From 51f44ab6824374e8390e8d1c1bdf29f85465c3d7 Mon Sep 17 00:00:00 2001
From: Mudit Ameta <zeusdeux@gmail.com>
Date: Wed, 29 Nov 2023 22:28:23 +0100
Subject: [PATCH] loader: speed up line length calc used by moduleProvider

When using a loader, for say TypeScript, the esm loader invokes the
`lineLengths` function via `maybeCacheSourceMap` when sourcemaps are
enabled. Therefore, `lineLengths` ends up getting called quite often
when running large servers written in TypeScript for example. Making
`lineLengths` faster should therefore speed up server startup times
for anyone using a loader with node with sourcemaps enabled.

The change itself is fairly simple and is all about removing creation
of unnecessary memory and iterating the whole source content only once
with the hope of making the function cache friendly.
---
 lib/internal/source_map/source_map_cache.js | 28 +++++++++++++++------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/lib/internal/source_map/source_map_cache.js b/lib/internal/source_map/source_map_cache.js
index 2813da21dfdc63..963b34ebcd241e 100644
--- a/lib/internal/source_map/source_map_cache.js
+++ b/lib/internal/source_map/source_map_cache.js
@@ -1,12 +1,12 @@
 'use strict';
 
 const {
-  ArrayPrototypeMap,
+  ArrayPrototypePush,
   JSONParse,
   ObjectKeys,
   RegExpPrototypeExec,
-  RegExpPrototypeSymbolSplit,
   SafeMap,
+  StringPrototypeCodePointAt,
   StringPrototypeSplit,
 } = primordials;
 
@@ -205,14 +205,26 @@ function dataFromUrl(sourceURL, sourceMappingURL) {
 // from. This allows translation from byte offset V8 coverage reports,
 // to line/column offset Source Map V3.
 function lineLengths(content) {
-  // We purposefully keep \r as part of the line-length calculation, in
-  // cases where there is a \r\n separator, so that this can be taken into
-  // account in coverage calculations.
-  return ArrayPrototypeMap(RegExpPrototypeSymbolSplit(/\n|\u2028|\u2029/, content), (line) => {
-    return line.length;
-  });
+  const contentLength = content.length;
+  const output = [];
+  let lineLength = 0;
+  for (let i = 0; i < contentLength; i++, lineLength++) {
+    const codePoint = StringPrototypeCodePointAt(content, i);
+
+    // We purposefully keep \r as part of the line-length calculation, in
+    // cases where there is a \r\n separator, so that this can be taken into
+    // account in coverage calculations.
+    // codepoints for \n (new line), \u2028 (line separator) and \u2029 (paragraph separator)
+    if (codePoint === 10 || codePoint === 0x2028 || codePoint === 0x2029) {
+      ArrayPrototypePush(output, lineLength);
+      lineLength = -1; // To not count the matched codePoint such as \n character
+    }
+  }
+  ArrayPrototypePush(output, lineLength);
+  return output;
 }
 
+
 function sourceMapFromFile(mapURL) {
   try {
     const fs = require('fs');