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    use super::plumbing::*;
    use super::*;
    use std::cell::Cell;
    use std::sync::atomic::{AtomicUsize, Ordering};

    #[cfg(test)]
    mod test;

    // The key optimization for find_first is that a consumer can stop its search if
    // some consumer to its left already found a match (and similarly for consumers
    // to the right for find_last). To make this work, all consumers need some
    // notion of their position in the data relative to other consumers, including
    // unindexed consumers that have no built-in notion of position.
    //
    // To solve this, we assign each consumer a lower and upper bound for an
    // imaginary "range" of data that it consumes. The initial consumer starts with
    // the range 0..usize::max_value(). The split divides this range in half so that
    // one resulting consumer has the range 0..(usize::max_value() / 2), and the
    // other has (usize::max_value() / 2)..usize::max_value(). Every subsequent
    // split divides the range in half again until it cannot be split anymore
    // (i.e. its length is 1), in which case the split returns two consumers with
    // the same range. In that case both consumers will continue to consume all
    // their data regardless of whether a better match is found, but the reducer
    // will still return the correct answer.

    #[derive(Copy, Clone)]
    enum MatchPosition {
        Leftmost,
        Rightmost,
    }

    /// Returns true if pos1 is a better match than pos2 according to MatchPosition
    #[inline]
    fn better_position(pos1: usize, pos2: usize, mp: MatchPosition) -> bool {
        match mp {
            MatchPosition::Leftmost => pos1 < pos2,
            MatchPosition::Rightmost => pos1 > pos2,
        }
    }

    pub(super) fn find_first<I, P>(pi: I, find_op: P) -> Option<I::Item>
    where
        I: ParallelIterator,
        P: Fn(&I::Item) -> bool + Sync,
    {
        let best_found = AtomicUsize::new(usize::max_value());
        let consumer = FindConsumer::new(&find_op, MatchPosition::Leftmost, &best_found);
        pi.drive_unindexed(consumer)
    }

    pub(super) fn find_last<I, P>(pi: I, find_op: P) -> Option<I::Item>
    where
        I: ParallelIterator,
        P: Fn(&I::Item) -> bool + Sync,
    {
        let best_found = AtomicUsize::new(0);
        let consumer = FindConsumer::new(&find_op, MatchPosition::Rightmost, &best_found);
        pi.drive_unindexed(consumer)
    }

    struct FindConsumer<'p, P: 'p> {
        find_op: &'p P,
        lower_bound: Cell<usize>,
        upper_bound: usize,
        match_position: MatchPosition,
        best_found: &'p AtomicUsize,
    }

    impl<'p, P> FindConsumer<'p, P> {
        fn new(find_op: &'p P, match_position: MatchPosition, best_found: &'p AtomicUsize) -> Self {
            FindConsumer {
                find_op,
                lower_bound: Cell::new(0),
                upper_bound: usize::max_value(),
                match_position,
                best_found,
            }
        }

        fn current_index(&self) -> usize {
            match self.match_position {
                MatchPosition::Leftmost => self.lower_bound.get(),
                MatchPosition::Rightmost => self.upper_bound,
            }
        }
    }

    impl<'p, T, P> Consumer<T> for FindConsumer<'p, P>
    where
        T: Send,
        P: Fn(&T) -> bool + Sync,
    {
        type Folder = FindFolder<'p, T, P>;
        type Reducer = FindReducer;
        type Result = Option<T>;

        fn split_at(self, _index: usize) -> (Self, Self, Self::Reducer) {
            let dir = self.match_position;
            (
                self.split_off_left(),
                self,
                FindReducer {
                    match_position: dir,
                },
            )
        }

        fn into_folder(self) -> Self::Folder {
            FindFolder {
                find_op: self.find_op,
                boundary: self.current_index(),
                match_position: self.match_position,
                best_found: self.best_found,
                item: None,
            }
        }

        fn full(&self) -> bool {
            // can stop consuming if the best found index so far is *strictly*
            // better than anything this consumer will find
            better_position(
                self.best_found.load(Ordering::Relaxed),
                self.current_index(),
                self.match_position,
            )
        }
    }

    impl<'p, T, P> UnindexedConsumer<T> for FindConsumer<'p, P>
    where
        T: Send,
        P: Fn(&T) -> bool + Sync,
    {
        fn split_off_left(&self) -> Self {
            // Upper bound for one consumer will be lower bound for the other. This
            // overlap is okay, because only one of the bounds will be used for
            // comparing against best_found; the other is kept only to be able to
            // divide the range in half.
            //
            // When the resolution of usize has been exhausted (i.e. when
            // upper_bound = lower_bound), both results of this split will have the
            // same range. When that happens, we lose the ability to tell one
            // consumer to stop working when the other finds a better match, but the
            // reducer ensures that the best answer is still returned (see the test
            // above).
            let old_lower_bound = self.lower_bound.get();
            let median = old_lower_bound + ((self.upper_bound - old_lower_bound) / 2);
            self.lower_bound.set(median);

            FindConsumer {
                find_op: self.find_op,
                lower_bound: Cell::new(old_lower_bound),
                upper_bound: median,
                match_position: self.match_position,
                best_found: self.best_found,
            }
        }

        fn to_reducer(&self) -> Self::Reducer {
            FindReducer {
                match_position: self.match_position,
            }
        }
    }

    struct FindFolder<'p, T, P: 'p> {
        find_op: &'p P,
        boundary: usize,
        match_position: MatchPosition,
        best_found: &'p AtomicUsize,
        item: Option<T>,
    }

    impl<'p, P: 'p + Fn(&T) -> bool, T> Folder<T> for FindFolder<'p, T, P> {
        type Result = Option<T>;

        fn consume(mut self, item: T) -> Self {
            let found_best_in_range = match self.match_position {
                MatchPosition::Leftmost => self.item.is_some(),
                MatchPosition::Rightmost => false,
            };

            if !found_best_in_range && (self.find_op)(&item) {
                // Continuously try to set best_found until we succeed or we
                // discover a better match was already found.
                let mut current = self.best_found.load(Ordering::Relaxed);
                loop {
                    if better_position(current, self.boundary, self.match_position) {
                        break;
                    }
                    match self.best_found.compare_exchange_weak(
                        current,
                        self.boundary,
                        Ordering::Relaxed,
                        Ordering::Relaxed,
                    ) {
                        Ok(_) => {
                            self.item = Some(item);
                            break;
                        }
                        Err(v) => current = v,
                    }
                }
            }
            self
        }

        fn complete(self) -> Self::Result {
            self.item
        }

        fn full(&self) -> bool {
            let found_best_in_range = match self.match_position {
                MatchPosition::Leftmost => self.item.is_some(),
                MatchPosition::Rightmost => false,
            };

            found_best_in_range
                || better_position(
                    self.best_found.load(Ordering::Relaxed),
                    self.boundary,
                    self.match_position,
                )
        }
    }

    struct FindReducer {
        match_position: MatchPosition,
    }

    impl<T> Reducer<Option<T>> for FindReducer {
        fn reduce(self, left: Option<T>, right: Option<T>) -> Option<T> {
            match self.match_position {
                MatchPosition::Leftmost => left.or(right),
                MatchPosition::Rightmost => right.or(left),
            }
        }
    }

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