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    // ignore-tidy-undocumented-unsafe

    use crate::cmp;

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

    /// An iterator that iterates two other iterators simultaneously.
    ///
    /// This `struct` is created by the [`zip`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`zip`]: trait.Iterator.html#method.zip
    /// [`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 Zip<A, B> {
        a: A,
        b: B,
        // index and len are only used by the specialized version of zip
        index: usize,
        len: usize,
    }
    impl<A: Iterator, B: Iterator> Zip<A, B> {
        pub(in super::super) fn new(a: A, b: B) -> Zip<A, B> {
            ZipImpl::new(a, b)
        }
        fn super_nth(&mut self, mut n: usize) -> Option<(A::Item, B::Item)> {
            while let Some(x) = Iterator::next(self) {
                if n == 0 { return Some(x) }
                n -= 1;
            }
            None
        }
    }

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

        #[inline]
        fn next(&mut self) -> Option<Self::Item> {
            ZipImpl::next(self)
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            ZipImpl::size_hint(self)
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<Self::Item> {
            ZipImpl::nth(self, n)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> DoubleEndedIterator for Zip<A, B> where
        A: DoubleEndedIterator + ExactSizeIterator,
        B: DoubleEndedIterator + ExactSizeIterator,
    {
        #[inline]
        fn next_back(&mut self) -> Option<(A::Item, B::Item)> {
            ZipImpl::next_back(self)
        }
    }

    // Zip specialization trait
    #[doc(hidden)]
    trait ZipImpl<A, B> {
        type Item;
        fn new(a: A, b: B) -> Self;
        fn next(&mut self) -> Option<Self::Item>;
        fn size_hint(&self) -> (usize, Option<usize>);
        fn nth(&mut self, n: usize) -> Option<Self::Item>;
        fn next_back(&mut self) -> Option<Self::Item>
            where A: DoubleEndedIterator + ExactSizeIterator,
                  B: DoubleEndedIterator + ExactSizeIterator;
    }

    // General Zip impl
    #[doc(hidden)]
    impl<A, B> ZipImpl<A, B> for Zip<A, B>
        where A: Iterator, B: Iterator
    {
        type Item = (A::Item, B::Item);
        default fn new(a: A, b: B) -> Self {
            Zip {
                a,
                b,
                index: 0, // unused
                len: 0, // unused
            }
        }

        #[inline]
        default fn next(&mut self) -> Option<(A::Item, B::Item)> {
            let x = self.a.next()?;
            let y = self.b.next()?;
            Some((x, y))
        }

        #[inline]
        default fn nth(&mut self, n: usize) -> Option<Self::Item> {
            self.super_nth(n)
        }

        #[inline]
        default fn next_back(&mut self) -> Option<(A::Item, B::Item)>
            where A: DoubleEndedIterator + ExactSizeIterator,
                  B: DoubleEndedIterator + ExactSizeIterator
        {
            let a_sz = self.a.len();
            let b_sz = self.b.len();
            if a_sz != b_sz {
                // Adjust a, b to equal length
                if a_sz > b_sz {
                    for _ in 0..a_sz - b_sz { self.a.next_back(); }
                } else {
                    for _ in 0..b_sz - a_sz { self.b.next_back(); }
                }
            }
            match (self.a.next_back(), self.b.next_back()) {
                (Some(x), Some(y)) => Some((x, y)),
                (None, None) => None,
                _ => unreachable!(),
            }
        }

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

            let lower = cmp::min(a_lower, b_lower);

            let upper = match (a_upper, b_upper) {
                (Some(x), Some(y)) => Some(cmp::min(x,y)),
                (Some(x), None) => Some(x),
                (None, Some(y)) => Some(y),
                (None, None) => None
            };

            (lower, upper)
        }
    }

    #[doc(hidden)]
    impl<A, B> ZipImpl<A, B> for Zip<A, B>
        where A: TrustedRandomAccess, B: TrustedRandomAccess
    {
        fn new(a: A, b: B) -> Self {
            let len = cmp::min(a.len(), b.len());
            Zip {
                a,
                b,
                index: 0,
                len,
            }
        }

        #[inline]
        fn next(&mut self) -> Option<(A::Item, B::Item)> {
            if self.index < self.len {
                let i = self.index;
                self.index += 1;
                unsafe {
                    Some((self.a.get_unchecked(i), self.b.get_unchecked(i)))
                }
            } else if A::may_have_side_effect() && self.index < self.a.len() {
                // match the base implementation's potential side effects
                unsafe {
                    self.a.get_unchecked(self.index);
                }
                self.index += 1;
                None
            } else {
                None
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let len = self.len - self.index;
            (len, Some(len))
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<Self::Item> {
            let delta = cmp::min(n, self.len - self.index);
            let end = self.index + delta;
            while self.index < end {
                let i = self.index;
                self.index += 1;
                if A::may_have_side_effect() {
                    unsafe { self.a.get_unchecked(i); }
                }
                if B::may_have_side_effect() {
                    unsafe { self.b.get_unchecked(i); }
                }
            }

            self.super_nth(n - delta)
        }

        #[inline]
        fn next_back(&mut self) -> Option<(A::Item, B::Item)>
            where A: DoubleEndedIterator + ExactSizeIterator,
                  B: DoubleEndedIterator + ExactSizeIterator
        {
            // Adjust a, b to equal length
            if A::may_have_side_effect() {
                let sz = self.a.len();
                if sz > self.len {
                    for _ in 0..sz - cmp::max(self.len, self.index) {
                        self.a.next_back();
                    }
                }
            }
            if B::may_have_side_effect() {
                let sz = self.b.len();
                if sz > self.len {
                    for _ in 0..sz - self.len {
                        self.b.next_back();
                    }
                }
            }
            if self.index < self.len {
                self.len -= 1;
                let i = self.len;
                unsafe {
                    Some((self.a.get_unchecked(i), self.b.get_unchecked(i)))
                }
            } else {
                None
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> ExactSizeIterator for Zip<A, B>
        where A: ExactSizeIterator, B: ExactSizeIterator {}

    #[doc(hidden)]
    unsafe impl<A, B> TrustedRandomAccess for Zip<A, B>
        where A: TrustedRandomAccess,
              B: TrustedRandomAccess,
    {
        unsafe fn get_unchecked(&mut self, i: usize) -> (A::Item, B::Item) {
            (self.a.get_unchecked(i), self.b.get_unchecked(i))
        }

        fn may_have_side_effect() -> bool {
            A::may_have_side_effect() || B::may_have_side_effect()
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<A, B> FusedIterator for Zip<A, B>
        where A: FusedIterator, B: FusedIterator, {}

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

    /// An iterator whose items are random-accessible efficiently
    ///
    /// # Safety
    ///
    /// The iterator's .len() and size_hint() must be exact.
    /// `.len()` must be cheap to call.
    ///
    /// .get_unchecked() must return distinct mutable references for distinct
    /// indices (if applicable), and must return a valid reference if index is in
    /// 0..self.len().
    pub(crate) unsafe trait TrustedRandomAccess : ExactSizeIterator {
        unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item;
        /// Returns `true` if getting an iterator element may have
        /// side effects. Remember to take inner iterators into account.
        fn may_have_side_effect() -> bool;
    }