projecteuler.net/037/main.rs

use std::collections::HashSet;

fn er() -> HashSet<u32> {
    let mut nums = Vec::new();
    let size = 1_000_000;
    for n in 0..size+1 { nums.push(true); }
    let mut p = 1;
    while true {
        let mut found = false;
        for i in p+1..size {
            if nums[i] {
                found = true;
                p = i;
                break;
            }
        }
        if !found { break; }
        for i in (2 * p .. size).step_by(p) {
            nums[i] = false;
        }
    }
    let mut res = HashSet::new();
    for i in 2..size {
        if nums[i] { res.insert(i as u32); }
    }
    return res;
}

fn trl(n: u32) -> u32 {
    let s = n.to_string();
    let mut chars = s.chars();
    let f = chars.next().unwrap();
    //chars.next_back();
    let mut res = String::from(chars.as_str());
    res.parse::<u32>().unwrap()
}

fn test(p: u32, hs: &HashSet<u32>) -> bool {
    let mut tl = p;
    while tl > 10 {
        if !hs.contains(&tl) { return false; }
        tl = trl(tl);
    }
    if !hs.contains(&tl) { return false; }
    let mut tr = p;
    while tr > 0 {
        if !hs.contains(&tr) { return false; }
        tr /= 10;
    }
    return true;
}

fn main() {
    let hs = er();
    let mut sum = 0;
    for p in hs.iter() {
        if *p < 10 { continue; }
        if test(*p, &hs) {
            println!("{}", p);
            sum += p;
        }
    }
    println!("sum: {}", sum);
}