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use crate::iter::plumbing::*;
use crate::iter::*;
/// Creates a parallel iterator that produces an element exactly once.
///
/// This admits no parallelism on its own, but it could be chained to existing
/// parallel iterators to extend their contents, or otherwise used for any code
/// that deals with generic parallel iterators.
///
/// # Examples
///
/// ```
/// use rayon::prelude::*;
/// use rayon::iter::once;
///
/// let pi = (0..1234).into_par_iter()
/// .chain(once(-1))
/// .chain(1234..10_000);
///
/// assert_eq!(pi.clone().count(), 10_001);
/// assert_eq!(pi.clone().filter(|&x| x < 0).count(), 1);
/// assert_eq!(pi.position_any(|x| x < 0), Some(1234));
/// ```
pub fn once<T: Send>(item: T) -> Once<T> {
Once { item }
}
/// Iterator adaptor for [the `once()` function](fn.once.html).
#[derive(Clone, Debug)]
pub struct Once<T: Send> {
item: T,
}
impl<T: Send> ParallelIterator for Once<T> {
type Item = T;
fn drive_unindexed<C>(self, consumer: C) -> C::Result
where
C: UnindexedConsumer<Self::Item>,
{
self.drive(consumer)
}
fn opt_len(&self) -> Option<usize> {
Some(1)
}
}
impl<T: Send> IndexedParallelIterator for Once<T> {
fn drive<C>(self, consumer: C) -> C::Result
where
C: Consumer<Self::Item>,
{
consumer.into_folder().consume(self.item).complete()
}
fn len(&self) -> usize {
1
}
fn with_producer<CB>(self, callback: CB) -> CB::Output
where
CB: ProducerCallback<Self::Item>,
{
// Let `OptionProducer` handle it.
Some(self.item).into_par_iter().with_producer(callback)
}
}