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use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use std::sync::Mutex;
use crate::iter::plumbing::{bridge_unindexed, Folder, UnindexedConsumer, UnindexedProducer};
use crate::iter::ParallelIterator;
use crate::{current_num_threads, current_thread_index};
/// Conversion trait to convert an `Iterator` to a `ParallelIterator`.
///
/// This creates a "bridge" from a sequential iterator to a parallel one, by distributing its items
/// across the Rayon thread pool. This has the advantage of being able to parallelize just about
/// anything, but the resulting `ParallelIterator` can be less efficient than if you started with
/// `par_iter` instead. However, it can still be useful for iterators that are difficult to
/// parallelize by other means, like channels or file or network I/O.
///
/// Iterator items are pulled by `next()` one at a time, synchronized from each thread that is
/// ready for work, so this may become a bottleneck if the serial iterator can't keep up with the
/// parallel demand. The items are not buffered by `IterBridge`, so it's fine to use this with
/// large or even unbounded iterators.
///
/// The resulting iterator is not guaranteed to keep the order of the original iterator.
///
/// # Examples
///
/// To use this trait, take an existing `Iterator` and call `par_bridge` on it. After that, you can
/// use any of the `ParallelIterator` methods:
///
/// ```
/// use rayon::iter::ParallelBridge;
/// use rayon::prelude::ParallelIterator;
/// use std::sync::mpsc::channel;
///
/// let rx = {
/// let (tx, rx) = channel();
///
/// tx.send("one!");
/// tx.send("two!");
/// tx.send("three!");
///
/// rx
/// };
///
/// let mut output: Vec<&'static str> = rx.into_iter().par_bridge().collect();
/// output.sort_unstable();
///
/// assert_eq!(&*output, &["one!", "three!", "two!"]);
/// ```
pub trait ParallelBridge: Sized {
/// Creates a bridge from this type to a `ParallelIterator`.
fn par_bridge(self) -> IterBridge<Self>;
}
impl<T: Iterator + Send> ParallelBridge for T
where
T::Item: Send,
{
fn par_bridge(self) -> IterBridge<Self> {
IterBridge { iter: self }
}
}
/// `IterBridge` is a parallel iterator that wraps a sequential iterator.
///
/// This type is created when using the `par_bridge` method on `ParallelBridge`. See the
/// [`ParallelBridge`] documentation for details.
///
/// [`ParallelBridge`]: trait.ParallelBridge.html
#[derive(Debug, Clone)]
pub struct IterBridge<Iter> {
iter: Iter,
}
impl<Iter: Iterator + Send> ParallelIterator for IterBridge<Iter>
where
Iter::Item: Send,
{
type Item = Iter::Item;
fn drive_unindexed<C>(self, consumer: C) -> C::Result
where
C: UnindexedConsumer<Self::Item>,
{
let num_threads = current_num_threads();
let threads_started: Vec<_> = (0..num_threads).map(|_| AtomicBool::new(false)).collect();
bridge_unindexed(
&IterParallelProducer {
split_count: AtomicUsize::new(num_threads),
iter: Mutex::new(self.iter.fuse()),
threads_started: &threads_started,
},
consumer,
)
}
}
struct IterParallelProducer<'a, Iter> {
split_count: AtomicUsize,
iter: Mutex<std::iter::Fuse<Iter>>,
threads_started: &'a [AtomicBool],
}
impl<Iter: Iterator + Send> UnindexedProducer for &IterParallelProducer<'_, Iter> {
type Item = Iter::Item;
fn split(self) -> (Self, Option<Self>) {
let mut count = self.split_count.load(Ordering::SeqCst);
loop {
// Check if the iterator is exhausted
if let Some(new_count) = count.checked_sub(1) {
match self.split_count.compare_exchange_weak(
count,
new_count,
Ordering::SeqCst,
Ordering::SeqCst,
) {
Ok(_) => return (self, Some(self)),
Err(last_count) => count = last_count,
}
} else {
return (self, None);
}
}
}
fn fold_with<F>(self, mut folder: F) -> F
where
F: Folder<Self::Item>,
{
// Guard against work-stealing-induced recursion, in case `Iter::next()`
// calls rayon internally, so we don't deadlock our mutex. We might also
// be recursing via `folder` methods, which doesn't present a mutex hazard,
// but it's lower overhead for us to just check this once, rather than
// updating additional shared state on every mutex lock/unlock.
// (If this isn't a rayon thread, then there's no work-stealing anyway...)
if let Some(i) = current_thread_index() {
// Note: If the number of threads in the pool ever grows dynamically, then
// we'll end up sharing flags and may falsely detect recursion -- that's
// still fine for overall correctness, just not optimal for parallelism.
let thread_started = &self.threads_started[i % self.threads_started.len()];
if thread_started.swap(true, Ordering::Relaxed) {
// We can't make progress with a nested mutex, so just return and let
// the outermost loop continue with the rest of the iterator items.
return folder;
}
}
loop {
if let Ok(mut iter) = self.iter.lock() {
if let Some(it) = iter.next() {
drop(iter);
folder = folder.consume(it);
if folder.full() {
return folder;
}
} else {
return folder;
}
} else {
// any panics from other threads will have been caught by the pool,
// and will be re-thrown when joined - just exit
return folder;
}
}
}
}
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