Add ParallelIterator::take_any_while and skip_any_while

These are like `Iterator::take_while` and `skip_while`, but the `any`
names emphasize that they're not a straightforward parallelization, as
they does not honor the original iterator order.

Author: cuviper

src/iter/mod.rs

@@ -142,11 +142,13 @@ mod repeat;
 mod rev;
 mod skip;
 mod skip_any;
+mod skip_any_while;
 mod splitter;
 mod step_by;
 mod sum;
 mod take;
 mod take_any;
+mod take_any_while;
 mod try_fold;
 mod try_reduce;
 mod try_reduce_with;
@@ -188,10 +190,12 @@ pub use self::{
     rev::Rev,
     skip::Skip,
     skip_any::SkipAny,
+    skip_any_while::SkipAnyWhile,
     splitter::{split, Split},
     step_by::StepBy,
     take::Take,
     take_any::TakeAny,
+    take_any_while::TakeAnyWhile,
     try_fold::{TryFold, TryFoldWith},
     update::Update,
     while_some::WhileSome,
@@ -2248,6 +2252,74 @@ pub trait ParallelIterator: Sized + Send {
         SkipAny::new(self, n)
     }
 
+    /// Creates an iterator that takes elements from *anywhere* in the original iterator
+    /// until the given `predicate` returns `false`.
+    ///
+    /// The `predicate` may be anything -- e.g. it could be checking a fact about the item, a
+    /// global condition unrelated to the item itself, or some combination thereof.
+    ///
+    /// If parallel calls to the `predicate` race and give different results, then the
+    /// `true` results will still take those particular items, while respecting the `false`
+    /// result from elsewhere to skip any further items.
+    ///
+    /// This is similar to [`Iterator::take_while`] without being constrained to the original
+    /// iterator order. The taken items will still maintain their relative order where that is
+    /// visible in `collect`, `reduce`, and similar outputs.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use rayon::prelude::*;
+    ///
+    /// let result: Vec<_> = (0..100)
+    ///     .into_par_iter()
+    ///     .take_any_while(|x| *x < 50)
+    ///     .collect();
+    ///
+    /// assert!(result.len() <= 50);
+    /// assert!(result.windows(2).all(|w| w[0] < w[1]));
+    /// ```
+    fn take_any_while<P>(self, predicate: P) -> TakeAnyWhile<Self, P>
+    where
+        P: Fn(&Self::Item) -> bool + Sync + Send,
+    {
+        TakeAnyWhile::new(self, predicate)
+    }
+
+    /// Creates an iterator that skips elements from *anywhere* in the original iterator
+    /// until the given `predicate` returns `false`.
+    ///
+    /// The `predicate` may be anything -- e.g. it could be checking a fact about the item, a
+    /// global condition unrelated to the item itself, or some combination thereof.
+    ///
+    /// If parallel calls to the `predicate` race and give different results, then the
+    /// `true` results will still skip those particular items, while respecting the `false`
+    /// result from elsewhere to skip any further items.
+    ///
+    /// This is similar to [`Iterator::skip_while`] without being constrained to the original
+    /// iterator order. The remaining items will still maintain their relative order where that is
+    /// visible in `collect`, `reduce`, and similar outputs.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use rayon::prelude::*;
+    ///
+    /// let result: Vec<_> = (0..100)
+    ///     .into_par_iter()
+    ///     .skip_any_while(|x| *x < 50)
+    ///     .collect();
+    ///
+    /// assert!(result.len() >= 50);
+    /// assert!(result.windows(2).all(|w| w[0] < w[1]));
+    /// ```
+    fn skip_any_while<P>(self, predicate: P) -> SkipAnyWhile<Self, P>
+    where
+        P: Fn(&Self::Item) -> bool + Sync + Send,
+    {
+        SkipAnyWhile::new(self, predicate)
+    }
+
     /// Internal method used to define the behavior of this parallel
     /// iterator. You should not need to call this directly.
     ///

