use crate::cmp;
use crate::fmt;
use crate::intrinsics;
use crate::ops::{Add, AddAssign, Try};
use crate::usize;
use super::{from_fn, LoopState};
use super::{DoubleEndedIterator, ExactSizeIterator, FusedIterator, Iterator, TrustedLen};
mod chain;
mod flatten;
mod zip;
pub use self::chain::Chain;
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::flatten::{FlatMap, Flatten};
pub(crate) use self::zip::TrustedRandomAccess;
pub use self::zip::Zip;
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Rev<T> {
    iter: T,
}
impl<T> Rev<T> {
    pub(super) fn new(iter: T) -> Rev<T> {
        Rev { iter }
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Rev<I>
where
    I: DoubleEndedIterator,
{
    type Item = <I as Iterator>::Item;
    #[inline]
    fn next(&mut self) -> Option<<I as Iterator>::Item> {
        self.iter.next_back()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
    #[inline]
    fn nth(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
        self.iter.nth_back(n)
    }
    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Ok = B>,
    {
        self.iter.try_rfold(init, f)
    }
    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.rfold(init, f)
    }
    #[inline]
    fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool,
    {
        self.iter.rfind(predicate)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> DoubleEndedIterator for Rev<I>
where
    I: DoubleEndedIterator,
{
    #[inline]
    fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
        self.iter.next()
    }
    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
        self.iter.nth(n)
    }
    fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Ok = B>,
    {
        self.iter.try_fold(init, f)
    }
    fn rfold<Acc, F>(self, init: Acc, f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.fold(init, f)
    }
    fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool,
    {
        self.iter.find(predicate)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Rev<I>
where
    I: ExactSizeIterator + DoubleEndedIterator,
{
    fn len(&self) -> usize {
        self.iter.len()
    }
    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Rev<I> where I: FusedIterator + DoubleEndedIterator {}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<I> TrustedLen for Rev<I> where I: TrustedLen + DoubleEndedIterator {}
#[stable(feature = "iter_copied", since = "1.36.0")]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[derive(Clone, Debug)]
pub struct Copied<I> {
    it: I,
}
impl<I> Copied<I> {
    pub(super) fn new(it: I) -> Copied<I> {
        Copied { it }
    }
}
fn copy_fold<T: Copy, Acc>(mut f: impl FnMut(Acc, T) -> Acc) -> impl FnMut(Acc, &T) -> Acc {
    move |acc, &elt| f(acc, elt)
}
fn copy_try_fold<T: Copy, Acc, R>(mut f: impl FnMut(Acc, T) -> R) -> impl FnMut(Acc, &T) -> R {
    move |acc, &elt| f(acc, elt)
}
#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> Iterator for Copied<I>
where
    I: Iterator<Item = &'a T>,
    T: Copy,
{
    type Item = T;
    fn next(&mut self) -> Option<T> {
        self.it.next().copied()
    }
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.it.size_hint()
    }
    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Ok = B>,
    {
        self.it.try_fold(init, copy_try_fold(f))
    }
    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.it.fold(init, copy_fold(f))
    }
}
#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> DoubleEndedIterator for Copied<I>
where
    I: DoubleEndedIterator<Item = &'a T>,
    T: Copy,
{
    fn next_back(&mut self) -> Option<T> {
        self.it.next_back().copied()
    }
    fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Ok = B>,
    {
        self.it.try_rfold(init, copy_try_fold(f))
    }
    fn rfold<Acc, F>(self, init: Acc, f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.it.rfold(init, copy_fold(f))
    }
}
#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> ExactSizeIterator for Copied<I>
where
    I: ExactSizeIterator<Item = &'a T>,
    T: Copy,
{
    fn len(&self) -> usize {
        self.it.len()
    }
    fn is_empty(&self) -> bool {
        self.it.is_empty()
    }
}
