use std::iter::Iterator;
use std::borrow::Borrow;

trait SequencePositionExt : Iterator
    where Self: Sized
{
    fn sequence_position<T>(mut self, seq: &[T]) -> Option<usize>
        where Self::Item : Borrow<T>, T: PartialEq
    {
        let mut pos = 0;
        let mut ring = Vec::with_capacity(seq.len());
        for _ in seq {
            ring.push(None);
        }
        let mut ring_pos = 0;
        while let Some(item) = self.next() {
            pos += 1;
            ring[ring_pos] = Some(item);
            ring_pos += 1;
            if ring_pos == ring.len() {
                ring_pos = 0;
            }
            if ring.iter().cycle().skip(ring_pos).take(ring.len()).map(|x| x.as_ref().map(|x| x.borrow())).eq(seq.iter().map(Some)) {
                return Some(pos - seq.len());
            }
        }
        None
    }
}

impl<T: Iterator + Sized> SequencePositionExt for T {}

fn main() {
    println!("{:?}", [2,4,6,3].iter().sequence_position(&[2,4]));
}