projecteuler.net/014/main.rs

use std::collections::HashMap;
type CZS = HashMap<u64, u64>;
struct Collatz { czs: CZS }

impl Collatz {
    pub fn new() -> Collatz {
        let mut res = Collatz { czs: HashMap::new() };
        res.czs.insert(1, 1);
        res
    }
    pub fn compute(&mut self, n: u64) -> u64 {
        match self.czs.get(&n) {
            Some(cl) => *cl,
            None => {
                //println!("compute {}", n);
                let cl_prev = if n % 2 == 0 {
                    self.compute(n / 2)
                } else {
                    self.compute(3 * n + 1)
                };
                self.czs.insert(n, cl_prev + 1);
                cl_prev + 1
            }
        }
    }
}

fn main() {
    let mut collatz = Collatz::new();

    let n = 1_000_000u64;
    let mut best_cl = 1;
    let mut best_n = 1;

    for k in (2..=n).step_by(1) {
        let clk = collatz.compute(k);
        if clk > best_cl {
            best_cl = clk;
            best_n = k;
        }
        //println!("{}: {}", k, clk);
    }
    println!("{} -> {}", best_n, best_cl);
}