#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> FusedIterator for Copied<I>
where
    I: FusedIterator<Item = &'a T>,
    T: Copy,
{
}
#[doc(hidden)]
unsafe impl<'a, I, T: 'a> TrustedRandomAccess for Copied<I>
where
    I: TrustedRandomAccess<Item = &'a T>,
    T: Copy,
{
    unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item {
        *self.it.get_unchecked(i)
    }
    #[inline]
    fn may_have_side_effect() -> bool {
        I::may_have_side_effect()
    }
}
#[stable(feature = "iter_copied", since = "1.36.0")]
unsafe impl<'a, I, T: 'a> TrustedLen for Copied<I>
where
    I: TrustedLen<Item = &'a T>,
    T: Copy,
{
}
#[stable(feature = "iter_cloned", since = "1.1.0")]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[derive(Clone, Debug)]
pub struct Cloned<I> {
    it: I,
}
impl<I> Cloned<I> {
    pub(super) fn new(it: I) -> Cloned<I> {
        Cloned { it }
    }
}
fn clone_try_fold<T: Clone, Acc, R>(mut f: impl FnMut(Acc, T) -> R) -> impl FnMut(Acc, &T) -> R {
    move |acc, elt| f(acc, elt.clone())
}
#[stable(feature = "iter_cloned", since = "1.1.0")]
impl<'a, I, T: 'a> Iterator for Cloned<I>
where
    I: Iterator<Item = &'a T>,
    T: Clone,
{
    type Item = T;
    fn next(&mut self) -> Option<T> {
        self.it.next().cloned()
    }
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.it.size_hint()
    }
    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Ok = B>,
    {
        self.it.try_fold(init, clone_try_fold(f))
    }
    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.it.map(T::clone).fold(init, f)
    }
}
#[stable(feature = "iter_cloned", since = "1.1.0")]
impl<'a, I, T: 'a> DoubleEndedIterator for Cloned<I>
where
    I: DoubleEndedIterator<Item = &'a T>,
    T: Clone,
{
    fn next_back(&mut self) -> Option<T> {
        self.it.next_back().cloned()
    }
    fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Ok = B>,
    {
        self.it.try_rfold(init, clone_try_fold(f))
    }
    fn rfold<Acc, F>(self, init: Acc, f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.it.map(T::clone).rfold(init, f)
    }
}
#[stable(feature = "iter_cloned", since = "1.1.0")]
impl<'a, I, T: 'a> ExactSizeIterator for Cloned<I>
where
    I: ExactSizeIterator<Item = &'a T>,
    T: Clone,
{
    fn len(&self) -> usize {
        self.it.len()
    }
    fn is_empty(&self) -> bool {
        self.it.is_empty()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<'a, I, T: 'a> FusedIterator for Cloned<I>
where
    I: FusedIterator<Item = &'a T>,
    T: Clone,
{
}
#[doc(hidden)]
unsafe impl<'a, I, T: 'a> TrustedRandomAccess for Cloned<I>
where
    I: TrustedRandomAccess<Item = &'a T>,
    T: Clone,
{
    default unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item {
        self.it.get_unchecked(i).clone()
    }
    #[inline]
    default fn may_have_side_effect() -> bool {
        true
    }
}
#[doc(hidden)]
unsafe impl<'a, I, T: 'a> TrustedRandomAccess for Cloned<I>
where
    I: TrustedRandomAccess<Item = &'a T>,
    T: Copy,
{
    unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item {
        *self.it.get_unchecked(i)
    }
    #[inline]
    fn may_have_side_effect() -> bool {
        I::may_have_side_effect()
    }
}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<'a, I, T: 'a> TrustedLen for Cloned<I>
where
    I: TrustedLen<Item = &'a T>,
    T: Clone,
{
}
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Cycle<I> {
    orig: I,
    iter: I,
}
impl<I: Clone> Cycle<I> {
    pub(super) fn new(iter: I) -> Cycle<I> {
        Cycle { orig: iter.clone(), iter }
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Cycle<I>
where
    I: Clone + Iterator,
{
    type Item = <I as Iterator>::Item;
    #[inline]
    fn next(&mut self) -> Option<<I as Iterator>::Item> {
        match self.iter.next() {
            None => {
                self.iter = self.orig.clone();
                self.iter.next()
            }
            y => y,
        }
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        
        match self.orig.size_hint() {
            sz @ (0, Some(0)) => sz,
            (0, _) => (0, None),
            _ => (usize::MAX, None),
        }
    }
    #[inline]
    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>,
    {
        
