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    use super::plumbing::*;
    use super::*;

    /// This trait abstracts the different ways we can "unzip" one parallel
    /// iterator into two distinct consumers, which we can handle almost
    /// identically apart from how to process the individual items.
    trait UnzipOp<T>: Sync + Send {
        /// The type of item expected by the left consumer.
        type Left: Send;

        /// The type of item expected by the right consumer.
        type Right: Send;

        /// Consume one item and feed it to one or both of the underlying folders.
        fn consume<FA, FB>(&self, item: T, left: FA, right: FB) -> (FA, FB)
        where
            FA: Folder<Self::Left>,
            FB: Folder<Self::Right>;

        /// Reports whether this op may support indexed consumers.
        /// - e.g. true for `unzip` where the item count passed through directly.
        /// - e.g. false for `partition` where the sorting is not yet known.
        fn indexable() -> bool {
            false
        }
    }

    /// Run an unzip-like operation into default `ParallelExtend` collections.
    fn execute<I, OP, FromA, FromB>(pi: I, op: OP) -> (FromA, FromB)
    where
        I: ParallelIterator,
        OP: UnzipOp<I::Item>,
        FromA: Default + Send + ParallelExtend<OP::Left>,
        FromB: Default + Send + ParallelExtend<OP::Right>,
    {
        let mut a = FromA::default();
        let mut b = FromB::default();
        execute_into(&mut a, &mut b, pi, op);
        (a, b)
    }

    /// Run an unzip-like operation into `ParallelExtend` collections.
    fn execute_into<I, OP, FromA, FromB>(a: &mut FromA, b: &mut FromB, pi: I, op: OP)
    where
        I: ParallelIterator,
        OP: UnzipOp<I::Item>,
        FromA: Send + ParallelExtend<OP::Left>,
        FromB: Send + ParallelExtend<OP::Right>,
    {
        // We have no idea what the consumers will look like for these
        // collections' `par_extend`, but we can intercept them in our own
        // `drive_unindexed`.  Start with the left side, type `A`:
        let iter = UnzipA { base: pi, op, b };
        a.par_extend(iter);
    }

    /// Unzips the items of a parallel iterator into a pair of arbitrary
    /// `ParallelExtend` containers.
    ///
    /// This is called by `ParallelIterator::unzip`.
    pub(super) fn unzip<I, A, B, FromA, FromB>(pi: I) -> (FromA, FromB)
    where
        I: ParallelIterator<Item = (A, B)>,
        FromA: Default + Send + ParallelExtend<A>,
        FromB: Default + Send + ParallelExtend<B>,
        A: Send,
        B: Send,
    {
        execute(pi, Unzip)
    }

    /// Unzip an `IndexedParallelIterator` into two arbitrary `Consumer`s.
    ///
    /// This is called by `super::collect::unzip_into_vecs`.
    pub(super) fn unzip_indexed<I, A, B, CA, CB>(pi: I, left: CA, right: CB) -> (CA::Result, CB::Result)
    where
        I: IndexedParallelIterator<Item = (A, B)>,
        CA: Consumer<A>,
        CB: Consumer<B>,
        A: Send,
        B: Send,
    {
        let consumer = UnzipConsumer {
            op: &Unzip,
            left,
            right,
        };
        pi.drive(consumer)
    }

    /// An `UnzipOp` that splits a tuple directly into the two consumers.
    struct Unzip;

    impl<A: Send, B: Send> UnzipOp<(A, B)> for Unzip {
        type Left = A;
        type Right = B;

        fn consume<FA, FB>(&self, item: (A, B), left: FA, right: FB) -> (FA, FB)
        where
            FA: Folder<A>,
            FB: Folder<B>,
        {
            (left.consume(item.0), right.consume(item.1))
        }

        fn indexable() -> bool {
            true
        }
    }

    /// Partitions the items of a parallel iterator into a pair of arbitrary
    /// `ParallelExtend` containers.
    ///
    /// This is called by `ParallelIterator::partition`.
    pub(super) fn partition<I, A, B, P>(pi: I, predicate: P) -> (A, B)
    where
        I: ParallelIterator,
        A: Default + Send + ParallelExtend<I::Item>,
        B: Default + Send + ParallelExtend<I::Item>,
        P: Fn(&I::Item) -> bool + Sync + Send,
    {
        execute(pi, Partition { predicate })
    }

    /// An `UnzipOp` that routes items depending on a predicate function.
    struct Partition<P> {
        predicate: P,
    }

    impl<P, T> UnzipOp<T> for Partition<P>
    where
        P: Fn(&T) -> bool + Sync + Send,
        T: Send,
    {
        type Left = T;
        type Right = T;

        fn consume<FA, FB>(&self, item: T, left: FA, right: FB) -> (FA, FB)
        where
            FA: Folder<T>,
            FB: Folder<T>,
        {
            if (self.predicate)(&item) {
                (left.consume(item), right)
            } else {
                (left, right.consume(item))
            }
        }
    }

