Cargo.toml

[package]
name = "substring-mutual-letters"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]

main.rs

use std::collections::HashSet;

fn main() {
    let repeatness = 1;
    let input = "aBBdaaa";
    let all_substrings = all_substrings(input);
    let processed = all_substrings
        .iter()
        .filter(|x| good_string(repeatness, x))
        .max_by_key(|x| x.len());
    if let Some(x) = processed {
        println!("{}, {:?}", &x.len(), x);
    } else {
        println!("0");
    }
}
fn all_substrings(s: &str) -> HashSet<&str> {
    let mut set = HashSet::new();
    for i in 0..s.len() {
        for j in i..s.len() {
            set.insert(&s[i..j]);
        }
    }
    set
}

fn good_string(repeatness: usize, input: &str) -> bool {
    let vec_char = Vec::from_iter(input.chars());

    let chunks = vec_char.chunks_exact(repeatness);

    if !chunks.remainder().is_empty() {
        return false;
    }

    let window_view = chunks
        .map(|x| {
            let all_upper = x.iter().all(|char| char.is_uppercase());
            let all_lower = x.iter().all(|char| char.is_lowercase());

            // first key is to keep track of whether each "chunk" is in a different state from each other
            // second key is to ensure that with each "chunk", every character is either lowercase or all uppercase
            (all_upper, all_upper || all_lower)
        })
        .collect::<Vec<_>>();
    if vec_char.len() == repeatness && !window_view[0].1 {
        return false;
    }
    window_view.windows(2).all(|x| x[0].1 && x[0].0 != x[1].0)
}