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use crate::iter::plumbing::*;
use crate::iter::*;
use std::fmt;
use std::marker::PhantomData;
/// Creates a parallel iterator that produces nothing.
///
/// This admits no parallelism on its own, but it could be used for code that
/// deals with generic parallel iterators.
///
/// # Examples
///
/// ```
/// use rayon::prelude::*;
/// use rayon::iter::empty;
///
/// let pi = (0..1234).into_par_iter()
/// .chain(empty())
/// .chain(1234..10_000);
///
/// assert_eq!(pi.count(), 10_000);
/// ```
pub fn empty<T: Send>() -> Empty<T> {
Empty {
marker: PhantomData,
}
}
/// Iterator adaptor for [the `empty()` function](fn.empty.html).
pub struct Empty<T: Send> {
marker: PhantomData<T>,
}
impl<T: Send> Clone for Empty<T> {
fn clone(&self) -> Self {
empty()
}
}
impl<T: Send> fmt::Debug for Empty<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("Empty")
}
}
impl<T: Send> ParallelIterator for Empty<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(0)
}
}
impl<T: Send> IndexedParallelIterator for Empty<T> {
fn drive<C>(self, consumer: C) -> C::Result
where
C: Consumer<Self::Item>,
{
consumer.into_folder().complete()
}
fn len(&self) -> usize {
0
}
fn with_producer<CB>(self, callback: CB) -> CB::Output
where
CB: ProducerCallback<Self::Item>,
{
callback.callback(EmptyProducer(PhantomData))
}
}
/// Private empty producer
struct EmptyProducer<T: Send>(PhantomData<T>);
impl<T: Send> Producer for EmptyProducer<T> {
type Item = T;
type IntoIter = std::iter::Empty<T>;
fn into_iter(self) -> Self::IntoIter {
std::iter::empty()
}
fn split_at(self, index: usize) -> (Self, Self) {
debug_assert_eq!(index, 0);
(self, EmptyProducer(PhantomData))
}
fn fold_with<F>(self, folder: F) -> F
where
F: Folder<Self::Item>,
{
folder
}
}