Excellent. For some analysis, please see this article on my blog:

Daniel Lemire, "Converting data to hexadecimal outputs quickly," in Daniel Lemire's blog, February 2, 2026, https://lemire.me/blog/2026/02/02/converting-data-to-hexadecimal-outputs-quickly/.

This is not the fastest way yet though, there should be more optimizations possible, but now I estimate that the most significant part of the slowdown is on the wrapping code.

@lemire pls s/Skovorora/Skovoroda/g 🙃

@ChALkeR I'll update your name. Sorry.

What you have here is great and I don't recommend doing more.

The better place to get fancier would be simdutf.

simdutf/simdutf#565

We do invite contributions.

@ChALkeR

Try

   uint8_t add = (x >= 10) ? ('a' - 10) : '0';
   return x + add;

GCC seems to like it better than what I originally proposed.

@lemire Seems slightly worse on Linux (gcc?) and slightly better on macOS (clang?)
nodejs/node#61609 (comment)

Compared to main:

01:03:52                                                 confidence improvement accuracy (*)   (**)  (***)
01:03:52 buffers/buffer-hex-decode.js n=1000000 len=1024        ***    167.49 %       ±1.45% ±1.94% ±2.56%
01:03:52 buffers/buffer-hex-decode.js n=1000000 len=64          ***     49.47 %       ±1.94% ±2.59% ±3.38%
01:03:52 buffers/buffer-hex-encode.js n=1000000 len=1024                 0.15 %       ±0.17% ±0.22% ±0.29%
01:03:52 buffers/buffer-hex-encode.js n=1000000 len=64                   0.06 %       ±0.99% ±1.32% ±1.72%

@ChALkeR

Seems slightly worse on Linux...

What I see is that my last proposal is better than your original version under GCC, but slightly worse with LLVM. However, both are autovectorized in a worthwhile manner which would make your PR safe and performant.

Also, the code is easier to understand.