Fast Hex

A fast, SIMD (vectorized) hex string encoder/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.

---

C++

Pull in src/hex.h and src/hex.cc, and adjust your build appropriately (GCC/Clang/ICC: -march=haswell for example; MSVC: set EnableAdvancedInstructionSet to "AVX2" or /arch:AVX2).

See hex.h for the exported functions. There are three decoder implementations and two encoder implementations, with the same signature:

// Decodes src hex string into dest bytes.
// len is number of dest bytes (1/2 the size of src).
void decodeHex___(uint8_t* __restrict__ dest, const uint8_t* __restrict__ src, size_t len);

// Encodes src bytes into dest hex string.
// len is number of src bytes (dest must be twice the size of src).
void encodeHex___(uint8_t* __restrict__ dest, const uint8_t* __restrict__ src, size_t len);

Benchmark

• Decoding ~12.5x over scalar.
• Encoding ~11.5x over scalar.

---

Node.js

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

Benchmark

• ~2.12x faster decoding for short string inputs (< 1,031,913 chars).
• ~12x faster decoding for some long strings, Buffers and TypedArrays.
• ~5.5x faster encoding.

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