exercism.io Rust nucleotide-count

lib.rs

use std::collections::HashMap;

const VALID: [char; 4] = ['A', 'C', 'G', 'T'];

pub fn count(nucleotide: char, dna: &str) -> Result<usize, char> {
    if !VALID.contains(&nucleotide) {
        return Err(nucleotide);
    }
    match dna.chars().find(|c| !VALID.contains(c)) {
        Some(c) => Err(c),
        None => Ok(dna.matches(nucleotide).count()),
    }
}

pub fn nucleotide_counts(dna: &str) -> Result<HashMap<char, usize>, char> {
    VALID
        .iter()
        .map(|&c| {
            let num = count(c, dna)?;
            Ok((c, num))
        })
        .collect()
}