perf: add fast path for single-char VLQ encode

lib/source-map-generator.js

@@ -323,6 +323,17 @@ SourceMapGenerator.prototype._validateMapping =
     }
   };
 
+// Fast VLQ encode lookup for values -15 to 15 (single char output)
+var vlqEncodeTable = [];
+(function() {
+  var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
+  for (var i = -15; i <= 15; i++) {
+    // VLQ signed: negative becomes odd, positive becomes even
+    var vlq = i < 0 ? ((-i) << 1) + 1 : (i << 1);
+    vlqEncodeTable[i + 15] = chars[vlq];
+  }
+})();
+
 /**
  * Serialize the accumulated mappings in to the stream of base 64 VLQs
  * specified by the source map format.
@@ -336,7 +347,7 @@ SourceMapGenerator.prototype._serializeMappings =
     var previousName = 0;
     var previousSource = 0;
     var result = '';
-    var next;
+    var next, val;
     var mapping;
     var nameIdx;
     var sourceIdx;
@@ -362,27 +373,29 @@ SourceMapGenerator.prototype._serializeMappings =
         }
       }
 
-      next += base64VLQ.encode(mapping.generatedColumn
-                                 - previousGeneratedColumn);
+      val = mapping.generatedColumn - previousGeneratedColumn;
+      next += (val >= -15 && val <= 15) ? vlqEncodeTable[val + 15] : base64VLQ.encode(val);
       previousGeneratedColumn = mapping.generatedColumn;
 
       if (mapping.source != null) {
         sourceIdx = this._sources.indexOf(mapping.source);
-        next += base64VLQ.encode(sourceIdx - previousSource);
+        val = sourceIdx - previousSource;
+        next += (val >= -15 && val <= 15) ? vlqEncodeTable[val + 15] : base64VLQ.encode(val);
         previousSource = sourceIdx;
 
         // lines are stored 0-based in SourceMap spec version 3
-        next += base64VLQ.encode(mapping.originalLine - 1
-                                   - previousOriginalLine);
+        val = mapping.originalLine - 1 - previousOriginalLine;
+        next += (val >= -15 && val <= 15) ? vlqEncodeTable[val + 15] : base64VLQ.encode(val);
         previousOriginalLine = mapping.originalLine - 1;
 
-        next += base64VLQ.encode(mapping.originalColumn
-                                   - previousOriginalColumn);
+        val = mapping.originalColumn - previousOriginalColumn;
+        next += (val >= -15 && val <= 15) ? vlqEncodeTable[val + 15] : base64VLQ.encode(val);
         previousOriginalColumn = mapping.originalColumn;
 
         if (mapping.name != null) {
           nameIdx = this._names.indexOf(mapping.name);
-          next += base64VLQ.encode(nameIdx - previousName);
+          val = nameIdx - previousName;
+          next += (val >= -15 && val <= 15) ? vlqEncodeTable[val + 15] : base64VLQ.encode(val);
           previousName = nameIdx;
         }
       }