Seven Minutes to Parse a Menu — JavaScript Bug Hunt

Briefing

Modelled on the GTA Online load-time investigation (2021): a player traced six-minute loads to a JSON parser that called sscanf on a 10 MB buffer for every one of its 63,000 items. sscanf measures the whole string first, so each item cost a full scan — a one-line fix cut load times by 70%.

catalog.js has the same shape: a helper re-walks the entire payload on every item.

Fix parseItems so the total work is linear in the payload length.

Bug report

BUG-LOADTIME · Priority: High · Reported by: performance

parseItems(payload) splits a comma-separated payload into trimmed item names.

Observed: parsing is quadratic. With a payload of 2,000 items the helper reads about 4,000,000 characters. The instrumented counter proves it.

Logs

[catalog] parsed 63000 items in 383s
[catalog] charReads=3968072 for a 1994-char payload

The code as shipped

src/catalog/catalog.js (editable)

var counter = require("./counter");

// Returns the length of the payload, counting every character it touches.
function measure(payload) {
  var n = 0;
  while (n < payload.length) {
    counter.charRead();
    n++;
  }
  return n;
}

exports.parseItems = function (payload) {
  var items = [];
  var current = "";
  for (var i = 0; i < payload.length; i++) {
    // Re-measures the whole payload on every single character.
    var total = measure(payload);
    if (i >= total) break;
    var c = payload.charAt(i);
    if (c === ",") {
      items.push(current);
      current = "";
    } else {
      current += c;
    }
  }
  if (current.length > 0) items.push(current);
  return items;
};

Read-only context: src/catalog/counter.js.