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    use crate::{intrinsics, iter::from_fn, ops::Try};

    /// An iterator for stepping iterators by a custom amount.
    ///
    /// This `struct` is created by the [`step_by`] method on [`Iterator`]. See
    /// its documentation for more.
    ///
    /// [`step_by`]: Iterator::step_by
    /// [`Iterator`]: trait.Iterator.html
    #[must_use = "iterators are lazy and do nothing unless consumed"]
    #[stable(feature = "iterator_step_by", since = "1.28.0")]
    #[derive(Clone, Debug)]
    pub struct StepBy<I> {
        iter: I,
        step: usize,
        first_take: bool,
    }

    impl<I> StepBy<I> {
        pub(in crate::iter) fn new(iter: I, step: usize) -> StepBy<I> {
            assert!(step != 0);
            StepBy { iter, step: step - 1, first_take: true }
        }
    }

    #[stable(feature = "iterator_step_by", since = "1.28.0")]
    impl<I> Iterator for StepBy<I>
    where
        I: Iterator,
    {
        type Item = I::Item;

        #[inline]
        fn next(&mut self) -> Option<Self::Item> {
            if self.first_take {
                self.first_take = false;
                self.iter.next()
            } else {
                self.iter.nth(self.step)
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            #[inline]
            fn first_size(step: usize) -> impl Fn(usize) -> usize {
                move |n| if n == 0 { 0 } else { 1 + (n - 1) / (step + 1) }
            }

            #[inline]
            fn other_size(step: usize) -> impl Fn(usize) -> usize {
                move |n| n / (step + 1)
            }

            let (low, high) = self.iter.size_hint();

            if self.first_take {
                let f = first_size(self.step);
                (f(low), high.map(f))
            } else {
                let f = other_size(self.step);
                (f(low), high.map(f))
            }
        }

        #[inline]
        fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
            if self.first_take {
                self.first_take = false;
                let first = self.iter.next();
                if n == 0 {
                    return first;
                }
                n -= 1;
            }
            // n and self.step are indices, we need to add 1 to get the amount of elements
            // When calling `.nth`, we need to subtract 1 again to convert back to an index
            // step + 1 can't overflow because `.step_by` sets `self.step` to `step - 1`
            let mut step = self.step + 1;
            // n + 1 could overflow
            // thus, if n is usize::MAX, instead of adding one, we call .nth(step)
            if n == usize::MAX {
                self.iter.nth(step - 1);
            } else {
                n += 1;
            }

            // overflow handling
            loop {
                let mul = n.checked_mul(step);
                {
                    if intrinsics::likely(mul.is_some()) {
                        return self.iter.nth(mul.unwrap() - 1);
                    }
                }
                let div_n = usize::MAX / n;
                let div_step = usize::MAX / step;
                let nth_n = div_n * n;
                let nth_step = div_step * step;
                let nth = if nth_n > nth_step {
                    step -= div_n;
                    nth_n
                } else {
                    n -= div_step;
                    nth_step
                };
                self.iter.nth(nth - 1);
            }
        }

        fn try_fold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
        where
            F: FnMut(Acc, Self::Item) -> R,
            R: Try<Ok = Acc>,
        {
            #[inline]
            fn nth<I: Iterator>(iter: &mut I, step: usize) -> impl FnMut() -> Option<I::Item> + '_ {
                move || iter.nth(step)
            }

            if self.first_take {
                self.first_take = false;
                match self.iter.next() {
                    None => return try { acc },
                    Some(x) => acc = f(acc, x)?,
                }
            }
            from_fn(nth(&mut self.iter, self.step)).try_fold(acc, f)
        }

        fn fold<Acc, F>(mut self, mut acc: Acc, mut f: F) -> Acc
        where
            F: FnMut(Acc, Self::Item) -> Acc,
        {
            #[inline]
            fn nth<I: Iterator>(iter: &mut I, step: usize) -> impl FnMut() -> Option<I::Item> + '_ {
                move || iter.nth(step)
            }

            if self.first_take {
                self.first_take = false;
                match self.iter.next() {
                    None => return acc,
                    Some(x) => acc = f(acc, x),
                }
            }
            from_fn(nth(&mut self.iter, self.step)).fold(acc, f)
        }
    }

    impl<I> StepBy<I>
    where
        I: ExactSizeIterator,
    {
        // The zero-based index starting from the end of the iterator of the
        // last element. Used in the `DoubleEndedIterator` implementation.
        fn next_back_index(&self) -> usize {
            let rem = self.iter.len() % (self.step + 1);
            if self.first_take {
                if rem == 0 { self.step } else { rem - 1 }
            } else {
                rem
            }
        }
    }

    #[stable(feature = "double_ended_step_by_iterator", since = "1.38.0")]
    impl<I> DoubleEndedIterator for StepBy<I>
    where
        I: DoubleEndedIterator + ExactSizeIterator,
    {
        #[inline]
        fn next_back(&mut self) -> Option<Self::Item> {
            self.iter.nth_back(self.next_back_index())
        }

        #[inline]
        fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
            // `self.iter.nth_back(usize::MAX)` does the right thing here when `n`
            // is out of bounds because the length of `self.iter` does not exceed
            // `usize::MAX` (because `I: ExactSizeIterator`) and `nth_back` is
            // zero-indexed
            let n = n.saturating_mul(self.step + 1).saturating_add(self.next_back_index());
            self.iter.nth_back(n)
        }

        fn try_rfold<Acc, F, R>(&mut self, init: Acc, mut f: F) -> R
        where
            F: FnMut(Acc, Self::Item) -> R,
            R: Try<Ok = Acc>,
        {
            #[inline]
            fn nth_back<I: DoubleEndedIterator>(
                iter: &mut I,
                step: usize,
            ) -> impl FnMut() -> Option<I::Item> + '_ {
                move || iter.nth_back(step)
            }

            match self.next_back() {
                None => try { init },
                Some(x) => {
                    let acc = f(init, x)?;
                    from_fn(nth_back(&mut self.iter, self.step)).try_fold(acc, f)
                }
            }
        }

        #[inline]
        fn rfold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
        where
            Self: Sized,
            F: FnMut(Acc, Self::Item) -> Acc,
        {
            #[inline]
            fn nth_back<I: DoubleEndedIterator>(
                iter: &mut I,
                step: usize,
            ) -> impl FnMut() -> Option<I::Item> + '_ {
                move || iter.nth_back(step)
            }

            match self.next_back() {
                None => init,
                Some(x) => {
                    let acc = f(init, x);
                    from_fn(nth_back(&mut self.iter, self.step)).fold(acc, f)
                }
            }
        }
    }

    // StepBy can only make the iterator shorter, so the len will still fit.
    #[stable(feature = "iterator_step_by", since = "1.28.0")]
    impl<I> ExactSizeIterator for StepBy<I> where I: ExactSizeIterator {}