014 ok

014/main.rs

@@ -1,15 +1,16 @@
-type CZS = Vec<Option<u32>>;
+use std::collections::HashMap;
+type CZS = HashMap<u64, u64>;
 struct Collatz { czs: CZS }
 
 impl Collatz {
     pub fn new() -> Collatz {
-        let cl0 = Some(0); // for indexing
-        let cl1 = Some(1);
-        Collatz { czs: vec![cl0, cl1] }
+        let mut res = Collatz { czs: HashMap::new() };
+        res.czs.insert(1, 1);
+        res
     }
-    pub fn compute(&mut self, n: usize) -> u32 {
-        match self.ensure_get(n) {
-            Some(cl) => cl,
+    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 {
@@ -17,22 +18,27 @@ impl Collatz {
                 } else {
                     self.compute(3 * n + 1)
                 };
-                self.czs[n] = Some(cl_prev + 1);
+                self.czs.insert(n, cl_prev + 1);
                 cl_prev + 1
             }
         }
     }
-    pub fn ensure_get(&mut self, i: usize) -> Option<u32> {
-        while self.czs.len() <= i { self.czs.push(None); }
-        self.czs[i]
-    }
 }
 
 fn main() {
     let mut collatz = Collatz::new();
 
-    for k in 0..100000usize {
+    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);
-        println!("{}: {}", k, clk);
+        if clk > best_cl {
+            best_cl = clk;
+            best_n = k;
+        }
+        //println!("{}: {}", k, clk);
     }
+    println!("{} -> {}", best_n, best_cl);
 }