projecteuler.net/046/main.rs

use std::collections::HashSet;

type HS = HashSet<u32>;

fn er() -> HS {
    let mut nums = Vec::new();
    let size = 1_000_000;
    for n in 0..size+1 { nums.push(true); }
    let mut p = 1;
    loop {
        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 sqs() -> HS {
    let mut res = HS::new();
    for n in 1..1_000_0 {
        res.insert(n*n);
    }
    return res;
}

fn test(n: u32, ps: &HS, sq: &HS) -> bool {
    for k in 7..n {
        if !ps.contains(&k) { continue }
        let s = (n - k) / 2;
        if !sq.contains(&s) { continue }
        return true;
    }
    return false;
}

fn main() {
    let ps = er();
    let sq = sqs();
    for k in (9..100000).step_by(2) {
        if ps.contains(&k) { continue; }
        if !test(k, &ps, &sq) {
            println!("{}", k);
            break;
        }
    }
}