How to convert string to hex at compile time?

help
1

Aim: convert string "1A2B3344" to hex array [1Au8, 2Bu8, 33u8, 44u8] at compile time, and that string may be stored in one file.

For now, hex crate and hex-literal crate provide the functionality to convert hex string to hex array. However, these two crates can't achieve my goal because:

  • hex crate provides decode by hex::decode function, which is not a const fn, so we can't do it at compile time.
  • hex-literal crate provides decode by hex macro, which only accepts string literal, so I can't do something like
const hex_array: &[u8] = {
    let hex_string = include_str!("path/to/hex_file");
    hex_literal::hex!(hex_string)
}
2

Assuming you control the file,

const hex_array: &[u8] = &u32::to_be_bytes(include!("hex_file"));

where hex_file contains

0x1A2B3344

would be one way.

3

This hex string may be very long, 1A2B3344 is just an example. Hex that larger than u64 can't be done in this way.

4

You could write a build script that combines the hex-literal macro and your file.

5

On the nightly compiler, you could do something like this:


#![feature(const_panic, min_const_generics)]

const fn hex_to_nybble(i:u8)->u8 {
    match i as char {
        '0' => 0,
        '1' => 1,
        '2' => 2,
        '3' => 3,
        '4' => 4,
        '5' => 5,
        /* ... */
        _ => panic!("invalid char")
    }
}

const fn hex_to_byte(h: u8, l:u8)->u8 { (hex_to_nybble(h) << 4) + hex_to_nybble(l) }

const fn hex_to_arr<const N:usize>(i_buf: &[u8])->[u8;N] {
    let mut o_buf = [0;N];
    let mut i:usize = 0;
    let mut o:usize = 0;
    while o<N {
        o_buf[o] = hex_to_byte(i_buf[i], i_buf[i+1]);
        o += 1;
        i += 2;
    }
    o_buf
}

const INFILE: &[u8] = b"12341234";
const FROMHEX_LEN:usize = INFILE.len()/2;
const FROMHEX: &[u8] = &hex_to_arr::<FROMHEX_LEN>(INFILE);

(Playground)

6

And for stable Rust, I've released a helper crate that allows chaining custom macros with some of the built-in ones: