lib: replace StringCharIterator with simpler functions for character access

Author: miguelmarcondesf

lib/internal/source_map/source_map.js

@@ -72,7 +72,6 @@ const {
   ArrayPrototypeSlice,
   ArrayPrototypeSort,
   ObjectPrototypeHasOwnProperty,
-  StringPrototypeCharAt,
   Symbol,
 } = primordials;
 
@@ -86,38 +85,6 @@ const VLQ_CONTINUATION_MASK = 1 << 5;
 
 const kMappings = Symbol('kMappings');
 
-class StringCharIterator {
-  /**
-   * @constructor
-   * @param {string} string
-   */
-  constructor(string) {
-    this._string = string;
-    this._position = 0;
-  }
-
-  /**
-   * @return {string}
-   */
-  next() {
-    return StringPrototypeCharAt(this._string, this._position++);
-  }
-
-  /**
-   * @return {string}
-   */
-  peek() {
-    return StringPrototypeCharAt(this._string, this._position);
-  }
-
-  /**
-   * @return {boolean}
-   */
-  hasNext() {
-    return this._position < this._string.length;
-  }
-}
-
 /**
  * Implements Source Map V3 model.
  * See https://github.com/google/closure-compiler/wiki/Source-Maps
@@ -279,46 +246,48 @@ class SourceMap {
         this.#sourceContentByURL[url] = mapSourcesContent[i];
     }
 
-    const stringCharIterator = new StringCharIterator(mapMappings);
+    let pos = 0;
+    const peek = () => mapMappings.charAt(pos);
+    const next = () => mapMappings.charAt(pos++);
+    const hasNext = () => pos < mapMappings.length;
+
     let sourceURL = sources[sourceIndex];
     while (true) {
-      if (stringCharIterator.peek() === ',')
-        stringCharIterator.next();
-      else {
-        while (stringCharIterator.peek() === ';') {
+      if (peek() === ',') {
+        next();
+      } else {
+        while (peek() === ';') {
           lineNumber += 1;
           columnNumber = 0;
-          stringCharIterator.next();
+          next();
         }
-        if (!stringCharIterator.hasNext())
-          break;
+        if (!hasNext()) break;
       }
 
-      columnNumber += decodeVLQ(stringCharIterator);
-      if (isSeparator(stringCharIterator.peek())) {
-        ArrayPrototypePush(this.#mappings, [lineNumber, columnNumber]);
+      columnNumber += decodeVLQ(next);
+      if (isSeparator(peek())) {
+        this.#mappings.push([lineNumber, columnNumber]);
         continue;
       }
 
-      const sourceIndexDelta = decodeVLQ(stringCharIterator);
+      const sourceIndexDelta = decodeVLQ(next);
       if (sourceIndexDelta) {
         sourceIndex += sourceIndexDelta;
         sourceURL = sources[sourceIndex];
       }
-      sourceLineNumber += decodeVLQ(stringCharIterator);
-      sourceColumnNumber += decodeVLQ(stringCharIterator);
+      sourceLineNumber += decodeVLQ(next);
+      sourceColumnNumber += decodeVLQ(next);
 
       let name;
-      if (!isSeparator(stringCharIterator.peek())) {
-        nameIndex += decodeVLQ(stringCharIterator);
+      if (!isSeparator(peek())) {
+        nameIndex += decodeVLQ(next);
         name = mapNames?.[nameIndex];
       }
 
-      ArrayPrototypePush(
-        this.#mappings,
-        [lineNumber, columnNumber, sourceURL, sourceLineNumber,
-         sourceColumnNumber, name],
-      );
+      this.#mappings.push([
+        lineNumber, columnNumber, sourceURL,
+        sourceLineNumber, sourceColumnNumber, name,
+      ]);
     }
   }
 }
@@ -332,16 +301,16 @@ function isSeparator(char) {
 }
 
 /**
- * @param {SourceMap.StringCharIterator} stringCharIterator
+ * @param {() => string} next
  * @return {number}
  */
-function decodeVLQ(stringCharIterator) {
+function decodeVLQ(next) {
   // Read unsigned value.
   let result = 0;
   let shift = 0;
   let digit;
   do {
-    digit = base64Map[stringCharIterator.next()];
+    digit = base64Map[next()];
     result += (digit & VLQ_BASE_MASK) << shift;
     shift += VLQ_BASE_SHIFT;
   } while (digit & VLQ_CONTINUATION_MASK);