        
        acc = self.iter.try_fold(acc, &mut f)?;
        self.iter = self.orig.clone();
        
        
        
        let mut is_empty = true;
        acc = self.iter.try_fold(acc, |acc, x| {
            is_empty = false;
            f(acc, x)
        })?;
        if is_empty {
            return Try::from_ok(acc);
        }
        loop {
            self.iter = self.orig.clone();
            acc = self.iter.try_fold(acc, &mut f)?;
        }
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Cycle<I> where I: Clone + Iterator {}
#[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(super) 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;
        }
        
        
        
        let mut step = self.step + 1;
        
        
        if n == usize::MAX {
            self.iter.nth(step - 1);
        } else {
            n += 1;
        }
        
        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::from_ok(acc),
                Some(x) => acc = f(acc, x)?,
            }
        }
        from_fn(nth(&mut self.iter, self.step)).try_fold(acc, f)
    }
}
impl<I> StepBy<I>
where
    I: ExactSizeIterator,
{
    
    
    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> {
        
        
        
        
        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::from_ok(init),
            Some(x) => {
                let acc = f(init, x)?;
                from_fn(nth_back(&mut self.iter, self.step)).try_fold(acc, f)
            }
        }
    }
}
#[stable(feature = "iterator_step_by", since = "1.28.0")]
impl<I> ExactSizeIterator for StepBy<I> where I: ExactSizeIterator {}
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct Map<I, F> {
    iter: I,
    f: F,
}
impl<I, F> Map<I, F> {
    pub(super) fn new(iter: I, f: F) -> Map<I, F> {
        Map { iter, f }
    }
}
#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, F> fmt::Debug for Map<I, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Map").field("iter", &self.iter).finish()
    }
}
fn map_fold<T, B, Acc>(
    mut f: impl FnMut(T) -> B,
    mut g: impl FnMut(Acc, B) -> Acc,
) -> impl FnMut(Acc, T) -> Acc {
    move |acc, elt| g(acc, f(elt))
}
fn map_try_fold<'a, T, B, Acc, R>(
    f: &'a mut impl FnMut(T) -> B,
    mut g: impl FnMut(Acc, B) -> R + 'a,
) -> impl FnMut(Acc, T) -> R + 'a {
    move |acc, elt| g(acc, f(elt))
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: Iterator, F> Iterator for Map<I, F>
where
    F: FnMut(I::Item) -> B,
{
    type Item = B;
    #[inline]
    fn next(&mut self) -> Option<B> {
        self.iter.next().map(&mut self.f)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
    fn try_fold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
    where
        Self: Sized,
        G: FnMut(Acc, Self::Item) -> R,
        R: Try<Ok = Acc>,
    {
        self.iter.try_fold(init, map_try_fold(&mut self.f, g))
    }
    fn fold<Acc, G>(self, init: Acc, g: G) -> Acc
    where
        G: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.fold(init, map_fold(self.f, g))
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for Map<I, F>
where
    F: FnMut(I::Item) -> B,
{
    #[inline]
    fn next_back(&mut self) -> Option<B> {
        self.iter.next_back().map(&mut self.f)
    }
    fn try_rfold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
    where
        Self: Sized,
        G: FnMut(Acc, Self::Item) -> R,
        R: Try<Ok = Acc>,
    {
        self.iter.try_rfold(init, map_try_fold(&mut self.f, g))
    }
    fn rfold<Acc, G>(self, init: Acc, g: G) -> Acc
    where
        G: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.rfold(init, map_fold(self.f, g))
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F>
where
    F: FnMut(I::Item) -> B,
{
    fn len(&self) -> usize {
        self.iter.len()
    }
    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<B, I: FusedIterator, F> FusedIterator for Map<I, F> where F: FnMut(I::Item) -> B {}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<B, I, F> TrustedLen for Map<I, F>
where
    I: TrustedLen,
    F: FnMut(I::Item) -> B,
{
}
#[doc(hidden)]
unsafe impl<B, I, F> TrustedRandomAccess for Map<I, F>
where
    I: TrustedRandomAccess,
    F: FnMut(I::Item) -> B,
{
    unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item {
        (self.f)(self.iter.get_unchecked(i))
    }
    #[inline]
    fn may_have_side_effect() -> bool {
        true
    }
}
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct Filter<I, P> {
    iter: I,
    predicate: P,
}
impl<I, P> Filter<I, P> {
    pub(super) fn new(iter: I, predicate: P) -> Filter<I, P> {
        Filter { iter, predicate }
    }
}
#[stable(feature = "core_impl_debug", since = "1.9.0")]
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()
    }
}
fn filter_fold<T, Acc>(
    mut predicate: impl FnMut(&T) -> bool,
    mut fold: impl FnMut(Acc, T) -> Acc,
) -> impl FnMut(Acc, T) -> Acc {
    move |acc, item| if predicate(&item) { fold(acc, item) } else { acc }
}
fn filter_try_fold<'a, T, Acc, R: Try<Ok = Acc>>(
    predicate: &'a mut impl FnMut(&T) -> bool,
    mut fold: impl FnMut(Acc, T) -> R + 'a,
) -> impl FnMut(Acc, T) -> R + 'a {
    move |acc, item| if predicate(&item) { fold(acc, item) } else { R::from_ok(acc) }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, P> Iterator for Filter<I, P>
where
    P: FnMut(&I::Item) -> bool,
{
    type Item = I::Item;
    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        self.iter.find(&mut self.predicate)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) 
    }
    
    
    
    
    
    
    
    
    
    
    
    #[inline]
    fn count(self) -> usize {
        #[inline]
        fn to_usize<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut(T) -> usize {
            move |x| predicate(&x) as usize
        }
        self.iter.map(to_usize(self.predicate)).sum()
    }
    #[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<Ok = Acc>,
    {
        self.iter.try_fold(init, filter_try_fold(&mut self.predicate, fold))
    }
    #[inline]
    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.fold(init, filter_fold(self.predicate, fold))
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: DoubleEndedIterator, P> DoubleEndedIterator for Filter<I, P>
where
    P: FnMut(&I::Item) -> bool,
{
    #[inline]
    fn next_back(&mut self) -> Option<I::Item> {
        self.iter.rfind(&mut self.predicate)
    }
    #[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<Ok = Acc>,
    {
        self.iter.try_rfold(init, filter_try_fold(&mut self.predicate, fold))
    }
    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.rfold(init, filter_fold(self.predicate, fold))
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<I: FusedIterator, P> FusedIterator for Filter<I, P> where P: FnMut(&I::Item) -> bool {}
#[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(super) 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<Ok = 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 => R::from_ok(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) 
    }
    #[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<Ok = 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) -> LoopState<(), B> + '_ {
            move |(), x| match f(x) {
                Some(x) => LoopState::Break(x),
                None => LoopState::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<Ok = 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> {}
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Enumerate<I> {
    iter: I,
    count: usize,
}
impl<I> Enumerate<I> {
    pub(super) 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]
    fn next(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
        let a = self.iter.next()?;
        let i = self.count;
        
