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    use crate::ops::Try;
    use crate::usize;

    use super::super::{Iterator, DoubleEndedIterator, FusedIterator, TrustedLen};

    /// An iterator that links two iterators together, in a chain.
    ///
    /// This `struct` is created by the [`chain`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`chain`]: trait.Iterator.html#method.chain
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterators are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Chain<A, B> {
        a: A,
        b: B,
        state: ChainState,
    }
    impl<A, B> Chain<A, B> {
        pub(in super::super) fn new(a: A, b: B) -> Chain<A, B> {
            Chain { a, b, state: ChainState::Both }
        }
    }

    // The iterator protocol specifies that iteration ends with the return value
    // `None` from `.next()` (or `.next_back()`) and it is unspecified what
    // further calls return. The chain adaptor must account for this since it uses
    // two subiterators.
    //
    //  It uses three states:
    //
    //  - Both: `a` and `b` are remaining
    //  - Front: `a` remaining
    //  - Back: `b` remaining
    //
    //  The fourth state (neither iterator is remaining) only occurs after Chain has
    //  returned None once, so we don't need to store this state.
    #[derive(Clone, Debug)]
    enum ChainState {
        // both front and back iterator are remaining
        Both,
        // only front is remaining
        Front,
        // only back is remaining
        Back,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> Iterator for Chain<A, B> where
        A: Iterator,
        B: Iterator<Item = A::Item>
    {
        type Item = A::Item;

        #[inline]
        fn next(&mut self) -> Option<A::Item> {
            match self.state {
                ChainState::Both => match self.a.next() {
                    elt @ Some(..) => elt,
                    None => {
                        self.state = ChainState::Back;
                        self.b.next()
                    }
                },
                ChainState::Front => self.a.next(),
                ChainState::Back => self.b.next(),
            }
        }

        #[inline]
        #[rustc_inherit_overflow_checks]
        fn count(self) -> usize {
            match self.state {
                ChainState::Both => self.a.count() + self.b.count(),
                ChainState::Front => self.a.count(),
                ChainState::Back => self.b.count(),
            }
        }

        fn try_fold<Acc, F, R>(&mut self, init: Acc, mut f: F) -> R where
            Self: Sized, F: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
        {
            let mut accum = init;
            match self.state {
                ChainState::Both | ChainState::Front => {
                    accum = self.a.try_fold(accum, &mut f)?;
                    if let ChainState::Both = self.state {
                        self.state = ChainState::Back;
                    }
                }
                _ => { }
            }
            if let ChainState::Back = self.state {
                accum = self.b.try_fold(accum, &mut f)?;
            }
            Try::from_ok(accum)
        }

        fn fold<Acc, F>(self, init: Acc, mut f: F) -> Acc
            where F: FnMut(Acc, Self::Item) -> Acc,
        {
            let mut accum = init;
            match self.state {
                ChainState::Both | ChainState::Front => {
                    accum = self.a.fold(accum, &mut f);
                }
                _ => { }
            }
            match self.state {
                ChainState::Both | ChainState::Back => {
                    accum = self.b.fold(accum, &mut f);
                }
                _ => { }
            }
            accum
        }

        #[inline]
        fn nth(&mut self, mut n: usize) -> Option<A::Item> {
            match self.state {
                ChainState::Both | ChainState::Front => {
                    for x in self.a.by_ref() {
                        if n == 0 {
                            return Some(x)
                        }
                        n -= 1;
                    }
                    if let ChainState::Both = self.state {
                        self.state = ChainState::Back;
                    }
                }
                ChainState::Back => {}
            }
            if let ChainState::Back = self.state {
                self.b.nth(n)
            } else {
                None
            }
        }

        #[inline]
        fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item> where
            P: FnMut(&Self::Item) -> bool,
        {
            match self.state {
                ChainState::Both => match self.a.find(&mut predicate) {
                    None => {
                        self.state = ChainState::Back;
                        self.b.find(predicate)
                    }
                    v => v
                },
                ChainState::Front => self.a.find(predicate),
                ChainState::Back => self.b.find(predicate),
            }
        }

        #[inline]
        fn last(self) -> Option<A::Item> {
            match self.state {
                ChainState::Both => {
                    // Must exhaust a before b.
                    let a_last = self.a.last();
                    let b_last = self.b.last();
                    b_last.or(a_last)
                },
                ChainState::Front => self.a.last(),
                ChainState::Back => self.b.last()
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            match self.state {
                ChainState::Both => {
                    let (a_lower, a_upper) = self.a.size_hint();
                    let (b_lower, b_upper) = self.b.size_hint();

                    let lower = a_lower.saturating_add(b_lower);

                    let upper = match (a_upper, b_upper) {
                        (Some(x), Some(y)) => x.checked_add(y),
                        _ => None
                    };

                    (lower, upper)
                }
                ChainState::Front => self.a.size_hint(),
                ChainState::Back => self.b.size_hint(),
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> DoubleEndedIterator for Chain<A, B> where
        A: DoubleEndedIterator,
        B: DoubleEndedIterator<Item=A::Item>,
    {
        #[inline]
        fn next_back(&mut self) -> Option<A::Item> {
            match self.state {
                ChainState::Both => match self.b.next_back() {
                    elt @ Some(..) => elt,
                    None => {
                        self.state = ChainState::Front;
                        self.a.next_back()
                    }
                },
                ChainState::Front => self.a.next_back(),
                ChainState::Back => self.b.next_back(),
            }
        }

        #[inline]
        fn nth_back(&mut self, mut n: usize) -> Option<A::Item> {
            match self.state {
                ChainState::Both | ChainState::Back => {
                    for x in self.b.by_ref().rev() {
                        if n == 0 {
                            return Some(x)
                        }
                        n -= 1;
                    }
                    if let ChainState::Both = self.state {
                        self.state = ChainState::Front;
                    }
                }
                ChainState::Front => {}
            }
            if let ChainState::Front = self.state {
                self.a.nth_back(n)
            } else {
                None
            }
        }

        fn try_rfold<Acc, F, R>(&mut self, init: Acc, mut f: F) -> R where
            Self: Sized, F: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
        {
            let mut accum = init;
            match self.state {
                ChainState::Both | ChainState::Back => {
                    accum = self.b.try_rfold(accum, &mut f)?;
                    if let ChainState::Both = self.state {
                        self.state = ChainState::Front;
                    }
                }
                _ => { }
            }
            if let ChainState::Front = self.state {
                accum = self.a.try_rfold(accum, &mut f)?;
            }
            Try::from_ok(accum)
        }

        fn rfold<Acc, F>(self, init: Acc, mut f: F) -> Acc
            where F: FnMut(Acc, Self::Item) -> Acc,
        {
            let mut accum = init;
            match self.state {
                ChainState::Both | ChainState::Back => {
                    accum = self.b.rfold(accum, &mut f);
                }
                _ => { }
            }
            match self.state {
                ChainState::Both | ChainState::Front => {
                    accum = self.a.rfold(accum, &mut f);
                }
                _ => { }
            }
            accum
        }

    }

    // Note: *both* must be fused to handle double-ended iterators.
    #[stable(feature = "fused", since = "1.26.0")]
    impl<A, B> FusedIterator for Chain<A, B>
        where A: FusedIterator,
              B: FusedIterator<Item=A::Item>,
    {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A, B> TrustedLen for Chain<A, B>
        where A: TrustedLen, B: TrustedLen<Item=A::Item>,
    {}