struct RunLength<I>
where
    I: Iterator,
{
    iter: I,
    saved: Option<I::Item>,
}

impl<I> RunLength<I>
where
    I: Iterator,
{
    fn new(mut iter: I) -> Self {
        let saved = iter.next();
        Self { iter, saved }
    }
}

impl<I> Iterator for RunLength<I>
where
    I: Iterator,
    I::Item: PartialEq,
{
    type Item = (I::Item, usize);

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        let c = self.saved.take().or_else(|| self.iter.next())?;

        let mut count = 1;
        while let Some(n) = self.iter.next() {
            if n == c {
                count += 1
            } else {
                self.saved = Some(n);
                break;
            }
        }

        Some((c, count))
    }

    #[inline]
    fn fold<B, F>(mut self, init: B, mut f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B,
    {
        let mut c = match self.saved.take().or_else(|| self.iter.next()) {
            Some(c) => c,
            None => return init,
        };

        let mut count = 1;
        let mut value = init;

        for n in self.iter {
            if n == c {
                count += 1
            } else {
                value = f(value, (c, count));
                c = n;
                count = 1;
            }
        }

        f(value, (c, count))
    }
}

pub fn encode_tiny(data: &str) -> String {
    use std::fmt::Write;

    RunLength::new(data.chars()).fold(String::new(), |mut s, (c, count)| {
        match count {
            1 => s.push(c),
            n => write!(&mut s, "{}{}", n, c).unwrap(),
        }
        s
    })
}

// encode (procedural)     time:   [18.343 ms 18.581 ms 18.838 ms]
// Found 13 outliers among 100 measurements (13.00%)
//   9 (9.00%) high mild
//   4 (4.00%) high severe
//
// encode (tiny)           time:   [21.206 ms 21.486 ms 21.787 ms]
// Found 5 outliers among 100 measurements (5.00%)
//   5 (5.00%) high mild