use crate::iter::adapters::zip::try_get_unchecked;
use crate::iter::adapters::{SourceIter, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
use crate::iter::{FusedIterator, InPlaceIterable, TrustedFused, TrustedLen};
use crate::num::NonZero;
use crate::ops::Try;
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "Enumerate")]
pub struct Enumerate<I> {
    iter: I,
    count: usize,
}
impl<I> Enumerate<I> {
    pub(in crate::iter) fn new(iter: I) -> Enumerate<I> {
        Enumerate { iter, count: 0 }
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Enumerate<I>
where
    I: Iterator,
{
    type Item = (usize, <I as Iterator>::Item);
    #[inline]
    #[rustc_inherit_overflow_checks]
    fn next(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
        let a = self.iter.next()?;
        let i = self.count;
        self.count += 1;
        Some((i, a))
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
    #[inline]
    #[rustc_inherit_overflow_checks]
    fn nth(&mut self, n: usize) -> Option<(usize, I::Item)> {
        let a = self.iter.nth(n)?;
        let i = self.count + n;
        self.count = i + 1;
        Some((i, a))
    }
    #[inline]
    fn count(self) -> usize {
        self.iter.count()
    }
    #[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>,
    {
        #[inline]
        fn enumerate<'a, T, Acc, R>(
            count: &'a mut usize,
            mut fold: impl FnMut(Acc, (usize, T)) -> R + 'a,
        ) -> impl FnMut(Acc, T) -> R + 'a {
            #[rustc_inherit_overflow_checks]
            move |acc, item| {
                let acc = fold(acc, (*count, item));
                *count += 1;
                acc
            }
        }
        self.iter.try_fold(init, enumerate(&mut self.count, fold))
    }
    #[inline]
    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        #[inline]
        fn enumerate<T, Acc>(
            mut count: usize,
            mut fold: impl FnMut(Acc, (usize, T)) -> Acc,
        ) -> impl FnMut(Acc, T) -> Acc {
            #[rustc_inherit_overflow_checks]
            move |acc, item| {
                let acc = fold(acc, (count, item));
                count += 1;
                acc
            }
        }
        self.iter.fold(init, enumerate(self.count, fold))
    }
    #[inline]
    #[rustc_inherit_overflow_checks]
    fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
        let remaining = self.iter.advance_by(n);
        let advanced = match remaining {
            Ok(()) => n,
            Err(rem) => n - rem.get(),
        };
        self.count += advanced;
        remaining
    }
    #[rustc_inherit_overflow_checks]
    #[inline]
    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> <Self as Iterator>::Item
    where
        Self: TrustedRandomAccessNoCoerce,
    {
        let value = unsafe { try_get_unchecked(&mut self.iter, idx) };
        (self.count + idx, value)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> DoubleEndedIterator for Enumerate<I>
where
    I: ExactSizeIterator + DoubleEndedIterator,
{
    #[inline]
    fn next_back(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
        let a = self.iter.next_back()?;
        let len = self.iter.len();
        Some((self.count + len, a))
    }
    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<(usize, <I as Iterator>::Item)> {
        let a = self.iter.nth_back(n)?;
        let len = self.iter.len();
        Some((self.count + len, a))
    }
    #[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>,
    {
        fn enumerate<T, Acc, R>(
            mut count: usize,
            mut fold: impl FnMut(Acc, (usize, T)) -> R,
        ) -> impl FnMut(Acc, T) -> R {
            move |acc, item| {
                count -= 1;
                fold(acc, (count, item))
            }
        }
        let count = self.count + self.iter.len();
        self.iter.try_rfold(init, enumerate(count, fold))
    }
    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        fn enumerate<T, Acc>(
            mut count: usize,
            mut fold: impl FnMut(Acc, (usize, T)) -> Acc,
        ) -> impl FnMut(Acc, T) -> Acc {
            move |acc, item| {
                count -= 1;
                fold(acc, (count, item))
            }
        }
        let count = self.count + self.iter.len();
        self.iter.rfold(init, enumerate(count, fold))
    }
    #[inline]
    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
        self.iter.advance_back_by(n)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Enumerate<I>
where
    I: ExactSizeIterator,
{
    fn len(&self) -> usize {
        self.iter.len()
    }
    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}
#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<I> TrustedRandomAccess for Enumerate<I> where I: TrustedRandomAccess {}
#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<I> TrustedRandomAccessNoCoerce for Enumerate<I>
where
    I: TrustedRandomAccessNoCoerce,
{
    const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
}
#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Enumerate<I> where I: FusedIterator {}
#[unstable(issue = "none", feature = "trusted_fused")]
unsafe impl<I: TrustedFused> TrustedFused for Enumerate<I> {}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<I> TrustedLen for Enumerate<I> where I: TrustedLen {}
#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<I> SourceIter for Enumerate<I>
where
    I: SourceIter,
{
    type Source = I::Source;
    #[inline]
    unsafe fn as_inner(&mut self) -> &mut I::Source {
        unsafe { SourceIter::as_inner(&mut self.iter) }
    }
}
#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<I: InPlaceIterable> InPlaceIterable for Enumerate<I> {
    const EXPAND_BY: Option<NonZero<usize>> = I::EXPAND_BY;
    const MERGE_BY: Option<NonZero<usize>> = I::MERGE_BY;
}
#[stable(feature = "default_iters", since = "1.70.0")]
impl<I: Default> Default for Enumerate<I> {
    fn default() -> Self {
        Enumerate::new(Default::default())
    }
}