Quickly detecting JSON Escapes with SWAR

Most JSON string data is boring. It is just ASCII text that does not contain " or \, does not dip below 0x20, and does not force any special-case escaping at all. The annoying part is that a serializer still has to prove that. The naive loop is obvious: for (let i = 0; i < src.length; i++) { const code = src.charCodeAt(i); if (code == 34 || code == 92 || code < 32) { // escape } } That is correct, but it is also one branch per code unit in the hottest part of the serializer. For json-as, I wanted the fast path to ask a cheaper question: Does this whole chunk contain anything interesting at all? That is where SWAR fits nicely. For this post, the companion code is here: 00-detector-functions 01-c-and-wasm-benchmark 02-assemblyscript-inspection What needs detecting? When serializing a JSON string, these are the lanes that matter: " because it must become \" \ because it must become \\ control characters < 0x20 non-ASCII UTF-16 code units, because they cannot stay on the pure ASCI