        AddAssign::add_assign(&mut self.count, 1);
        Some((i, a))
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
    #[inline]
    fn nth(&mut self, n: usize) -> Option<(usize, I::Item)> {
        let a = self.iter.nth(n)?;
        
        let i = Add::add(self.count, n);
        self.count = Add::add(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<Ok = 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 {
            move |acc, item| {
                let acc = fold(acc, (*count, item));
                
                AddAssign::add_assign(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 {
            move |acc, item| {
                let acc = fold(acc, (count, item));
                
                AddAssign::add_assign(&mut count, 1);
                acc
            }
        }
        self.iter.fold(init, enumerate(self.count, fold))
    }
}
#[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<Ok = 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))
    }
}
#[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)]
unsafe impl<I> TrustedRandomAccess for Enumerate<I>
where
    I: TrustedRandomAccess,
{
    unsafe fn get_unchecked(&mut self, i: usize) -> (usize, I::Item) {
        (self.count + i, self.iter.get_unchecked(i))
    }
    fn 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(feature = "trusted_len", issue = "37572")]
unsafe impl<I> TrustedLen for Enumerate<I> where I: TrustedLen {}
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Peekable<I: Iterator> {
    iter: I,
    
    peeked: Option<Option<I::Item>>,
}
impl<I: Iterator> Peekable<I> {
    pub(super) fn new(iter: I) -> Peekable<I> {
        Peekable { iter, peeked: None }
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator> Iterator for Peekable<I> {
    type Item = I::Item;
    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        match self.peeked.take() {
            Some(v) => v,
            None => self.iter.next(),
        }
    }
    #[inline]
    #[rustc_inherit_overflow_checks]
    fn count(mut self) -> usize {
        match self.peeked.take() {
            Some(None) => 0,
            Some(Some(_)) => 1 + self.iter.count(),
            None => self.iter.count(),
        }
    }
    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        match self.peeked.take() {
            Some(None) => None,
            Some(v @ Some(_)) if n == 0 => v,
            Some(Some(_)) => self.iter.nth(n - 1),
            None => self.iter.nth(n),
        }
    }
    #[inline]
    fn last(mut self) -> Option<I::Item> {
        let peek_opt = match self.peeked.take() {
            Some(None) => return None,
            Some(v) => v,
            None => None,
        };
        self.iter.last().or(peek_opt)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let peek_len = match self.peeked {
            Some(None) => return (0, Some(0)),
            Some(Some(_)) => 1,
            None => 0,
        };
        let (lo, hi) = self.iter.size_hint();
        let lo = lo.saturating_add(peek_len);
        let hi = match hi {
            Some(x) => x.checked_add(peek_len),
            None => None,
        };
        (lo, hi)
    }
    #[inline]
    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Ok = B>,
    {
        let acc = match self.peeked.take() {
            Some(None) => return Try::from_ok(init),
            Some(Some(v)) => f(init, v)?,
            None => init,
        };
        self.iter.try_fold(acc, f)
    }
    #[inline]
    fn fold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        let acc = match self.peeked {
            Some(None) => return init,
            Some(Some(v)) => fold(init, v),
            None => init,
        };
        self.iter.fold(acc, fold)
    }
}
#[stable(feature = "double_ended_peek_iterator", since = "1.38.0")]
impl<I> DoubleEndedIterator for Peekable<I>
where
    I: DoubleEndedIterator,
{
    #[inline]
    fn next_back(&mut self) -> Option<Self::Item> {
        self.iter.next_back().or_else(|| self.peeked.take().and_then(|x| x))
    }
    #[inline]
    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Ok = B>,
    {
        match self.peeked.take() {
            Some(None) => Try::from_ok(init),
            Some(Some(v)) => match self.iter.try_rfold(init, &mut f).into_result() {
                Ok(acc) => f(acc, v),
                Err(e) => {
                    self.peeked = Some(Some(v));
                    Try::from_error(e)
                }
            },
            None => self.iter.try_rfold(init, f),
        }
    }
    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        match self.peeked {
            Some(None) => init,
            Some(Some(v)) => {
                let acc = self.iter.rfold(init, &mut fold);
                fold(acc, v)
            }
            None => self.iter.rfold(init, fold),
        }
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I> {}
#[stable(feature = "fused", since = "1.26.0")]
impl<I: FusedIterator> FusedIterator for Peekable<I> {}
impl<I: Iterator> Peekable<I> {
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn peek(&mut self) -> Option<&I::Item> {
        let iter = &mut self.iter;
        self.peeked.get_or_insert_with(|| iter.next()).as_ref()
    }
}
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct SkipWhile<I, P> {
    iter: I,
    flag: bool,
    predicate: P,
}
impl<I, P> SkipWhile<I, P> {
    pub(super) fn new(iter: I, predicate: P) -> SkipWhile<I, P> {
        SkipWhile { iter, flag: false, predicate }
    }
}
#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, P> fmt::Debug for SkipWhile<I, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SkipWhile").field("iter", &self.iter).field("flag", &self.flag).finish()
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, P> Iterator for SkipWhile<I, P>
where
    P: FnMut(&I::Item) -> bool,
{
    type Item = I::Item;
    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        fn check<'a, T>(
            flag: &'a mut bool,
            pred: &'a mut impl FnMut(&T) -> bool,
