Update DOMTextScanner to support UTF-16 surrogate pairs (#2213)

ext/js/dom/dom-text-scanner.js

@@ -145,6 +145,44 @@ class DOMTextScanner {
 
     // Private
 
+    /**
+     * Reads a code point in a string in the forward direction.
+     * @param {string} text The text to read the code point from.
+     * @param {number} position The index of the first character to read.
+     * @returns {string} The code point from the string.
+     */
+    _readCodePointForward(text, position) {
+        let char = text[position];
+        const charCode = char.charCodeAt(0);
+        if (charCode >= 0xd800 && charCode < 0xdc00 && ++position < text.length) {
+            const char2 = text[position];
+            const charCode2 = char2.charCodeAt(0);
+            if (charCode2 >= 0xdc00 && charCode2 < 0xe000) {
+                char += char2;
+            }
+        }
+        return char;
+    }
+
+    /**
+     * Reads a code point in a string in the backward direction.
+     * @param {string} text The text to read the code point from.
+     * @param {number} position The index of the first character to read.
+     * @returns {string} The code point from the string.
+     */
+    _readCodePointBackward(text, position) {
+        let char = text[position];
+        const charCode = char.charCodeAt(0);
+        if (charCode >= 0xdc00 && charCode < 0xe000 && position > 0) {
+            const char2 = text[position - 1];
+            const charCode2 = char2.charCodeAt(0);
+            if (charCode2 >= 0xd800 && charCode2 < 0xdc00) {
+                char = char2 + char;
+            }
+        }
+        return char;
+    }
+
     /**
      * Seeks forward in a text node.
      * @param {Text} textNode The text node to use.
@@ -164,9 +202,9 @@ class DOMTextScanner {
         let newlines = this._newlines;
 
         while (offset < nodeValueLength) {
-            const char = nodeValue[offset];
+            const char = this._readCodePointForward(nodeValue, offset);
+            offset += char.length;
             const charAttributes = DOMTextScanner.getCharacterAttributes(char, preserveNewlines, preserveWhitespace);
-            ++offset;
 
             if (charAttributes === 0) {
                 // Character should be ignored
@@ -188,7 +226,7 @@ class DOMTextScanner {
                     lineHasContent = false;
                     lineHasWhitespace = false;
                     if (remainder <= 0) {
-                        --offset; // Revert character offset
+                        offset -= char.length; // Revert character offset
                         break;
                     }
                 }
@@ -200,7 +238,7 @@ class DOMTextScanner {
                         content += ' ';
                         lineHasWhitespace = false;
                         if (--remainder <= 0) {
-                            --offset; // Revert character offset
+                            offset -= char.length; // Revert character offset
                             break;
                         }
                     } else {
@@ -250,8 +288,8 @@ class DOMTextScanner {
         let newlines = this._newlines;
 
         while (offset > 0) {
-            --offset;
+            const char = this._readCodePointBackward(nodeValue, offset - 1);
-            const char = nodeValue[offset];
+            offset -= char.length;
             const charAttributes = DOMTextScanner.getCharacterAttributes(char, preserveNewlines, preserveWhitespace);
 
             if (charAttributes === 0) {
@@ -274,7 +312,7 @@ class DOMTextScanner {
                     lineHasContent = false;
                     lineHasWhitespace = false;
                     if (remainder <= 0) {
-                        ++offset; // Revert character offset
+                        offset += char.length; // Revert character offset
                         break;
                     }
                 }
@@ -286,7 +324,7 @@ class DOMTextScanner {
                         content = ' ' + content;
                         lineHasWhitespace = false;
                         if (--remainder <= 0) {
-                            ++offset; // Revert character offset
+                            offset += char.length; // Revert character offset
                             break;
                         }
                     } else {