gat-lending-iterator 0.1.8

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

/// A lending iterator that filters the elements of `iter` with `predicate`.
///
/// This `struct` is created by the [`filter`] method on [`LendingIterator`]. See
/// its documentation for more.
///
/// [`LendingIterator`]: crate::LendingIterator
/// [`filter`]: crate::LendingIterator::filter
#[derive(Clone)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
pub struct Filter<I, P> {
    iter: I,
    predicate: P,
}

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

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

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

    #[inline]
    fn next(&mut self) -> Option<Self::Item<'_>> {
        loop {
            // SAFETY: see https://docs.rs/polonius-the-crab/0.3.1/polonius_the_crab/#the-arcanemagic
            let self_ = unsafe { &mut *(self as *mut Self) };
            if let Some(item) = self_.iter.next() {
                if (self_.predicate)(&item) {
                    return Some(item);
                }
            } else {
                return None;
            }
        }
    }
}

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

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

    #[test]
    fn filter_basic() {
        let result: Vec<_> = (0..10)
            .into_lending()
            .filter(|&x| x % 2 == 0)
            .map(identity)
            .into_iter()
            .collect();
        assert_eq!(result, vec![0, 2, 4, 6, 8]);
    }
}