    /// Partitions and maps the items of a parallel iterator into a pair of
    /// arbitrary `ParallelExtend` containers.
    ///
    /// This called by `ParallelIterator::partition_map`.
    pub(super) fn partition_map<I, A, B, P, L, R>(pi: I, predicate: P) -> (A, B)
    where
        I: ParallelIterator,
        A: Default + Send + ParallelExtend<L>,
        B: Default + Send + ParallelExtend<R>,
        P: Fn(I::Item) -> Either<L, R> + Sync + Send,
        L: Send,
        R: Send,
    {
        execute(pi, PartitionMap { predicate })
    }

    /// An `UnzipOp` that routes items depending on how they are mapped `Either`.
    struct PartitionMap<P> {
        predicate: P,
    }

    impl<P, L, R, T> UnzipOp<T> for PartitionMap<P>
    where
        P: Fn(T) -> Either<L, R> + Sync + Send,
        L: Send,
        R: Send,
    {
        type Left = L;
        type Right = R;

        fn consume<FA, FB>(&self, item: T, left: FA, right: FB) -> (FA, FB)
        where
            FA: Folder<L>,
            FB: Folder<R>,
        {
            match (self.predicate)(item) {
                Either::Left(item) => (left.consume(item), right),
                Either::Right(item) => (left, right.consume(item)),
            }
        }
    }

    /// A fake iterator to intercept the `Consumer` for type `A`.
    struct UnzipA<'b, I, OP, FromB: 'b> {
        base: I,
        op: OP,
        b: &'b mut FromB,
    }

    impl<'b, I, OP, FromB> ParallelIterator for UnzipA<'b, I, OP, FromB>
    where
        I: ParallelIterator,
        OP: UnzipOp<I::Item>,
        FromB: Send + ParallelExtend<OP::Right>,
    {
        type Item = OP::Left;

        fn drive_unindexed<C>(self, consumer: C) -> C::Result
        where
            C: UnindexedConsumer<Self::Item>,
        {
            let mut result = None;
            {
                // Now it's time to find the consumer for type `B`
                let iter = UnzipB {
                    base: self.base,
                    op: self.op,
                    left_consumer: consumer,
                    left_result: &mut result,
                };
                self.b.par_extend(iter);
            }
            // NB: If for some reason `b.par_extend` doesn't actually drive the
            // iterator, then we won't have a result for the left side to return
            // at all.  We can't fake an arbitrary consumer's result, so panic.
            result.expect("unzip consumers didn't execute!")
        }

        fn opt_len(&self) -> Option<usize> {
            if OP::indexable() {
                self.base.opt_len()
            } else {
                None
            }
        }
    }

    /// A fake iterator to intercept the `Consumer` for type `B`.
    struct UnzipB<'r, I, OP, CA>
    where
        I: ParallelIterator,
        OP: UnzipOp<I::Item>,
        CA: UnindexedConsumer<OP::Left>,
        CA::Result: 'r,
    {
        base: I,
        op: OP,
        left_consumer: CA,
        left_result: &'r mut Option<CA::Result>,
    }

    impl<'r, I, OP, CA> ParallelIterator for UnzipB<'r, I, OP, CA>
    where
        I: ParallelIterator,
        OP: UnzipOp<I::Item>,
        CA: UnindexedConsumer<OP::Left>,
    {
        type Item = OP::Right;

        fn drive_unindexed<C>(self, consumer: C) -> C::Result
        where
            C: UnindexedConsumer<Self::Item>,
        {
            // Now that we have two consumers, we can unzip the real iterator.
            let consumer = UnzipConsumer {
                op: &self.op,
                left: self.left_consumer,
                right: consumer,
            };

            let result = self.base.drive_unindexed(consumer);
            *self.left_result = Some(result.0);
            result.1
        }

        fn opt_len(&self) -> Option<usize> {
            if OP::indexable() {
                self.base.opt_len()
            } else {
                None
            }
        }
    }

    /// `Consumer` that unzips into two other `Consumer`s
    struct UnzipConsumer<'a, OP: 'a, CA, CB> {
        op: &'a OP,
        left: CA,
        right: CB,
    }

    impl<'a, T, OP, CA, CB> Consumer<T> for UnzipConsumer<'a, OP, CA, CB>
    where
        OP: UnzipOp<T>,
        CA: Consumer<OP::Left>,
        CB: Consumer<OP::Right>,
    {
        type Folder = UnzipFolder<'a, OP, CA::Folder, CB::Folder>;
        type Reducer = UnzipReducer<CA::Reducer, CB::Reducer>;
        type Result = (CA::Result, CB::Result);

        fn split_at(self, index: usize) -> (Self, Self, Self::Reducer) {
            let (left1, left2, left_reducer) = self.left.split_at(index);
            let (right1, right2, right_reducer) = self.right.split_at(index);

