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use core::fmt;

use crate::{higher_order::FnMutHKAOpt, DoubleEndedLender, FusedLender, Lender, Lending};
#[derive(Clone)]
#[must_use = "lenders are lazy and do nothing unless consumed"]
pub struct FilterMap<L, F> {
    lender: L,
    f: F,
}
impl<L, F> FilterMap<L, F> {
    pub(crate) fn new(lender: L, f: F) -> FilterMap<L, F> { FilterMap { lender, f } }
}
impl<L: fmt::Debug, F> fmt::Debug for FilterMap<L, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("FilterMap").field("lender", &self.lender).finish()
    }
}
impl<'lend, B, L, F> Lending<'lend> for FilterMap<L, F>
where
    F: FnMut(<L as Lending<'lend>>::Lend) -> Option<B>,
    B: 'lend,
    L: Lender,
{
    type Lend = B;
}

impl<L, F> Lender for FilterMap<L, F>
where
    for<'all> F: FnMutHKAOpt<'all, <L as Lending<'all>>::Lend>,
    L: Lender,
{
    fn next(&mut self) -> Option<<Self as Lending<'_>>::Lend> {
        while let Some(x) = self.lender.next() {
            if let Some(y) = (self.f)(x) {
                // SAFETY: polonius return
                return Some(unsafe { core::mem::transmute::<<Self as Lending<'_>>::Lend, <Self as Lending<'_>>::Lend>(y) });
            }
        }
        None
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.lender.size_hint();
        (0, upper)
    }
}
impl<L, F> DoubleEndedLender for FilterMap<L, F>
where
    for<'all> F: FnMutHKAOpt<'all, <L as Lending<'all>>::Lend>,
    L: DoubleEndedLender,
{
    fn next_back(&mut self) -> Option<<Self as Lending<'_>>::Lend> {
        while let Some(x) = self.lender.next_back() {
            if let Some(y) = (self.f)(x) {
                // SAFETY: polonius return
                return Some(unsafe { core::mem::transmute::<<Self as Lending<'_>>::Lend, <Self as Lending<'_>>::Lend>(y) });
            }
        }
        None
    }
}
impl<L, F> FusedLender for FilterMap<L, F>
where
    for<'all> F: FnMutHKAOpt<'all, <L as Lending<'all>>::Lend>,
    L: FusedLender,
{
}