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use std::collections::HashMap;
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type CZS = HashMap<u64, u64>;
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struct Collatz { czs: CZS }
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impl Collatz {
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pub fn new() -> Collatz {
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let mut res = Collatz { czs: HashMap::new() };
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res.czs.insert(1, 1);
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res
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}
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pub fn compute(&mut self, n: u64) -> u64 {
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match self.czs.get(&n) {
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Some(cl) => *cl,
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None => {
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//println!("compute {}", n);
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let cl_prev = if n % 2 == 0 {
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self.compute(n / 2)
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} else {
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self.compute(3 * n + 1)
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};
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self.czs.insert(n, cl_prev + 1);
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cl_prev + 1
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}
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}
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}
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}
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fn main() {
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let mut collatz = Collatz::new();
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let n = 1_000_000u64;
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let mut best_cl = 1;
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let mut best_n = 1;
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for k in (2..=n).step_by(1) {
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let clk = collatz.compute(k);
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if clk > best_cl {
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best_cl = clk;
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best_n = k;
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}
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//println!("{}: {}", k, clk);
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}
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println!("{} -> {}", best_n, best_cl);
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}
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