)
 
   return series
     .filter((s) => s.label !== reference.label)
     .map((driver) => {
-      const driverCumulative = buildCumulative(driver.lapTimes)
+      const driverCumulative = buildAlignedCumulative(driver.lapTimes, refLapTimes)
       const lapCount = Math.max(driver.lapTimes.length, refCumulative.length)
       const deltas: (number | null)[] = []
 
       for (let i = 0; i < lapCount; i++) {
-        if (driver.lapTimes[i] === null) {
+        if (refLapTimes[i] == null || driver.lapTimes[i] == null) {
           deltas.push(null)
           continue
         }
-        const refValue = refCumulative[i] ?? refCumulative[refCumulative.length - 1] ?? 0
+        deltas.push(driverCumulative[i] - refCumulative[i])
-        const driverValue =
-          driverCumulative[i] ?? driverCumulative[driverCumulative.length - 1] ?? 0
-        deltas.push(driverValue - refValue)
       }
 
       return {

frontend/src/test/delta-time-graph.test.tsx

@@ -47,7 +47,7 @@ describe('computeCumulativeDeltas', () => {
     expect(result[0].deltas[2]).toBeCloseTo(2)
   })
 
-  it('emits null for missing lap times while carrying cumulative forward', () => {
+  it('emits null for challenger missing lap times while carrying cumulative forward', () => {
     const withNull: DeltaSeries = {
       label: 'NOR',
       color: '#FF8000',
@@ -60,6 +60,41 @@ describe('computeCumulativeDeltas', () => {
     expect(result[0].deltas[2]).toBeCloseTo(-92)
   })
 
+  it('gaps all drivers when the reference lap is null and resumes without that window', () => {
+    const refWithNull: DeltaSeries = {
+      label: 'VER',
+      color: '#3671C6',
+      lapTimes: [90, null, 92],
+    }
+    const validChallenger: DeltaSeries = {
+      label: 'HAM',
+      color: '#E8002D',
+      lapTimes: [89, 91, 90],
+    }
+    const result = computeCumulativeDeltas([refWithNull, validChallenger])
+    expect(result[0].deltas[0]).toBeCloseTo(-1)
+    expect(result[0].deltas[1]).toBeNull()
+    // Lap 3 excludes the reference-null window for both: (89+90) - (90+92) = -3
+    expect(result[0].deltas[2]).toBeCloseTo(-3)
+  })
+
+  it('gaps challenger laps beyond a shorter reference series', () => {
+    const shortReference: DeltaSeries = {
+      label: 'VER',
+      color: '#3671C6',
+      lapTimes: [90, 91],
+    }
+    const longerChallenger: DeltaSeries = {
+      label: 'HAM',
+      color: '#E8002D',
+      lapTimes: [89, 92, 90],
+    }
+    const result = computeCumulativeDeltas([shortReference, longerChallenger])
+    expect(result[0].deltas[0]).toBeCloseTo(-1)
+    expect(result[0].deltas[1]).toBeCloseTo(0)
+    expect(result[0].deltas[2]).toBeNull()
+  })
+
   it('returns an empty array when only one series is provided', () => {
     expect(computeCumulativeDeltas([reference])).toEqual([])
   })
@@ -97,6 +132,19 @@ describe('DeltaTimeGraph', () => {
     expect(screen.queryByTestId('delta-line-VER')).not.toBeInTheDocument()
   })
 
+  it('splits polylines at reference-null laps', () => {
+    const refWithNull: DeltaSeries = {
+      label: 'VER',
+      color: '#3671C6',
+      lapTimes: [90, null, 92],
+    }
+    const { container } = render(
+      <DeltaTimeGraph series={[refWithNull, challenger]} />,
+    )
+    const lines = container.querySelectorAll('.delta-graph-driver-line')
+    expect(lines.length).toBeGreaterThan(1)
+  })
+
   it('shows a crosshair tooltip on hover', () => {
     vi.spyOn(SVGSVGElement.prototype, 'getBoundingClientRect').mockReturnValue({
       x: 0,
@@ -118,4 +166,31 @@ describe('DeltaTimeGraph', () => {
     expect(screen.getByText(/Lap 1/)).toBeInTheDocument()
     vi.restoreAllMocks()
   })
+
+  it('omits tooltip rows on reference-null laps', () => {
+    vi.spyOn(SVGSVGElement.prototype, 'getBoundingClientRect').mockReturnValue({
+      x: 0,
+      y: 0,
+      left: 0,
+      top: 0,
+      width: 640,
+      height: 220,
+      right: 640,
+      bottom: 220,
+      toJSON: () => ({}),
+    })
+    const refWithNull: DeltaSeries = {
+      label: 'VER',
+      color: '#3671C6',
+      lapTimes: [90, null, 92],
+    }
+    const { container } = render(
+      <DeltaTimeGraph series={[refWithNull, challenger]} />,
+    )
+    const hoverLayer = container.querySelector('.delta-graph-hover-layer')
+    fireEvent.mouseMove(hoverLayer!, { clientX: 352, clientY: 100 })
+    expect(screen.getByTestId('delta-crosshair')).toBeInTheDocument()
+    expect(screen.queryByTestId('delta-tooltip')).not.toBeInTheDocument()
+    vi.restoreAllMocks()
+  })
 })