fix: handleincompletesingleunderscoreitalic is not accounting for the usage inside math equations (vercel#38)

* Fix underscores in math equations

* Create cuddly-goats-warn.md

Author: haydenbleasel

.changeset/cuddly-goats-warn.md

@@ -0,0 +1,5 @@
+---
+"streamdown": patch
+---
+
+fix: handleIncompleteSingleUnderscoreItalic is not accounting for the usage inside math equations

packages/streamdown/__tests__/parse-incomplete-markdown.test.ts

@@ -626,6 +626,73 @@ describe('parseIncompleteMarkdown', () => {
     });
   });
 
+  describe('math blocks with underscores', () => {
+    it('should not complete underscores within inline math blocks', () => {
+      const text = 'The variable $x_1$ represents the first element';
+      expect(parseIncompleteMarkdown(text)).toBe(text);
+      
+      const text2 = 'Formula: $a_b + c_d = e_f$';
+      expect(parseIncompleteMarkdown(text2)).toBe(text2);
+    });
+
+    it('should not complete underscores within block math', () => {
+      const text = '$$x_1 + y_2 = z_3$$';
+      expect(parseIncompleteMarkdown(text)).toBe(text);
+      
+      const text2 = '$$\na_1 + b_2\nc_3 + d_4\n$$';
+      expect(parseIncompleteMarkdown(text2)).toBe(text2);
+    });
+
+    it('should not add underscore when math block has incomplete underscore', () => {
+      // Incomplete math blocks get completed by handleIncompleteInlineKatex
+      // The underscore inside should not be treated as italic
+      const text = 'Math expression $x_';
+      expect(parseIncompleteMarkdown(text)).toBe('Math expression $x_$');
+      
+      const text2 = '$$formula_';
+      expect(parseIncompleteMarkdown(text2)).toBe('$$formula_$$');
+    });
+
+    it('should handle underscores outside math blocks normally', () => {
+      const text = 'Text with _italic_ and math $x_1$';
+      expect(parseIncompleteMarkdown(text)).toBe(text);
+      
+      const text2 = '_italic text_ followed by $a_b$';
+      expect(parseIncompleteMarkdown(text2)).toBe(text2);
+    });
+
+    it('should complete italic underscore outside math but not inside', () => {
+      const text = 'Start _italic with $x_1$';
+      expect(parseIncompleteMarkdown(text)).toBe('Start _italic with $x_1$_');
+    });
+
+    it('should handle complex math expressions with multiple underscores', () => {
+      const text = '$x_1 + x_2 + x_3 = y_1$';
+      expect(parseIncompleteMarkdown(text)).toBe(text);
+      
+      const text2 = '$$\\sum_{i=1}^{n} x_i = \\prod_{j=1}^{m} y_j$$';
+      expect(parseIncompleteMarkdown(text2)).toBe(text2);
+    });
+
+    it('should handle escaped dollar signs correctly', () => {
+      const text = 'Price is \\$50 and _this is italic_';
+      expect(parseIncompleteMarkdown(text)).toBe(text);
+      
+      const text2 = 'Cost \\$100 with _incomplete';
+      expect(parseIncompleteMarkdown(text2)).toBe('Cost \\$100 with _incomplete_');
+    });
+
+    it('should handle mixed inline and block math', () => {
+      const text = 'Inline $x_1$ and block $$y_2$$ math';
+      expect(parseIncompleteMarkdown(text)).toBe(text);
+    });
+
+    it('should not interfere with complete math blocks when adding underscores outside', () => {
+      const text = '_italic start $x_1$ italic end_';
+      expect(parseIncompleteMarkdown(text)).toBe(text);
+    });
+  });
+
   describe('edge cases', () => {
     it('should handle text ending with formatting characters', () => {
       expect(parseIncompleteMarkdown('Text ending with *')).toBe(

packages/streamdown/lib/parse-incomplete-markdown.ts

@@ -81,7 +81,36 @@ const handleIncompleteSingleAsteriskItalic = (text: string): string => {
   return text;
 };
 
-// Counts single underscores that are not part of double underscores and not escaped
+// Check if a position is within a math block (between $ or $$)
+const isWithinMathBlock = (text: string, position: number): boolean => {
+  // Count dollar signs before this position
+  let inInlineMath = false;
+  let inBlockMath = false;
+  
+  for (let i = 0; i < text.length && i < position; i++) {
+    // Skip escaped dollar signs
+    if (text[i] === '\\' && text[i + 1] === '$') {
+      i++; // Skip the next character
+      continue;
+    }
+    
+    if (text[i] === '$') {
+      // Check for block math ($$)
+      if (text[i + 1] === '$') {
+        inBlockMath = !inBlockMath;
+        i++; // Skip the second $
+        inInlineMath = false; // Block math takes precedence
+      } else if (!inBlockMath) {
+        // Only toggle inline math if not in block math
+        inInlineMath = !inInlineMath;
+      }
+    }
+  }
+  
+  return inInlineMath || inBlockMath;
+};
+
+// Counts single underscores that are not part of double underscores, not escaped, and not in math blocks
 const countSingleUnderscores = (text: string): number => {
   return text.split('').reduce((acc, char, index) => {
     if (char === '_') {
@@ -91,6 +120,10 @@ const countSingleUnderscores = (text: string): number => {
       if (prevChar === '\\') {
         return acc;
       }
+      // Skip if within math block
+      if (isWithinMathBlock(text, index)) {
+        return acc;
+      }
       if (prevChar !== '_' && nextChar !== '_') {
         return acc + 1;
       }