        ) -> impl FnMut(&T) -> bool + 'a {
            move |x| {
                if *flag || !pred(x) {
                    *flag = true;
                    true
                } else {
                    false
                }
            }
        }
        let flag = &mut self.flag;
        let pred = &mut self.predicate;
        self.iter.find(check(flag, pred))
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) 
    }
    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Ok = Acc>,
    {
        if !self.flag {
            match self.next() {
                Some(v) => init = fold(init, v)?,
                None => return Try::from_ok(init),
            }
        }
        self.iter.try_fold(init, fold)
    }
    #[inline]
    fn fold<Acc, Fold>(mut self, mut init: Acc, mut fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        if !self.flag {
            match self.next() {
                Some(v) => init = fold(init, v),
                None => return init,
            }
        }
        self.iter.fold(init, fold)
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<I, P> FusedIterator for SkipWhile<I, P>
where
    I: FusedIterator,
    P: FnMut(&I::Item) -> bool,
{
}
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct TakeWhile<I, P> {
    iter: I,
    flag: bool,
    predicate: P,
}
impl<I, P> TakeWhile<I, P> {
    pub(super) fn new(iter: I, predicate: P) -> TakeWhile<I, P> {
        TakeWhile { iter, flag: false, predicate }
    }
}
#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, P> fmt::Debug for TakeWhile<I, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("TakeWhile").field("iter", &self.iter).field("flag", &self.flag).finish()
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, P> Iterator for TakeWhile<I, P>
where
    P: FnMut(&I::Item) -> bool,
{
    type Item = I::Item;
    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        if self.flag {
            None
        } else {
            let x = self.iter.next()?;
            if (self.predicate)(&x) {
                Some(x)
            } else {
                self.flag = true;
                None
            }
        }
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.flag {
            (0, Some(0))
        } else {
            let (_, upper) = self.iter.size_hint();
            (0, upper) 
        }
    }
    #[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<Ok = Acc>,
    {
        fn check<'a, T, Acc, R: Try<Ok = Acc>>(
            flag: &'a mut bool,
            p: &'a mut impl FnMut(&T) -> bool,
            mut fold: impl FnMut(Acc, T) -> R + 'a,
        ) -> impl FnMut(Acc, T) -> LoopState<Acc, R> + 'a {
            move |acc, x| {
                if p(&x) {
                    LoopState::from_try(fold(acc, x))
                } else {
                    *flag = true;
                    LoopState::Break(Try::from_ok(acc))
                }
            }
        }
        if self.flag {
            Try::from_ok(init)
        } else {
            let flag = &mut self.flag;
            let p = &mut self.predicate;
            self.iter.try_fold(init, check(flag, p, fold)).into_try()
        }
    }
}
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
#[derive(Clone)]
pub struct MapWhile<I, P> {
    iter: I,
    finished: bool,
    predicate: P,
}
impl<I, P> MapWhile<I, P> {
    pub(super) fn new(iter: I, predicate: P) -> MapWhile<I, P> {
        MapWhile { iter, finished: false, predicate }
    }
}
#[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
impl<I: fmt::Debug, P> fmt::Debug for MapWhile<I, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("MapWhile").field("iter", &self.iter).field("flag", &self.finished).finish()
    }
}
#[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
impl<B, I: Iterator, P> Iterator for MapWhile<I, P>
where
    P: FnMut(I::Item) -> Option<B>,
{
    type Item = B;
    #[inline]
    fn next(&mut self) -> Option<B> {
        if self.finished {
            None
        } else {
            let x = self.iter.next()?;
            let ret = (self.predicate)(x);
            self.finished = ret.is_none();
            ret
        }
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.finished {
            (0, Some(0))
        } else {
            let (_, upper) = self.iter.size_hint();
            (0, upper) 
        }
    }
    #[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<Ok = Acc>,
    {
        fn check<'a, B, T, Acc, R: Try<Ok = Acc>>(
            flag: &'a mut bool,
            p: &'a mut impl FnMut(T) -> Option<B>,
            mut fold: impl FnMut(Acc, B) -> R + 'a,
        ) -> impl FnMut(Acc, T) -> LoopState<Acc, R> + 'a {
            move |acc, x| match p(x) {
                Some(item) => LoopState::from_try(fold(acc, item)),
                None => {
                    *flag = true;
                    LoopState::Break(Try::from_ok(acc))
                }
            }
        }
        if self.finished {
            Try::from_ok(init)
        } else {
            let flag = &mut self.finished;
            let p = &mut self.predicate;
            self.iter.try_fold(init, check(flag, p, fold)).into_try()
        }
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<I, P> FusedIterator for TakeWhile<I, P>
where
    I: FusedIterator,
    P: FnMut(&I::Item) -> bool,
{
}
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Skip<I> {
    iter: I,
    n: usize,
}
impl<I> Skip<I> {
    pub(super) fn new(iter: I, n: usize) -> Skip<I> {
        Skip { iter, n }
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Skip<I>
where
    I: Iterator,
{
    type Item = <I as Iterator>::Item;
    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        if self.n == 0 {
            self.iter.next()
        } else {
            let old_n = self.n;
            self.n = 0;
            self.iter.nth(old_n)
        }
    }
    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        
        if self.n > 0 {
            let to_skip = self.n;
            self.n = 0;
            
