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

    use std::fmt::{self, Debug};

    /// `FlatMap` maps each element to an iterator, then flattens these iterators together.
    /// This struct is created by the [`flat_map()`] method on [`ParallelIterator`]
    ///
    /// [`flat_map()`]: trait.ParallelIterator.html#method.flat_map
    /// [`ParallelIterator`]: trait.ParallelIterator.html
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[derive(Clone)]
    pub struct FlatMap<I: ParallelIterator, F> {
        base: I,
        map_op: F,
    }

    impl<I: ParallelIterator + Debug, F> Debug for FlatMap<I, F> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("FlatMap").field("base", &self.base).finish()
        }
    }

    impl<I: ParallelIterator, F> FlatMap<I, F> {
        /// Create a new `FlatMap` iterator.
        pub(super) fn new(base: I, map_op: F) -> Self {
            FlatMap { base, map_op }
        }
    }

    impl<I, F, PI> ParallelIterator for FlatMap<I, F>
    where
        I: ParallelIterator,
        F: Fn(I::Item) -> PI + Sync + Send,
        PI: IntoParallelIterator,
    {
        type Item = PI::Item;

        fn drive_unindexed<C>(self, consumer: C) -> C::Result
        where
            C: UnindexedConsumer<Self::Item>,
        {
            let consumer = FlatMapConsumer {
                base: consumer,
                map_op: &self.map_op,
            };
            self.base.drive_unindexed(consumer)
        }
    }

    /// ////////////////////////////////////////////////////////////////////////
    /// Consumer implementation

    struct FlatMapConsumer<'f, C, F: 'f> {
        base: C,
        map_op: &'f F,
    }

    impl<'f, C, F> FlatMapConsumer<'f, C, F> {
        fn new(base: C, map_op: &'f F) -> Self {
            FlatMapConsumer { base, map_op }
        }
    }

    impl<'f, T, U, C, F> Consumer<T> for FlatMapConsumer<'f, C, F>
    where
        C: UnindexedConsumer<U::Item>,
        F: Fn(T) -> U + Sync,
        U: IntoParallelIterator,
    {
        type Folder = FlatMapFolder<'f, C, F, C::Result>;
        type Reducer = C::Reducer;
        type Result = C::Result;

        fn split_at(self, index: usize) -> (Self, Self, C::Reducer) {
            let (left, right, reducer) = self.base.split_at(index);
            (
                FlatMapConsumer::new(left, self.map_op),
                FlatMapConsumer::new(right, self.map_op),
                reducer,
            )
        }

        fn into_folder(self) -> Self::Folder {
            FlatMapFolder {
                base: self.base,
                map_op: self.map_op,
                previous: None,
            }
        }

        fn full(&self) -> bool {
            self.base.full()
        }
    }

    impl<'f, T, U, C, F> UnindexedConsumer<T> for FlatMapConsumer<'f, C, F>
    where
        C: UnindexedConsumer<U::Item>,
        F: Fn(T) -> U + Sync,
        U: IntoParallelIterator,
    {
        fn split_off_left(&self) -> Self {
            FlatMapConsumer::new(self.base.split_off_left(), self.map_op)
        }

        fn to_reducer(&self) -> Self::Reducer {
            self.base.to_reducer()
        }
    }

    struct FlatMapFolder<'f, C, F: 'f, R> {
        base: C,
        map_op: &'f F,
        previous: Option<R>,
    }

    impl<'f, T, U, C, F> Folder<T> for FlatMapFolder<'f, C, F, C::Result>
    where
        C: UnindexedConsumer<U::Item>,
        F: Fn(T) -> U + Sync,
        U: IntoParallelIterator,
    {
        type Result = C::Result;

        fn consume(self, item: T) -> Self {
            let map_op = self.map_op;
            let par_iter = map_op(item).into_par_iter();
            let result = par_iter.drive_unindexed(self.base.split_off_left());

            // We expect that `previous` is `None`, because we drive
            // the cost up so high, but just in case.
            let previous = match self.previous {
                None => Some(result),
                Some(previous) => {
                    let reducer = self.base.to_reducer();
                    Some(reducer.reduce(previous, result))
                }
            };

            FlatMapFolder {
                base: self.base,
                map_op,
                previous,
            }
        }

        fn complete(self) -> Self::Result {
            match self.previous {
                Some(previous) => previous,
                None => self.base.into_folder().complete(),
            }
        }

        fn full(&self) -> bool {
            self.base.full()
        }
    }

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