gat-lending-iterator 0.1.8

use crate::LendingIterator;
use core::fmt;

/// A lending iterator that yields items based on a predicate.
///
/// This iterator is fused.
#[derive(Clone)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
pub struct TakeWhile<I, P> {
    iter: I,
    predicate: P,
    done: bool,
}

impl<I, P> TakeWhile<I, P> {
    pub(crate) fn new(iter: I, predicate: P) -> Self {
        Self {
            iter,
            predicate,
            done: false,
        }
    }
}

impl<I: fmt::Debug, P> fmt::Debug for TakeWhile<I, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("TakeWhile")
            .field("iter", &self.iter)
            .field("done", &self.done)
            .finish_non_exhaustive()
    }
}

impl<I, P> LendingIterator for TakeWhile<I, P>
where
    I: LendingIterator,
    P: for<'a> FnMut(&I::Item<'a>) -> bool,
{
    type Item<'a>
        = I::Item<'a>
    where
        I: 'a,
        P: 'a;

    #[inline]
    fn next(&mut self) -> Option<Self::Item<'_>> {
        if self.done {
            None
        } else {
            let item = self.iter.next()?;
            if (self.predicate)(&item) {
                Some(item)
            } else {
                self.done = true;
                None
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.done {
            (0, Some(0))
        } else {
            (0, self.iter.size_hint().1)
        }
    }
}

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

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

    #[test]
    fn take_while_basic() {
        let result: Vec<_> = (0..10)
            .into_lending()
            .take_while(|&x| x < 5)
            .map(identity)
            .into_iter()
            .collect();
        assert_eq!(result, vec![0, 1, 2, 3, 4]);
    }
}