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    use super::plumbing::*;
    use super::*;
    use std::iter;
    use std::ops::Range;
    use std::usize;

    /// `Enumerate` is an iterator that returns the current count along with the element.
    /// This struct is created by the [`enumerate()`] method on [`IndexedParallelIterator`]
    ///
    /// [`enumerate()`]: trait.IndexedParallelIterator.html#method.enumerate
    /// [`IndexedParallelIterator`]: trait.IndexedParallelIterator.html
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[derive(Debug, Clone)]
    pub struct Enumerate<I: IndexedParallelIterator> {
        base: I,
    }

    impl<I> Enumerate<I>
    where
        I: IndexedParallelIterator,
    {
        /// Create a new `Enumerate` iterator.
        pub(super) fn new(base: I) -> Self {
            Enumerate { base }
        }
    }

    impl<I> ParallelIterator for Enumerate<I>
    where
        I: IndexedParallelIterator,
    {
        type Item = (usize, I::Item);

        fn drive_unindexed<C>(self, consumer: C) -> C::Result
        where
            C: UnindexedConsumer<Self::Item>,
        {
            bridge(self, consumer)
        }

        fn opt_len(&self) -> Option<usize> {
            Some(self.len())
        }
    }

    impl<I> IndexedParallelIterator for Enumerate<I>
    where
        I: IndexedParallelIterator,
    {
        fn drive<C: Consumer<Self::Item>>(self, consumer: C) -> C::Result {
            bridge(self, consumer)
        }

        fn len(&self) -> usize {
            self.base.len()
        }

        fn with_producer<CB>(self, callback: CB) -> CB::Output
        where
            CB: ProducerCallback<Self::Item>,
        {
            return self.base.with_producer(Callback { callback });

            struct Callback<CB> {
                callback: CB,
            }

            impl<I, CB> ProducerCallback<I> for Callback<CB>
            where
                CB: ProducerCallback<(usize, I)>,
            {
                type Output = CB::Output;
                fn callback<P>(self, base: P) -> CB::Output
                where
                    P: Producer<Item = I>,
                {
                    let producer = EnumerateProducer { base, offset: 0 };
                    self.callback.callback(producer)
                }
            }
        }
    }

    /// ////////////////////////////////////////////////////////////////////////
    /// Producer implementation

    struct EnumerateProducer<P> {
        base: P,
        offset: usize,
    }

    impl<P> Producer for EnumerateProducer<P>
    where
        P: Producer,
    {
        type Item = (usize, P::Item);
        type IntoIter = iter::Zip<Range<usize>, P::IntoIter>;

        fn into_iter(self) -> Self::IntoIter {
            // Enumerate only works for IndexedParallelIterators. Since those
            // have a max length of usize::MAX, their max index is
            // usize::MAX - 1, so the range 0..usize::MAX includes all
            // possible indices.
            //
            // However, we should to use a precise end to the range, otherwise
            // reversing the iterator may have to walk back a long ways before
            // `Zip::next_back` can produce anything.
            let base = self.base.into_iter();
            let end = self.offset + base.len();
            (self.offset..end).zip(base)
        }

        fn min_len(&self) -> usize {
            self.base.min_len()
        }
        fn max_len(&self) -> usize {
            self.base.max_len()
        }

        fn split_at(self, index: usize) -> (Self, Self) {
            let (left, right) = self.base.split_at(index);
            (
                EnumerateProducer {
                    base: left,
                    offset: self.offset,
                },
                EnumerateProducer {
                    base: right,
                    offset: self.offset + index,
                },
            )
        }
    }

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