use std::iter::Peekable;

pub(crate) fn intersperse_option<I: Iterator>(iterator: I) -> impl Iterator<Item = Option<I::Item>>
where
    I::Item: Clone,
{
    intersperse(iterator.map(Some), None)
}

pub(crate) fn intersperse<I: Iterator>(iterator: I, separator: I::Item) -> Intersperse<I>
where
    I::Item: Clone,
{
    Intersperse {
        inner: iterator.peekable(),
        separator,
        needs_sep: false,
    }
}

pub struct Intersperse<I>
where
    I: Iterator,
{
    inner: Peekable<I>,
    separator: I::Item,
    needs_sep: bool,
}

impl<I> Iterator for Intersperse<I>
where
    I: Iterator,
    I::Item: Clone,
{
    type Item = I::Item;

    fn next(&mut self) -> Option<Self::Item> {
        if self.needs_sep && self.inner.peek().is_some() {
            self.needs_sep = false;
            Some(self.separator.clone())
        } else {
            self.needs_sep = true;
            self.inner.next()
        }
    }
}