fix: cache breakable fit advances per mode

src/layout.test.ts

@@ -12,6 +12,8 @@ type LayoutModule = typeof import('./layout.ts')
 type LineBreakModule = typeof import('./line-break.ts')
 type RichInlineModule = typeof import('./rich-inline.ts')
 type AnalysisModule = typeof import('./analysis.ts')
+type MeasurementModule = typeof import('./measurement.ts')
+type SegmentMetrics = ReturnType<MeasurementModule['getSegmentMetrics']>
 
 let prepare: LayoutModule['prepare']
 let prepareWithSegments: LayoutModule['prepareWithSegments']
@@ -32,6 +34,7 @@ let materializeRichInlineLineRange: RichInlineModule['materializeRichInlineLineR
 let measureRichInlineStats: RichInlineModule['measureRichInlineStats']
 let walkRichInlineLineRanges: RichInlineModule['walkRichInlineLineRanges']
 let isCJK: AnalysisModule['isCJK']
+let getSegmentBreakableFitAdvances: MeasurementModule['getSegmentBreakableFitAdvances']
 
 const emojiPresentationRe = /\p{Emoji_Presentation}/u
 const punctuationRe = /[.,!?;:%)\]}'"”’»›…—-]/u
@@ -262,11 +265,12 @@ class TestOffscreenCanvas {
 
 beforeAll(async () => {
   Reflect.set(globalThis, 'OffscreenCanvas', TestOffscreenCanvas)
-  const [analysisMod, mod, lineBreakMod, richInlineMod] = await Promise.all([
+  const [analysisMod, mod, lineBreakMod, richInlineMod, measurementMod] = await Promise.all([
     import('./analysis.ts'),
     import('./layout.ts'),
     import('./line-break.ts'),
     import('./rich-inline.ts'),
+    import('./measurement.ts'),
   ])
   ;({ isCJK } = analysisMod)
   ;({
@@ -283,13 +287,33 @@ beforeAll(async () => {
   } = mod)
   ;({ countPreparedLines, measurePreparedLineGeometry, stepPreparedLineGeometry, walkPreparedLines } = lineBreakMod)
   ;({ prepareRichInline, materializeRichInlineLineRange, measureRichInlineStats, walkRichInlineLineRanges } = richInlineMod)
+  ;({ getSegmentBreakableFitAdvances } = measurementMod)
 })
 
 beforeEach(() => {
   setLocale(undefined)
   clearCache()
 })
 
+describe('measurement invariants', () => {
+  test('breakable fit advance cache is keyed by fit mode', () => {
+    const metrics: SegmentMetrics = { width: 80, containsCJK: false }
+    const cache: Map<string, SegmentMetrics> = new Map([
+      ['a', { width: 10, containsCJK: false }],
+      ['b', { width: 20, containsCJK: false }],
+      ['c', { width: 30, containsCJK: false }],
+      ['ab', { width: 35, containsCJK: false }],
+      ['bc', { width: 60, containsCJK: false }],
+      ['abc', metrics],
+    ])
+
+    expect(getSegmentBreakableFitAdvances('abc', metrics, cache, 0, 'sum-graphemes')).toEqual([10, 20, 30])
+    expect(getSegmentBreakableFitAdvances('abc', metrics, cache, 0, 'pair-context')).toEqual([10, 25, 40])
+    expect(getSegmentBreakableFitAdvances('abc', metrics, cache, 0, 'segment-prefixes')).toEqual([10, 25, 45])
+    expect(getSegmentBreakableFitAdvances('abc', metrics, cache, 0, 'sum-graphemes')).toEqual([10, 20, 30])
+  })
+})
+
 describe('prepare invariants', () => {
   test('whitespace-only input stays empty', () => {
     const prepared = prepare('  \t\n  ', FONT)

src/measurement.ts

@@ -4,8 +4,7 @@ export type SegmentMetrics = {
   width: number
   containsCJK: boolean
   emojiCount?: number
-  breakableFitMode?: BreakableFitMode
-  breakableFitAdvances?: number[] | null
+  breakableFitAdvancesByMode?: Partial<Record<BreakableFitMode, number[] | null>>
 }
 
 export type EngineProfile = {
@@ -204,32 +203,41 @@ export function getSegmentBreakableFitAdvances(
   emojiCorrection: number,
   mode: BreakableFitMode,
 ): number[] | null {
-  if (metrics.breakableFitAdvances !== undefined && metrics.breakableFitMode === mode) {
-    return metrics.breakableFitAdvances
-  }
-  metrics.breakableFitMode = mode
-
   const graphemeSegmenter = getSharedGraphemeSegmenter()
   const graphemes: string[] = []
   for (const gs of graphemeSegmenter.segment(seg)) {
     graphemes.push(gs.segment)
   }
+
+  const cacheMode =
+    mode === 'segment-prefixes' && graphemes.length > MAX_PREFIX_FIT_GRAPHEMES
+      ? 'pair-context'
+      : mode
+  const cached = metrics.breakableFitAdvancesByMode?.[cacheMode]
+  if (cached !== undefined) {
+    return cached
+  }
+
+  function cacheAdvances(advances: number[] | null): number[] | null {
+    const byMode = metrics.breakableFitAdvancesByMode ??= {}
+    byMode[cacheMode] = advances
+    return advances
+  }
+
   if (graphemes.length <= 1) {
-    metrics.breakableFitAdvances = null
-    return metrics.breakableFitAdvances
+    return cacheAdvances(null)
   }
 
-  if (mode === 'sum-graphemes') {
+  if (cacheMode === 'sum-graphemes') {
     const advances: number[] = []
     for (const grapheme of graphemes) {
       const graphemeMetrics = getSegmentMetrics(grapheme, cache)
       advances.push(getCorrectedSegmentWidth(grapheme, graphemeMetrics, emojiCorrection))
     }
-    metrics.breakableFitAdvances = advances
-    return metrics.breakableFitAdvances
+    return cacheAdvances(advances)
   }
 
-  if (mode === 'pair-context' || graphemes.length > MAX_PREFIX_FIT_GRAPHEMES) {
+  if (cacheMode === 'pair-context') {
     const advances: number[] = []
     let previousGrapheme: string | null = null
     let previousWidth = 0
@@ -250,8 +258,7 @@ export function getSegmentBreakableFitAdvances(
       previousWidth = currentWidth
     }
 
-    metrics.breakableFitAdvances = advances
-    return metrics.breakableFitAdvances
+    return cacheAdvances(advances)
   }
 
   const advances: number[] = []
@@ -266,8 +273,7 @@ export function getSegmentBreakableFitAdvances(
     prefixWidth = nextPrefixWidth
   }
 
-  metrics.breakableFitAdvances = advances
-  return metrics.breakableFitAdvances
+  return cacheAdvances(advances)
 }
 
 export function getFontMeasurementState(font: string, needsEmojiCorrection: boolean): {