1 files changed, 68 insertions, 0 deletions
diff --git a/src/iter/once.rs b/src/iter/once.rs
new file mode 100644
index 0000000..5140b6b
--- /dev/null
+++ b/src/iter/once.rs
@@ -0,0 +1,68 @@
+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)
+    }
+}