rayon 0.8.2

use super::internal::*;
use super::*;
use std::cmp;
use std::iter;
use rayon_core::join;

/// `Chain` is an iterator that joins `b` after `a` in one continuous iterator.
/// This struct is created by the [`chain()`] method on [`ParallelIterator`]
///
/// [`chain()`]: trait.ParallelIterator.html#method.chain
/// [`ParallelIterator`]: trait.ParallelIterator.html
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
pub struct Chain<A, B>
    where A: ParallelIterator,
          B: ParallelIterator<Item = A::Item>
{
    a: A,
    b: B,
}

/// Create a new `Chain` iterator.
///
/// NB: a free fn because it is NOT part of the end-user API.
pub fn new<A, B>(a: A, b: B) -> Chain<A, B>
    where A: ParallelIterator,
          B: ParallelIterator<Item = A::Item>
{
    Chain { a: a, b: b }
}

impl<A, B> ParallelIterator for Chain<A, B>
    where A: ParallelIterator,
          B: ParallelIterator<Item = A::Item>
{
    type Item = A::Item;

    fn drive_unindexed<C>(self, consumer: C) -> C::Result
        where C: UnindexedConsumer<Self::Item>
    {
        let Chain { mut a, b } = self;

        // If we returned a value from our own `opt_len`, then the collect consumer in particular
        // will balk at being treated like an actual `UnindexedConsumer`.  But when we do know the
        // length, we can use `Consumer::split_at` instead, and this is still harmless for other
        // truly-unindexed consumers too.
        let (left, right, reducer) = if let Some(len) = a.opt_len() {
            consumer.split_at(len)
        } else {
            let reducer = consumer.to_reducer();
            (consumer.split_off_left(), consumer, reducer)
        };

        let (a, b) = join(|| a.drive_unindexed(left), || b.drive_unindexed(right));
        reducer.reduce(a, b)
    }

    fn opt_len(&mut self) -> Option<usize> {
        match (self.a.opt_len(), self.b.opt_len()) {
            (Some(a_len), Some(b_len)) => a_len.checked_add(b_len),
            _ => None,
        }
    }
}

impl<A, B> IndexedParallelIterator for Chain<A, B>
    where A: IndexedParallelIterator,
          B: IndexedParallelIterator<Item = A::Item>
{
    fn drive<C>(self, consumer: C) -> C::Result
        where C: Consumer<Self::Item>
    {
        let Chain { mut a, b } = self;
        let (left, right, reducer) = consumer.split_at(a.len());
        let (a, b) = join(|| a.drive(left), || b.drive(right));
        reducer.reduce(a, b)
    }

    fn len(&mut self) -> usize {
        self.a
            .len()
            .checked_add(self.b.len())
            .expect("overflow")
    }

    fn with_producer<CB>(mut self, callback: CB) -> CB::Output
        where CB: ProducerCallback<Self::Item>
    {
        let a_len = self.a.len();
        return self.a.with_producer(CallbackA {
                                        callback: callback,
                                        a_len: a_len,
                                        b: self.b,
                                    });

        struct CallbackA<CB, B> {
            callback: CB,
            a_len: usize,
            b: B,
        }

        impl<CB, B> ProducerCallback<B::Item> for CallbackA<CB, B>
            where B: IndexedParallelIterator,
                  CB: ProducerCallback<B::Item>
        {
            type Output = CB::Output;

            fn callback<A>(self, a_producer: A) -> Self::Output
                where A: Producer<Item = B::Item>
            {
                return self.b.with_producer(CallbackB {
                                                callback: self.callback,
                                                a_len: self.a_len,
                                                a_producer: a_producer,
                                            });
            }
        }

        struct CallbackB<CB, A> {
            callback: CB,
            a_len: usize,
            a_producer: A,
        }

        impl<CB, A> ProducerCallback<A::Item> for CallbackB<CB, A>
            where A: Producer,
                  CB: ProducerCallback<A::Item>
        {
            type Output = CB::Output;

            fn callback<B>(self, b_producer: B) -> Self::Output
                where B: Producer<Item = A::Item>
            {
                let producer = ChainProducer::new(self.a_len, self.a_producer, b_producer);
                self.callback.callback(producer)
            }
        }

    }
}

/// ////////////////////////////////////////////////////////////////////////

struct ChainProducer<A, B>
    where A: Producer,
          B: Producer<Item = A::Item>
{
    a_len: usize,
    a: A,
    b: B,
}

impl<A, B> ChainProducer<A, B>
    where A: Producer,
          B: Producer<Item = A::Item>
{
    fn new(a_len: usize, a: A, b: B) -> Self {
        ChainProducer {
            a_len: a_len,
            a: a,
            b: b,
        }
    }
}

impl<A, B> Producer for ChainProducer<A, B>
    where A: Producer,
          B: Producer<Item = A::Item>
{
    type Item = A::Item;
    type IntoIter = ChainSeq<A::IntoIter, B::IntoIter>;

    fn into_iter(self) -> Self::IntoIter {
        ChainSeq::new(self.a.into_iter(), self.b.into_iter())
    }

    fn min_len(&self) -> usize {
        cmp::max(self.a.min_len(), self.b.min_len())
    }

    fn max_len(&self) -> usize {
        cmp::min(self.a.max_len(), self.b.max_len())
    }

    fn split_at(self, index: usize) -> (Self, Self) {
        if index <= self.a_len {
            let a_rem = self.a_len - index;
            let (a_left, a_right) = self.a.split_at(index);
            let (b_left, b_right) = self.b.split_at(0);
            (ChainProducer::new(index, a_left, b_left), ChainProducer::new(a_rem, a_right, b_right))
        } else {
            let (a_left, a_right) = self.a.split_at(self.a_len);
            let (b_left, b_right) = self.b.split_at(index - self.a_len);
            (ChainProducer::new(self.a_len, a_left, b_left),
             ChainProducer::new(0, a_right, b_right))
        }
    }

    fn fold_with<F>(self, mut folder: F) -> F
        where F: Folder<A::Item>
    {
        folder = self.a.fold_with(folder);
        if folder.full() {
            folder
        } else {
            self.b.fold_with(folder)
        }
    }
}

/// ////////////////////////////////////////////////////////////////////////
/// Wrapper for Chain to implement ExactSizeIterator

struct ChainSeq<A, B> {
    chain: iter::Chain<A, B>,
}

impl<A, B> ChainSeq<A, B> {
    fn new(a: A, b: B) -> ChainSeq<A, B>
        where A: ExactSizeIterator,
              B: ExactSizeIterator<Item = A::Item>
    {
        ChainSeq { chain: a.chain(b) }
    }
}

impl<A, B> Iterator for ChainSeq<A, B>
    where A: Iterator,
          B: Iterator<Item = A::Item>
{
    type Item = A::Item;

    fn next(&mut self) -> Option<Self::Item> {
        self.chain.next()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.chain.size_hint()
    }
}

impl<A, B> ExactSizeIterator for ChainSeq<A, B>
    where A: ExactSizeIterator,
          B: ExactSizeIterator<Item = A::Item>
{
}

impl<A, B> DoubleEndedIterator for ChainSeq<A, B>
    where A: DoubleEndedIterator,
          B: DoubleEndedIterator<Item = A::Item>
{
    fn next_back(&mut self) -> Option<Self::Item> {
        self.chain.next_back()
    }
}