use num_traits::{NumAssign, NumCast, ToPrimitive};
use std::ops::*;

trait Averageable: Iterator {
    fn mean<T>(self) -> T
    where
        Self: Sized,
        Self::Item: NumAssign + ToPrimitive,
        T: NumCast + Div<Output = T>,
    {
        let mut iter = self.enumerate();
        let mut len;
        let mut sum;

        if let Some((i, n)) = iter.next() {
            sum = n;
            len = i;
        } else {
            panic!("Empty iterator!");
        }

        for (i, n) in iter {
            sum += n;
            len = i;
        }
        len += 1;

        let len: T = NumCast::from(len).unwrap();
        let sum: T = NumCast::from(sum).unwrap();

        sum / len
    }
}

impl<T> Averageable for T where T: Iterator {}

fn main() {
    let v: Vec<i32> = vec![10, 10, 20, 20];
    let mean: i32 = v.into_iter().mean();
    println!("{}", mean);
}