finish 14

src/euler14/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "euler14"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

src/euler14/src/main.rs

@@ -0,0 +1,53 @@
+fn main() {
+    let mut starts_longest = 0;
+    let mut max_length = 0;
+    for i in 1..1_000_000 {
+        let length = Collatz::new(i).count();
+        if length > max_length {
+            max_length = length;
+            starts_longest = i;
+        }
+    }
+    println!("{starts_longest}");
+}
+
+struct Collatz {
+    n: usize,
+    done: bool,
+}
+
+impl Collatz {
+    fn new(n: usize) -> Self {
+        Self { n, done: false }
+    }
+}
+
+impl Iterator for Collatz {
+    type Item = usize;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.done {
+            return None;
+        }
+        if self.n == 1 {
+            self.done = true;
+            return Some(self.n);
+        }
+        if self.n % 2 == 0 {
+            self.n /= 2
+        } else {
+            self.n = 3 * self.n + 1;
+        }
+        Some(self.n)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::Collatz;
+
+    #[test]
+    fn collatz() {
+        assert_eq!(Collatz::new(13).count(), 10);
+    }
+}