            self.iter.nth(to_skip - 1)?;
        }
        self.iter.nth(n)
    }
    #[inline]
    fn count(mut self) -> usize {
        if self.n > 0 {
            
            if self.iter.nth(self.n - 1).is_none() {
                return 0;
            }
        }
        self.iter.count()
    }
    #[inline]
    fn last(mut self) -> Option<I::Item> {
        if self.n > 0 {
            
            self.iter.nth(self.n - 1)?;
        }
        self.iter.last()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (lower, upper) = self.iter.size_hint();
        let lower = lower.saturating_sub(self.n);
        let upper = match upper {
            Some(x) => Some(x.saturating_sub(self.n)),
            None => None,
        };
        (lower, upper)
    }
    #[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<Ok = Acc>,
    {
        let n = self.n;
        self.n = 0;
        if n > 0 {
            
            if self.iter.nth(n - 1).is_none() {
                return Try::from_ok(init);
            }
        }
        self.iter.try_fold(init, fold)
    }
    #[inline]
    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        if self.n > 0 {
            
            if self.iter.nth(self.n - 1).is_none() {
                return init;
            }
        }
        self.iter.fold(init, fold)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator {}
#[stable(feature = "double_ended_skip_iterator", since = "1.9.0")]
impl<I> DoubleEndedIterator for Skip<I>
where
    I: DoubleEndedIterator + ExactSizeIterator,
{
    fn next_back(&mut self) -> Option<Self::Item> {
        if self.len() > 0 { self.iter.next_back() } else { None }
    }
    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<I::Item> {
        let len = self.len();
        if n < len {
            self.iter.nth_back(n)
        } else {
            if len > 0 {
                
