fix: render inline math while typing

src/utils/mathMarkdown.test.ts

@@ -4,6 +4,7 @@ import {
   MATH_INLINE_TYPE,
   injectMathInBlocks,
   preProcessMathMarkdown,
+  readCompletedInlineMathAtEnd,
   serializeMathAwareBlocks,
 } from './mathMarkdown'
 
@@ -107,4 +108,18 @@ describe('math markdown round-trip', () => {
 
     expect(preProcessMathMarkdown({ markdown })).toBe(markdown)
   })
+
+  it('recognizes completed inline math at the end of text', () => {
+    expect(readCompletedInlineMathAtEnd({ text: 'Energy is $E=mc^2$' })).toEqual({
+      latex: 'E=mc^2',
+      start: 10,
+      end: 17,
+    })
+  })
+
+  it('ignores incomplete, escaped, and display-style dollar sequences at text end', () => {
+    expect(readCompletedInlineMathAtEnd({ text: 'Energy is $E=mc^2' })).toBeNull()
+    expect(readCompletedInlineMathAtEnd({ text: String.raw`Energy is $E=mc^2\$` })).toBeNull()
+    expect(readCompletedInlineMathAtEnd({ text: '$$x^2$$' })).toBeNull()
+  })
 })

src/utils/mathMarkdown.ts

@@ -55,6 +55,10 @@ interface InlineMathMatch extends LatexPayload {
   end: number
 }
 
+interface CompletedInlineMathMatch extends InlineMathMatch {
+  start: number
+}
+
 interface MarkdownSource {
   markdown: string
 }
@@ -137,6 +141,30 @@ function readInlineMath({ text, index }: TextPosition): InlineMathMatch | null {
   return isValidInlineLatex({ latex }) ? { latex, end } : null
 }
 
+function isCompletedInlineMathEnd({ text, index }: TextPosition): boolean {
+  return isInlineMathEnd({ text, index }) && text[index - 1] !== '$'
+}
+
+function findCompletedInlineMathStart({ text, index: end }: TextPosition): number {
+  for (let i = end - 1; i >= 0; i--) {
+    if (isInlineMathEnd({ text, index: i }) && text[i - 1] !== '$') {
+      return i
+    }
+  }
+  return -1
+}
+
+export function readCompletedInlineMathAtEnd({ text }: { text: string }): CompletedInlineMathMatch | null {
+  const end = text.length - 1
+  if (end < 1 || !isCompletedInlineMathEnd({ text, index: end })) return null
+
+  const start = findCompletedInlineMathStart({ text, index: end })
+  if (start === -1) return null
+
+  const latex = text.slice(start + 1, end)
+  return isValidInlineLatex({ latex }) ? { latex, start, end } : null
+}
+
 function replaceInlineMath({ line }: MarkdownLine): string {
   let result = ''
   let index = 0