Is there a way to get C99 array designators or alternative in Rust?

Question

I've just completed a CodeWars kata on vowel-counting in C & Rust. The sample code is easy & obvious. Arrays can be used as fast mappings. Here, I map characters to logical values (0 or 1).

C implementation:

#include <stddef.h>
#include <stdint.h>

// :) This const array design looks smart & IMO-readable. Use of C99 feature.
const uint8_t areVowels[256]= {['a']=1, ['e']=1, ['i']=1, ['o']=1,  ['u']=1};

size_t get_count(const unsigned char *s)
{
  auto size_t count= 0;
  for (;*s!='\0';s++){
    count+= areVowels[*s];
  }
  return count;
}

Rust implementation:

// :( Here is me pain. Unreadable, python3-generated array content.
const ARE_VOWELS:[u8;256]= [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0];

fn get_count(s: &str) -> usize {
  let mut vowels_count: usize = 0;

  for c in s.bytes(){
      vowels_count+= ARE_VOWELS[c as usize] as usize;
  }
  
  return vowels_count;
}

I wish I knew an alternative for array designators in Rust, which are a useful C99 feature. Initialization of the same byte array is much more awkward in my Rust code.

Littleton

Ian answer is good. But this solution to the problem is not rusty, imo. – Quinacrine

Answer 1

static mut is unnecessary here (and also bad practice). You can assign the value of a constant to the value of a code block which is capable of being evaluated in a const context

const ARE_VOWELS: [u8;256] = {
    let mut data = [0u8; 256];
    data['a' as usize] = 1;
    data['e' as usize] = 1;
    data['i' as usize] = 1;
    data['o' as usize] = 1;
    data['u' as usize] = 1;
    data
};

See this answer for prettying this up with a macro.

Chanukah

Answer 2

An alternative is a macro that provides the syntax you want, its actually not too hard to make a general purpose implementation:

macro_rules! array {
    ($def:expr; $len:expr; $([$idx:expr]=$val:expr),* $(,)?) => { {
        let mut a = [$def; $len];
        $(a[$idx] = $val;)*
        a
    } }
}

let data = array![0; 256; ['a' as usize]=1, ['e' as usize]=1];

You can of course move the as usize into the macro if you like.

Daven

Oh hey look, there's a crate that does this and more: array-lit – Daven

I really like that macro. It's easy & readable, and I've just rewritten it in my way. – Littleton

Answer 3

I found some (unsafe ⇒ amusing) work around with "mut static value runtime initialization".

const VOWELS:&[u8;5]= b"aeiou";
static mut ARE_VOWELS:[u8;256]= [0;256];

fn init(){
    unsafe{
        let ptr: *mut u8= &mut ARE_VOWELS[0];
        for b in VOWELS{
            *(ptr.offset(*b as isize))= 1;
        }
    }
}

fn get_count(s: &str) -> usize {
    init();
    
    let mut vowels_count: usize = 0;

    for c in s.bytes(){
        unsafe{
            vowels_count+= ARE_VOWELS[c as usize] as usize;
        }
    }
  
    return vowels_count;
}

Although I have serious doubts about the performance & maintainability of such piece of code.

Littleton