                self.iter.nth_back(len - 1);
            }
            None
        }
    }
    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Ok = Acc>,
    {
        fn check<T, Acc, R: Try<Ok = Acc>>(
            mut n: usize,
            mut fold: impl FnMut(Acc, T) -> R,
        ) -> impl FnMut(Acc, T) -> LoopState<Acc, R> {
            move |acc, x| {
                n -= 1;
                let r = fold(acc, x);
                if n == 0 { LoopState::Break(r) } else { LoopState::from_try(r) }
            }
        }
        let n = self.len();
        if n == 0 {
            Try::from_ok(init)
        } else {
            self.iter.try_rfold(init, check(n, fold)).into_try()
        }
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Skip<I> where I: FusedIterator {}
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Take<I> {
    pub(super) iter: I,
    pub(super) n: usize,
}
impl<I> Take<I> {
    pub(super) fn new(iter: I, n: usize) -> Take<I> {
        Take { iter, n }
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Take<I>
where
    I: Iterator,
{
    type Item = <I as Iterator>::Item;
    #[inline]
    fn next(&mut self) -> Option<<I as Iterator>::Item> {
        if self.n != 0 {
            self.n -= 1;
            self.iter.next()
        } else {
            None
        }
    }
    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        if self.n > n {
            self.n -= n + 1;
            self.iter.nth(n)
        } else {
            if self.n > 0 {
                self.iter.nth(self.n - 1);
                self.n = 0;
            }
            None
        }
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.n == 0 {
            return (0, Some(0));
        }
        let (lower, upper) = self.iter.size_hint();
        let lower = cmp::min(lower, self.n);
        let upper = match upper {
            Some(x) if x < self.n => Some(x),
            _ => Some(self.n),
        };
        (lower, upper)
    }
    #[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<Ok = Acc>,
    {
        fn check<'a, T, Acc, R: Try<Ok = Acc>>(
            n: &'a mut usize,
            mut fold: impl FnMut(Acc, T) -> R + 'a,
        ) -> impl FnMut(Acc, T) -> LoopState<Acc, R> + 'a {
            move |acc, x| {
                *n -= 1;
                let r = fold(acc, x);
                if *n == 0 { LoopState::Break(r) } else { LoopState::from_try(r) }
            }
        }
        if self.n == 0 {
            Try::from_ok(init)
        } else {
            let n = &mut self.n;
            self.iter.try_fold(init, check(n, fold)).into_try()
        }
    }
}
#[stable(feature = "double_ended_take_iterator", since = "1.38.0")]
impl<I> DoubleEndedIterator for Take<I>
where
    I: DoubleEndedIterator + ExactSizeIterator,
{
    #[inline]
    fn next_back(&mut self) -> Option<Self::Item> {
        if self.n == 0 {
            None
        } else {
            let n = self.n;
            self.n -= 1;
            self.iter.nth_back(self.iter.len().saturating_sub(n))
        }
    }
    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let len = self.iter.len();
        if self.n > n {
            let m = len.saturating_sub(self.n) + n;
            self.n -= n + 1;
            self.iter.nth_back(m)
        } else {
            if len > 0 {
                self.iter.nth_back(len - 1);
            }
            None
        }
    }
    #[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<Ok = Acc>,
    {
        if self.n == 0 {
            Try::from_ok(init)
        } else {
            let len = self.iter.len();
            if len > self.n && self.iter.nth_back(len - self.n - 1).is_none() {
                Try::from_ok(init)
            } else {
                self.iter.try_rfold(init, fold)
            }
        }
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Take<I> where I: ExactSizeIterator {}
#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Take<I> where I: FusedIterator {}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<I: TrustedLen> TrustedLen for Take<I> {}
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct Scan<I, St, F> {
    iter: I,
    f: F,
    state: St,
}
impl<I, St, F> Scan<I, St, F> {
    pub(super) fn new(iter: I, state: St, f: F) -> Scan<I, St, F> {
        Scan { iter, state, f }
    }
}
#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, St: fmt::Debug, F> fmt::Debug for Scan<I, St, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Scan").field("iter", &self.iter).field("state", &self.state).finish()
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I, St, F> Iterator for Scan<I, St, F>
where
    I: Iterator,
    F: FnMut(&mut St, I::Item) -> Option<B>,
{
    type Item = B;
    #[inline]
    fn next(&mut self) -> Option<B> {
        let a = self.iter.next()?;
        (self.f)(&mut self.state, a)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) 
    }
    #[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<Ok = Acc>,
    {
        fn scan<'a, T, St, B, Acc, R: Try<Ok = Acc>>(
            state: &'a mut St,
            f: &'a mut impl FnMut(&mut St, T) -> Option<B>,
            mut fold: impl FnMut(Acc, B) -> R + 'a,
        ) -> impl FnMut(Acc, T) -> LoopState<Acc, R> + 'a {
            move |acc, x| match f(state, x) {
                None => LoopState::Break(Try::from_ok(acc)),
                Some(x) => LoopState::from_try(fold(acc, x)),
            }
        }
        let state = &mut self.state;
        let f = &mut self.f;
        self.iter.try_fold(init, scan(state, f, fold)).into_try()
    }
}
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Fuse<I> {
    iter: I,
    done: bool,
}
impl<I> Fuse<I> {
    pub(super) fn new(iter: I) -> Fuse<I> {
        Fuse { iter, done: false }
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Fuse<I> where I: Iterator {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Fuse<I>
where
    I: Iterator,
{
    type Item = <I as Iterator>::Item;
    #[inline]
    default fn next(&mut self) -> Option<<I as Iterator>::Item> {
        if self.done {
            None
        } else {
            let next = self.iter.next();
            self.done = next.is_none();
            next
        }
    }
    #[inline]
    default fn nth(&mut self, n: usize) -> Option<I::Item> {
        if self.done {
            None
        } else {
            let nth = self.iter.nth(n);
            self.done = nth.is_none();
            nth
        }
    }
    #[inline]
    default fn last(self) -> Option<I::Item> {
        if self.done { None } else { self.iter.last() }
    }
    #[inline]
    default fn count(self) -> usize {
        if self.done { 0 } else { self.iter.count() }
    }
    #[inline]
    default fn size_hint(&self) -> (usize, Option<usize>) {
        if self.done { (0, Some(0)) } else { self.iter.size_hint() }
    }
    #[inline]
