Fast Hex

A fast, SIMD (vectorized) hex string decoder, available as a stand-alone C++ module and as a Node.js module.

• Supports upper-case and lower-case characters.
• Does not validate input. (I'm open to a PR to add this as an option.)
• Requires AVX2 (Haswell or later).

I think these implementations are close to optimal, but PRs are welcome for optimizations.

Node.js

const {decodeHexVec} = require("fast-hex");
decodeHexVec(output: Uint8Array|Buffer, input: Uint8Array|Buffer): void;

Benchmark

• ~2.12x faster decoding for short string inputs (< 1,031,913 chars).
• ~12x faster decoding for some long strings, Buffers and TypedArrays.

Development notes:

• Accessing string bytes from v8 is slow. Node uses external strings in some cases, but so far the only scenarios in which I've found them used are (a) via buffer.toString(), (b) via the C++ API v8::Local<v8::Value> node::Encode(v8::Isolate* isolate, const char* buf, size_t len, enum encoding encoding). Haven't looked yet at what cases cause v8 to automatically use external strings.
• stream.write uses StringBytes::Encode internally. Can't replace this without modifying node.js, unless we intercept stream.write in JS.
• Creating strings is also expensive. Still need to look at if there's a better way to do this, and definitely need to use external strings over some length threshold.

Supported compilers:

• MSVC 2015 or later, or Visual C++ Build Tools 2015 or later
• Clang 3.4.x or later
• GCC 4.8.x or later
• ICC 16 or later