use crate::fmt;
use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
use crate::ops::{ControlFlow, Try};

/// An iterator that uses `f` to both filter and map elements from `iter`.
///
/// This `struct` is created by the [`filter_map`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`filter_map`]: Iterator::filter_map
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct FilterMap<I, F> {
    iter: I,
    f: F,
}
impl<I, F> FilterMap<I, F> {
    pub(in crate::iter) fn new(iter: I, f: F) -> FilterMap<I, F> {
        FilterMap { iter, f }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, F> fmt::Debug for FilterMap<I, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("FilterMap").field("iter", &self.iter).finish()
    }
}

fn filter_map_fold<T, B, Acc>(
    mut f: impl FnMut(T) -> Option<B>,
    mut fold: impl FnMut(Acc, B) -> Acc,
) -> impl FnMut(Acc, T) -> Acc {
    move |acc, item| match f(item) {
        Some(x) => fold(acc, x),
        None => acc,
    }
}

fn filter_map_try_fold<'a, T, B, Acc, R: Try<Output = Acc>>(
    f: &'a mut impl FnMut(T) -> Option<B>,
    mut fold: impl FnMut(Acc, B) -> R + 'a,
) -> impl FnMut(Acc, T) -> R + 'a {
    move |acc, item| match f(item) {
        Some(x) => fold(acc, x),
        None => try { acc },
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: Iterator, F> Iterator for FilterMap<I, F>
where
    F: FnMut(I::Item) -> Option<B>,
{
    type Item = B;

    #[inline]
    fn next(&mut self) -> Option<B> {
        self.iter.find_map(&mut self.f)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) // can't know a lower bound, due to the predicate
    }

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        self.iter.try_fold(init, filter_map_try_fold(&mut self.f, fold))
    }

    #[inline]
    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.fold(init, filter_map_fold(self.f, fold))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for FilterMap<I, F>
where
    F: FnMut(I::Item) -> Option<B>,
{
    #[inline]
    fn next_back(&mut self) -> Option<B> {
        #[inline]
        fn find<T, B>(
            f: &mut impl FnMut(T) -> Option<B>,
        ) -> impl FnMut((), T) -> ControlFlow<B> + '_ {
            move |(), x| match f(x) {
                Some(x) => ControlFlow::Break(x),
                None => ControlFlow::CONTINUE,
            }
        }

        self.iter.try_rfold((), find(&mut self.f)).break_value()
    }

    #[inline]
    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        self.iter.try_rfold(init, filter_map_try_fold(&mut self.f, fold))
    }

    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.rfold(init, filter_map_fold(self.f, fold))
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<B, I: FusedIterator, F> FusedIterator for FilterMap<I, F> where F: FnMut(I::Item) -> Option<B> {}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<I, F> SourceIter for FilterMap<I, F>
where
    I: SourceIter,
{
    type Source = I::Source;

    #[inline]
    unsafe fn as_inner(&mut self) -> &mut I::Source {
        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
        unsafe { SourceIter::as_inner(&mut self.iter) }
    }
}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<B, I: InPlaceIterable, F> InPlaceIterable for FilterMap<I, F> where
    F: FnMut(I::Item) -> Option<B>
{
}