Fix fromHex invalid hex behavior

[junderw]

In NodeJS, the behavior of Buffer.from(x, 'hex') differed slightly on its handling of bad hex parsing.

My fix also increased performance. I tried parsing a hex string '7c'.repeat(5e7) and it went from 2.8 s to ~800ms.

// node
> Buffer.from('abc def01','hex')
<Buffer ab>

// this library
> require('./index.js').Buffer.from('abc def01','hex')
<Buffer ab 0c de f0>

[junderw]

Note: This new implementation also mimics the code for NodeJS hex parser used in the from(x, 'hex') portion.

https://github.com/nodejs/node/blob/v14.18.1/src/string_bytes.cc#L246-L261

[LinusU]

buf here is a 4 bytes long buffer, so these asserts can never succeed? Did you try this locally? 🤔

[junderw]

buf should NOT be a 4 byte buffer. That's the point.

If it is a 4 byte buffer, than the code is broken.

a non-hex character should end the hex parsing and only the 2-hex pairs up until the first non-hex is output as a Buffer.

in this case, [0xab]

[LinusU]

But you are doing .write on an already existing Buffer that is 4 bytes long. write cannot truncate the length of the buffer.

With my updated tests it checks that write only wrote 1 byte, and that it left the three bytes after as 00 00 00.

[LinusU]

This broke both the linting and the tests, I've submitted PRs to fix that: #340, #341

ping @dcousens

[dcousens]

With CI passing now, I'm slightly more confident with this going out (see #335)

[chjj]

@dcousens, please look at #343 before tagging a release. This PR (#303) added a massive slowdown.