From c3e3ef219918c25d208e1ade95f45b7fa9856a82 Mon Sep 17 00:00:00 2001
From: Valentin Semirulnik <v7rulnik@gmail.com>
Date: Mon, 11 May 2026 03:47:27 +0400
Subject: [PATCH] perf: inline base64VLQ.decode slow path in _parseMappings

---
 lib/source-map-consumer.js | 37 +++++++++++++++++++++++++++++++------
 1 file changed, 31 insertions(+), 6 deletions(-)

diff --git a/lib/source-map-consumer.js b/lib/source-map-consumer.js
index f56ffcdb..d25b4944 100644
--- a/lib/source-map-consumer.js
+++ b/lib/source-map-consumer.js
@@ -8,7 +8,12 @@
 var util = require('./util');
 var binarySearch = require('./binary-search');
 var ArraySet = require('./array-set').ArraySet;
-var base64VLQ = require('./base64-vlq');
+// `base64DecodeTable` maps a base64 character code → its 6-bit decoded value,
+// with `255` as the invalid-char sentinel. Imported directly so the inlined
+// multi-byte VLQ slow path in `_parseMappings` can index it without going
+// through a wrapper function — same direct-table pattern PR #67 applied to
+// `_names._array`.
+var base64DecodeTable = require('./base64').charToIntMap;
 
 function SourceMapConsumer(aSourceMap, aSourceMapURL) {
   var sourceMap = aSourceMap;
@@ -697,7 +702,6 @@ BasicSourceMapConsumer.prototype._parseMappings =
     var previousName = 0;
     var length = aStr.length;
     var index = 0;
-    var temp = {};
     var value, charCode;
     // Reuse segment array to avoid allocations per mapping
     var segment = [0, 0, 0, 0, 0];
@@ -738,10 +742,31 @@ BasicSourceMapConsumer.prototype._parseMappings =
             index++;
             segment[segmentLength++] = value;
           } else {
-            base64VLQ.decode(aStr, index, temp);
-            value = temp.value;
-            index = temp.rest;
-            segment[segmentLength++] = value;
+            // Multi-byte VLQ decode — inlined. Indirect-dispatched
+            // base64VLQ.decode + the temp output object's `.value`/`.rest`
+            // round trip is more cost than the body itself for typical
+            // small deltas. Bit layout: bit 0 = sign, bits 1-4 = value
+            // (chunked 5-bit groups across continuation digits), bit 5 =
+            // continuation. `base64DecodeTable[c] === 255` marks an
+            // invalid base64 char.
+            var vlqResult = 0;
+            var vlqShift = 0;
+            var vlqDigit;
+            do {
+              if (index >= length) {
+                throw new Error('Expected more digits in base 64 VLQ value.');
+              }
+              var vlqC = aStr.charCodeAt(index++);
+              vlqDigit = vlqC < 128 ? base64DecodeTable[vlqC] : 255;
+              if (vlqDigit === 255) {
+                throw new Error('Invalid base64 digit: ' + aStr.charAt(index - 1));
+              }
+              vlqResult = vlqResult + ((vlqDigit & 31) << vlqShift);
+              vlqShift += 5;
+            } while ((vlqDigit & 32) !== 0);
+            segment[segmentLength++] = (vlqResult & 1) === 1
+              ? -(vlqResult >> 1)
+              : (vlqResult >> 1);
           }
         }