projecteuler.net/038/main.rs

use std::collections::HashSet;
use std::collections::VecDeque;

type Str = VecDeque<u8>;
type Res = HashSet<u32>;

fn take(n: usize, s: &mut Str) -> u32 {
    let mut res = 0u32;
    for _ in 0..n {
        res *= 10;
        res += s.pop_front().unwrap() as u32;
    }
    res
}

fn nstr(n: u32) -> Str {
    let mut res = Str::new();
    let mut nn = n;
    while nn > 0 {
        res.push_front((nn % 10) as u8);
        nn /= 10;
    }
    res
}

fn strn(s: &Str) -> u32 {
    let mut res = 0;
    for d in s.iter() {
        res *= 10;
        res += *d as u32;
    }
    res
}

fn len(n: u32) -> usize {
    let mut res = 0usize;
    let mut nn = n;
    while nn > 0 {
        res += 1;
        nn /= 10;
    }
    res
}

fn testn(s: &Str, n: usize) -> bool {
    let mut ss = s.clone();
    let k = take(n, &mut ss);
    let mut i = 2;
    while len(k * i) <= ss.len() {
        let kt = take(len(k*i), &mut ss);
        if kt != k*i { return false; }
        i += 1;
    }
    ss.len() == 0
}

fn test(s: Str) -> bool {
    for k in 1..s.len() - 4 {
        if testn(&s.clone(), k) { return true; }
    }
    return false;
}

fn sp(s: &Str, fx: usize, k: usize) -> Str {
    let mut res = Str::new();
    for i in 0..fx {
        res.push_back(s[i]);
    }
    res.push_back(s[k]);
    for i in fx..s.len() {
        if i != k { res.push_back(s[i]); }
    }
    res
}


fn lp(s: &Str, fx: usize, keep_first: bool, max: u32) -> u32 {
    let kf = 0; //if keep_first { 0usize } else { 1usize };
    let mut mymax = max;
    for k in (fx + kf)..s.len() {
        let ss = sp(&s, fx, k);
        if  ss != *s || keep_first {
            if test(ss.clone()) {
                let n = strn(&ss);
                if n > mymax { mymax = n }
            }
        }
        let m = lp(&ss, fx + 1, false, mymax);
        if m > mymax { mymax = m; }

    }
    mymax
}
fn main() {
    let mut s = nstr(123456789);
    //let mut s = nstr(192384576);
    //let mut s = nstr(918273654);

    //let mut hs = HashSet::new();
    let m = lp(&s, 0, true, 0);
    println!("{}", m);

    //let n = strn(&s);
    //println!("{}", n);
    
}