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perf: add fast path for single-byte VLQ decode #37

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perf: add fast path for single-byte VLQ decode #37

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33 changes: 28 additions & 5 deletions lib/source-map-consumer.js
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Original file line number Diff line number Diff line change
Expand Up @@ -502,6 +502,21 @@ function sortGenerated(array, start) {
quickSort(array, compareGenerated, start);
}
}
// Lookup table for single-byte VLQ decode (no continuation bit)
// Maps base64 char code -> decoded signed value, or undefined if multi-byte
var vlqTable = [];
(function() {
var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
for (var i = 0; i < 128; i++) vlqTable[i] = undefined;
// Only first 32 base64 values (A-f) are single-byte VLQ (no continuation bit)
for (var i = 0; i < 32; i++) {
var charCode = chars.charCodeAt(i);
// Single-byte VLQ: bit 0 is sign, bits 1-4 are value
var value = i >> 1;
vlqTable[charCode] = (i & 1) ? -value : value;
}
})();

BasicSourceMapConsumer.prototype._parseMappings =
function SourceMapConsumer_parseMappings(aStr, aSourceRoot) {
var generatedLine = 1;
Expand All @@ -516,7 +531,7 @@ BasicSourceMapConsumer.prototype._parseMappings =
var temp = {};
var originalMappings = [];
var generatedMappings = [];
var mapping, str, segment, end, value;
var mapping, str, segment, end, value, charCode;

let subarrayStart = 0;
while (index < length) {
Expand Down Expand Up @@ -544,10 +559,18 @@ BasicSourceMapConsumer.prototype._parseMappings =

segment = [];
while (index < end) {
base64VLQ.decode(aStr, index, temp);
value = temp.value;
index = temp.rest;
segment.push(value);
// Fast path for single-byte VLQ (most common case)
charCode = aStr.charCodeAt(index);
value = vlqTable[charCode];
if (value !== undefined) {
index++;
segment.push(value);
} else {
base64VLQ.decode(aStr, index, temp);
value = temp.value;
index = temp.rest;
segment.push(value);
}
}

if (segment.length === 2) {
Expand Down