use num_traits::{zero, Num, NumCast, RefNum, ToPrimitive};
use std::borrow::Borrow;
use std::ops::*;

trait Averageable<T>: Iterator {
    fn mean(self) -> T;
}

impl<T, U> Averageable<T> for U
where
    U: Sized + Iterator,
    U::Item: Num + ToPrimitive,
    T: NumCast + Div<Output = T>,
{
    fn mean(self) -> T {
        let (sum, len): (Self::Item, usize) =
            self.fold((zero(), 0), |(a, len), e| (a + e, len + 1));

        if len == 0 {
            panic!("Iterator is empty!");
        } else {
            let sum: T = NumCast::from(sum).expect("NumCast failed!");
            let len: T = NumCast::from(len).expect("NumCast failed!");
            sum / len
        }
    }
}

fn main() {
    let v: Vec<i32> = vec![10, 10, 20, 20];
    // let mean: f32 = v.iter().mean();
    // dbg!(mean);
    let mean: f32 = v.into_iter().mean();
    dbg!(mean);
}