moirai-iter 0.4.0

//! Peekable iterator adapter.

/// Iterator adapter that allows peeking at the next element.
#[derive(Clone)]
pub struct Peekable<I: Iterator> {
    iter: I,
    peeked: Option<Option<I::Item>>,
}

impl<I: Iterator> Peekable<I> {
    /// Creates a new `Peekable` adapter.
    #[inline]
    pub fn new(iter: I) -> Self {
        Peekable { iter, peeked: None }
    }

    /// Returns a reference to the next element without consuming it.
    #[inline]
    pub fn peek(&mut self) -> Option<&I::Item> {
        let iter = &mut self.iter;
        self.peeked.get_or_insert_with(|| iter.next()).as_ref()
    }

    /// Returns a mutable reference to the next element without consuming it.
    #[inline]
    pub fn peek_mut(&mut self) -> Option<&mut I::Item> {
        let iter = &mut self.iter;
        self.peeked.get_or_insert_with(|| iter.next()).as_mut()
    }
}

impl<I: Iterator> Iterator for Peekable<I> {
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        match self.peeked.take() {
            Some(v) => v,
            None => self.iter.next(),
        }
    }

    #[inline]
    fn count(mut self) -> usize {
        match self.peeked.take() {
            Some(None) => 0,
            Some(Some(_)) => 1 + self.iter.count(),
            None => self.iter.count(),
        }
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        match self.peeked.take() {
            Some(None) => None,
            Some(v @ Some(_)) if n == 0 => v,
            Some(Some(_)) => self.iter.nth(n - 1),
            None => self.iter.nth(n),
        }
    }

    #[inline]
    fn last(mut self) -> Option<I::Item> {
        let peek_opt = match self.peeked.take() {
            Some(None) => return None,
            Some(v) => v,
            None => None,
        };
        self.iter.last().or(peek_opt)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let peek_len = match self.peeked {
            Some(None) => return (0, Some(0)),
            Some(Some(_)) => 1,
            None => 0,
        };
        let (lo, hi) = self.iter.size_hint();
        let lo = lo.saturating_add(peek_len);
        let hi = match hi {
            Some(x) => x.checked_add(peek_len),
            None => None,
        };
        (lo, hi)
    }
}