Solution

/// Determine the length of the collatz sequence beginning at `n`.
fn collatz_length(mut n: i32) -> u32 {
    let mut len = 1;
    while n > 1 {
        n = if n % 2 == 0 { n / 2 } else { 3 * n + 1 };
        len += 1;
    }
    len
}
#[test]
fn test_collatz_length() {
    assert_eq!(collatz_length(11), 15);
}
fn main() {
    println!("Length: {}", collatz_length(11));
}