Exercise: Collatz Sequence

The Collatz Sequence is defined as follows, for an arbitrary n1 greater than zero:

• If ni is 1, then the sequence terminates at ni.
• If ni is even, then ni+1 = ni / 2.
• If ni is odd, then ni+1 = 3 * ni + 1.

For example, beginning with n1 = 3:

• 3 is odd, so n2 = 3 * 3 + 1 = 10;
• 10 is even, so n3 = 10 / 2 = 5;
• 5 is odd, so n4 = 3 * 5 + 1 = 16;
• 16 is even, so n5 = 16 / 2 = 8;
• 8 is even, so n6 = 8 / 2 = 4;
• 4 is even, so n7 = 4 / 2 = 2;
• 2 is even, so n8 = 1; and
• the sequence terminates.

Write a function to calculate the length of the collatz sequence for a given initial n.

1. ```
   /// Determine the length of the collatz sequence beginning at `n`.
   ```
2. ```
   fn collatz_length(mut n: i32) -> u32 {
   ```
3. ```
     todo!("Implement this")
   ```
4. ```
   }
   ```

6. ```
   #[test]
   ```
7. ```
   fn test_collatz_length() {
   ```
8. ```
       assert_eq!(collatz_length(11), 15);
   ```
9. ```
   }
   ```

11. ```
    fn main() {
    ```
12. ```
        println!("Length: {}", collatz_length(11));
    ```
13. ```
    }
    ```