fix: use first strong character for rich bidi metadata

Co-authored-by: Sepcnt <30561671+sepcnt@users.noreply.github.com>

Author: chenglou

src/bidi.ts

@@ -78,7 +78,21 @@ function computeBidiLevels(str: string): Int8Array | null {
 
   if (numBidi === 0) return null
 
-  const startLevel = (len / numBidi) < 0.3 ? 0 : 1
+  // Use the first strong character to pick the paragraph base direction.
+  // Rich-path bidi metadata is only an approximation, but this keeps mixed
+  // LTR/RTL text aligned with the common UBA paragraph rule.
+  let startLevel = 0
+  for (let i = 0; i < len; i++) {
+    const t = types[i]!
+    if (t === 'L') {
+      startLevel = 0
+      break
+    }
+    if (t === 'R' || t === 'AL') {
+      startLevel = 1
+      break
+    }
+  }
   const levels = new Int8Array(len)
   for (let i = 0; i < len; i++) levels[i] = startLevel
 

src/layout.test.ts

@@ -43,6 +43,7 @@ type TestLayoutCursor = {
 
 type TestPreparedTextWithSegments = {
   segments: string[]
+  segLevels?: Int8Array | null
 }
 
 type TestLayoutLine = {
@@ -213,6 +214,20 @@ function terminalCursor(prepared: TestPreparedTextWithSegments): TestLayoutCurso
   return { segmentIndex: prepared.segments.length, graphemeIndex: 0 }
 }
 
+function getNonSpaceSegmentLevels(
+  prepared: TestPreparedTextWithSegments,
+): Array<{ level: number, text: string }> {
+  if (prepared.segLevels === null || prepared.segLevels === undefined) return []
+
+  const levels: Array<{ level: number, text: string }> = []
+  for (let i = 0; i < prepared.segments.length; i++) {
+    const text = prepared.segments[i]!
+    if (text.trim().length === 0) continue
+    levels.push({ level: prepared.segLevels[i]!, text })
+  }
+  return levels
+}
+
 class TestCanvasRenderingContext2D {
   font = ''
 
@@ -543,6 +558,30 @@ describe('prepare invariants', () => {
     const latin = prepare('hello world', FONT)
     expect(layout(latin, 200, LINE_HEIGHT)).toEqual({ lineCount: 1, height: LINE_HEIGHT })
   })
+
+  test('pure LTR text skips rich bidi metadata', () => {
+    expect(prepareWithSegments('hello world', FONT).segLevels).toBeNull()
+  })
+
+  test('rich bidi metadata uses the first strong character for paragraph direction', () => {
+    const ltrFirst = prepareWithSegments('one اثنان three', FONT)
+    expect(ltrFirst.segLevels).not.toBeNull()
+    expect(ltrFirst.segLevels).toHaveLength(ltrFirst.segments.length)
+    expect(getNonSpaceSegmentLevels(ltrFirst)).toEqual([
+      { text: 'one', level: 0 },
+      { text: 'اثنان', level: 1 },
+      { text: 'three', level: 0 },
+    ])
+
+    const rtlFirst = prepareWithSegments('123 واحد three', FONT)
+    expect(rtlFirst.segLevels).not.toBeNull()
+    expect(rtlFirst.segLevels).toHaveLength(rtlFirst.segments.length)
+    expect(getNonSpaceSegmentLevels(rtlFirst)).toEqual([
+      { text: '123', level: 2 },
+      { text: 'واحد', level: 1 },
+      { text: 'three', level: 2 },
+    ])
+  })
 })
 
 describe('inline-flow invariants', () => {