gat-lending-iterator 0.1.8

use crate::LendingIterator;

/// A lending iterator that skips over the first `n` items of `iter`.
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
pub struct Skip<I> {
    iter: I,
    n: usize,
}

impl<I> Skip<I> {
    pub(crate) fn new(iter: I, n: usize) -> Skip<I> {
        Skip { iter, n }
    }
}

impl<I> LendingIterator for Skip<I>
where
    I: LendingIterator,
{
    type Item<'a>
        = I::Item<'a>
    where
        I: 'a;

    #[inline]
    fn next(&mut self) -> Option<I::Item<'_>> {
        if self.n > 0 {
            self.iter.nth(core::mem::take(&mut self.n))
        } else {
            self.iter.next()
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (lower, upper) = self.iter.size_hint();

        let lower = lower.saturating_sub(self.n);
        let upper = upper.map(|x| x.saturating_sub(self.n));
        (lower, upper)
    }
}

#[cfg(test)]
mod test {
    use crate::{LendingIterator, ToLendingIterator};

    fn identity(x: i32) -> i32 {
        x
    }

    #[test]
    fn test() {
        assert_eq!((0..5).into_lending().skip(1).nth(1), (0..5).skip(1).nth(1));
    }

    #[test]
    fn skip_basic() {
        let result: Vec<_> = (0..5)
            .into_lending()
            .skip(2)
            .map(identity)
            .into_iter()
            .collect();
        assert_eq!(result, vec![2, 3, 4]);
    }

    #[test]
    fn skip_more_than_available() {
        let result: Vec<_> = (0..3)
            .into_lending()
            .skip(10)
            .map(identity)
            .into_iter()
            .collect();
        assert_eq!(result, vec![]);
    }
}