            (
                UnzipConsumer {
                    op: self.op,
                    left: left1,
                    right: right1,
                },
                UnzipConsumer {
                    op: self.op,
                    left: left2,
                    right: right2,
                },
                UnzipReducer {
                    left: left_reducer,
                    right: right_reducer,
                },
            )
        }

        fn into_folder(self) -> Self::Folder {
            UnzipFolder {
                op: self.op,
                left: self.left.into_folder(),
                right: self.right.into_folder(),
            }
        }

        fn full(&self) -> bool {
            // don't stop until everyone is full
            self.left.full() && self.right.full()
        }
    }

    impl<'a, T, OP, CA, CB> UnindexedConsumer<T> for UnzipConsumer<'a, OP, CA, CB>
    where
        OP: UnzipOp<T>,
        CA: UnindexedConsumer<OP::Left>,
        CB: UnindexedConsumer<OP::Right>,
    {
        fn split_off_left(&self) -> Self {
            UnzipConsumer {
                op: self.op,
                left: self.left.split_off_left(),
                right: self.right.split_off_left(),
            }
        }

        fn to_reducer(&self) -> Self::Reducer {
            UnzipReducer {
                left: self.left.to_reducer(),
                right: self.right.to_reducer(),
            }
        }
    }

    /// `Folder` that unzips into two other `Folder`s
    struct UnzipFolder<'a, OP: 'a, FA, FB> {
        op: &'a OP,
        left: FA,
        right: FB,
    }

    impl<'a, T, OP, FA, FB> Folder<T> for UnzipFolder<'a, OP, FA, FB>
    where
        OP: UnzipOp<T>,
        FA: Folder<OP::Left>,
        FB: Folder<OP::Right>,
    {
        type Result = (FA::Result, FB::Result);

        fn consume(self, item: T) -> Self {
            let (left, right) = self.op.consume(item, self.left, self.right);
            UnzipFolder {
                op: self.op,
                left,
                right,
            }
        }

        fn complete(self) -> Self::Result {
            (self.left.complete(), self.right.complete())
        }

        fn full(&self) -> bool {
            // don't stop until everyone is full
            self.left.full() && self.right.full()
        }
    }

    /// `Reducer` that unzips into two other `Reducer`s
    struct UnzipReducer<RA, RB> {
        left: RA,
        right: RB,
    }

    impl<A, B, RA, RB> Reducer<(A, B)> for UnzipReducer<RA, RB>
    where
        RA: Reducer<A>,
        RB: Reducer<B>,
    {
        fn reduce(self, left: (A, B), right: (A, B)) -> (A, B) {
            (
                self.left.reduce(left.0, right.0),
                self.right.reduce(left.1, right.1),
            )
        }
    }

    impl<A, B, FromA, FromB> ParallelExtend<(A, B)> for (FromA, FromB)
    where
        A: Send,
        B: Send,
        FromA: Send + ParallelExtend<A>,
        FromB: Send + ParallelExtend<B>,
    {
        fn par_extend<I>(&mut self, pi: I)
        where
            I: IntoParallelIterator<Item = (A, B)>,
        {
            execute_into(&mut self.0, &mut self.1, pi.into_par_iter(), Unzip);
        }
    }

    impl<L, R, A, B> ParallelExtend<Either<L, R>> for (A, B)
    where
        L: Send,
        R: Send,
        A: Send + ParallelExtend<L>,
        B: Send + ParallelExtend<R>,
    {
        fn par_extend<I>(&mut self, pi: I)
        where
            I: IntoParallelIterator<Item = Either<L, R>>,
        {
            execute_into(&mut self.0, &mut self.1, pi.into_par_iter(), UnEither);
        }
    }

    /// An `UnzipOp` that routes items depending on their `Either` variant.
    struct UnEither;

    impl<L, R> UnzipOp<Either<L, R>> for UnEither
    where
        L: Send,
        R: Send,
    {
        type Left = L;
        type Right = R;

        fn consume<FL, FR>(&self, item: Either<L, R>, left: FL, right: FR) -> (FL, FR)
        where
            FL: Folder<L>,
            FR: Folder<R>,
        {
            match item {
                Either::Left(item) => (left.consume(item), right),
                Either::Right(item) => (left, right.consume(item)),
            }
        }
    }

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