    default fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Ok = Acc>,
    {
        if self.done {
            Try::from_ok(init)
        } else {
            let acc = self.iter.try_fold(init, fold)?;
            self.done = true;
            Try::from_ok(acc)
        }
    }
    #[inline]
    default fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        if self.done { init } else { self.iter.fold(init, fold) }
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> DoubleEndedIterator for Fuse<I>
where
    I: DoubleEndedIterator,
{
    #[inline]
    default fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
        if self.done {
            None
        } else {
            let next = self.iter.next_back();
            self.done = next.is_none();
            next
        }
    }
    #[inline]
    default fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
        if self.done {
            None
        } else {
            let nth = self.iter.nth_back(n);
            self.done = nth.is_none();
            nth
        }
    }
    #[inline]
    default fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Ok = Acc>,
    {
        if self.done {
            Try::from_ok(init)
        } else {
            let acc = self.iter.try_rfold(init, fold)?;
            self.done = true;
            Try::from_ok(acc)
        }
    }
    #[inline]
    default fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        if self.done { init } else { self.iter.rfold(init, fold) }
    }
}
unsafe impl<I> TrustedRandomAccess for Fuse<I>
where
    I: TrustedRandomAccess,
{
    unsafe fn get_unchecked(&mut self, i: usize) -> I::Item {
        self.iter.get_unchecked(i)
    }
    fn may_have_side_effect() -> bool {
        I::may_have_side_effect()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<I> Iterator for Fuse<I>
where
    I: FusedIterator,
{
    #[inline]
    fn next(&mut self) -> Option<<I as Iterator>::Item> {
        self.iter.next()
    }
    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        self.iter.nth(n)
    }
    #[inline]
    fn last(self) -> Option<I::Item> {
        self.iter.last()
    }
    #[inline]
    fn count(self) -> usize {
        self.iter.count()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
    #[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<Ok = Acc>,
    {
        self.iter.try_fold(init, fold)
    }
    #[inline]
    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.fold(init, fold)
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<I> DoubleEndedIterator for Fuse<I>
where
    I: DoubleEndedIterator + FusedIterator,
{
    #[inline]
    fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
        self.iter.next_back()
    }
    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
        self.iter.nth_back(n)
    }
    #[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<Ok = Acc>,
    {
        self.iter.try_rfold(init, fold)
    }
    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.rfold(init, fold)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Fuse<I>
where
    I: ExactSizeIterator,
{
    fn len(&self) -> usize {
        self.iter.len()
    }
    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct Inspect<I, F> {
    iter: I,
    f: F,
}
impl<I, F> Inspect<I, F> {
    pub(super) fn new(iter: I, f: F) -> Inspect<I, F> {
        Inspect { iter, f }
    }
}
#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, F> fmt::Debug for Inspect<I, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Inspect").field("iter", &self.iter).finish()
    }
}
impl<I: Iterator, F> Inspect<I, F>
where
    F: FnMut(&I::Item),
{
    #[inline]
    fn do_inspect(&mut self, elt: Option<I::Item>) -> Option<I::Item> {
        if let Some(ref a) = elt {
            (self.f)(a);
        }
        elt
    }
}
fn inspect_fold<T, Acc>(
    mut f: impl FnMut(&T),
    mut fold: impl FnMut(Acc, T) -> Acc,
) -> impl FnMut(Acc, T) -> Acc {
    move |acc, item| {
        f(&item);
        fold(acc, item)
    }
}
fn inspect_try_fold<'a, T, Acc, R>(
    f: &'a mut impl FnMut(&T),
    mut fold: impl FnMut(Acc, T) -> R + 'a,
) -> impl FnMut(Acc, T) -> R + 'a {
    move |acc, item| {
        f(&item);
        fold(acc, item)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, F> Iterator for Inspect<I, F>
where
    F: FnMut(&I::Item),
{
    type Item = I::Item;
    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        let next = self.iter.next();
        self.do_inspect(next)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
    #[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<Ok = Acc>,
    {
        self.iter.try_fold(init, inspect_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, inspect_fold(self.f, fold))
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: DoubleEndedIterator, F> DoubleEndedIterator for Inspect<I, F>
where
    F: FnMut(&I::Item),
{
    #[inline]
    fn next_back(&mut self) -> Option<I::Item> {
        let next = self.iter.next_back();
        self.do_inspect(next)
    }
    #[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<Ok = Acc>,
    {
        self.iter.try_rfold(init, inspect_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, inspect_fold(self.f, fold))
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: ExactSizeIterator, F> ExactSizeIterator for Inspect<I, F>
where
    F: FnMut(&I::Item),
{
    fn len(&self) -> usize {
        self.iter.len()
    }
    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<I: FusedIterator, F> FusedIterator for Inspect<I, F> where F: FnMut(&I::Item) {}
pub(crate) struct ResultShunt<'a, I, E> {
    iter: I,
    error: &'a mut Result<(), E>,
}
pub(crate) fn process_results<I, T, E, F, U>(iter: I, mut f: F) -> Result<U, E>
where
    I: Iterator<Item = Result<T, E>>,
    for<'a> F: FnMut(ResultShunt<'a, I, E>) -> U,
{
    let mut error = Ok(());
    let shunt = ResultShunt { iter, error: &mut error };
    let value = f(shunt);
    error.map(|()| value)
}
impl<I, T, E> Iterator for ResultShunt<'_, I, E>
where
    I: Iterator<Item = Result<T, E>>,
{
    type Item = T;
    fn next(&mut self) -> Option<Self::Item> {
        self.find(|_| true)
    }
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.error.is_err() {
            (0, Some(0))
        } else {
            let (_, upper) = self.iter.size_hint();
            (0, upper)
        }
    }
    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
    where
        F: FnMut(B, Self::Item) -> R,
        R: Try<Ok = B>,
    {
        let error = &mut *self.error;
        self.iter
            .try_fold(init, |acc, x| match x {
                Ok(x) => LoopState::from_try(f(acc, x)),
                Err(e) => {
                    *error = Err(e);
                    LoopState::Break(Try::from_ok(acc))
                }
            })
            .into_try()
    }
}