src/iter/skip_any_while.rs

@@ -0,0 +1,166 @@
+use super::plumbing::*;
+use super::*;
+use std::fmt;
+use std::sync::atomic::{AtomicBool, Ordering};
+
+/// `SkipAnyWhile` is an iterator that skips over elements from anywhere in `I`
+/// until the callback returns `false`.
+/// This struct is created by the [`skip_any_while()`] method on [`ParallelIterator`]
+///
+/// [`skip_any_while()`]: trait.ParallelIterator.html#method.skip_any_while
+/// [`ParallelIterator`]: trait.ParallelIterator.html
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+#[derive(Clone)]
+pub struct SkipAnyWhile<I: ParallelIterator, P> {
+    base: I,
+    predicate: P,
+}
+
+impl<I: ParallelIterator + fmt::Debug, P> fmt::Debug for SkipAnyWhile<I, P> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("SkipAnyWhile")
+            .field("base", &self.base)
+            .finish()
+    }
+}
+
+impl<I, P> SkipAnyWhile<I, P>
+where
+    I: ParallelIterator,
+{
+    /// Creates a new `SkipAnyWhile` iterator.
+    pub(super) fn new(base: I, predicate: P) -> Self {
+        SkipAnyWhile { base, predicate }
+    }
+}
+
+impl<I, P> ParallelIterator for SkipAnyWhile<I, P>
+where
+    I: ParallelIterator,
+    P: Fn(&I::Item) -> bool + Sync + Send,
+{
+    type Item = I::Item;
+
+    fn drive_unindexed<C>(self, consumer: C) -> C::Result
+    where
+        C: UnindexedConsumer<Self::Item>,
+    {
+        let consumer1 = SkipAnyWhileConsumer {
+            base: consumer,
+            predicate: &self.predicate,
+            skipping: &AtomicBool::new(true),
+        };
+        self.base.drive_unindexed(consumer1)
+    }
+}
+
+/// ////////////////////////////////////////////////////////////////////////
+/// Consumer implementation
+
+struct SkipAnyWhileConsumer<'p, C, P> {
+    base: C,
+    predicate: &'p P,
+    skipping: &'p AtomicBool,
+}
+
+impl<'p, T, C, P> Consumer<T> for SkipAnyWhileConsumer<'p, C, P>
+where
+    C: Consumer<T>,
+    P: Fn(&T) -> bool + Sync,
+{
+    type Folder = SkipAnyWhileFolder<'p, C::Folder, P>;
+    type Reducer = C::Reducer;
+    type Result = C::Result;
+
+    fn split_at(self, index: usize) -> (Self, Self, Self::Reducer) {
+        let (left, right, reducer) = self.base.split_at(index);
+        (
+            SkipAnyWhileConsumer { base: left, ..self },
+            SkipAnyWhileConsumer {
+                base: right,
+                ..self
+            },
+            reducer,
+        )
+    }
+
+    fn into_folder(self) -> Self::Folder {
+        SkipAnyWhileFolder {
+            base: self.base.into_folder(),
+            predicate: self.predicate,
+            skipping: self.skipping,
+        }
+    }
+
+    fn full(&self) -> bool {
+        self.base.full()
+    }
+}
+
+impl<'p, T, C, P> UnindexedConsumer<T> for SkipAnyWhileConsumer<'p, C, P>
+where
+    C: UnindexedConsumer<T>,
+    P: Fn(&T) -> bool + Sync,
+{
+    fn split_off_left(&self) -> Self {
+        SkipAnyWhileConsumer {
+            base: self.base.split_off_left(),
+            ..*self
+        }
+    }
+
+    fn to_reducer(&self) -> Self::Reducer {
+        self.base.to_reducer()
+    }
+}
+
+struct SkipAnyWhileFolder<'p, C, P> {
+    base: C,
+    predicate: &'p P,
+    skipping: &'p AtomicBool,
+}
+
+fn skip<T>(item: &T, skipping: &AtomicBool, predicate: &impl Fn(&T) -> bool) -> bool {
+    if !skipping.load(Ordering::Relaxed) {
+        return false;
+    }
+    if predicate(item) {
+        return true;
+    }
+    skipping.store(false, Ordering::Relaxed);
+    false
+}
+
+impl<'p, T, C, P> Folder<T> for SkipAnyWhileFolder<'p, C, P>
+where
+    C: Folder<T>,
+    P: Fn(&T) -> bool + 'p,
+{
+    type Result = C::Result;
+
+    fn consume(mut self, item: T) -> Self {
+        if !skip(&item, self.skipping, self.predicate) {
+            self.base = self.base.consume(item);
+        }
+        self
+    }
+
+    fn consume_iter<I>(mut self, iter: I) -> Self
+    where
+        I: IntoIterator<Item = T>,
+    {
+        self.base = self.base.consume_iter(
+            iter.into_iter()
+                .skip_while(move |x| skip(x, self.skipping, self.predicate)),
+        );
+        self
+    }
+
+    fn complete(self) -> C::Result {
+        self.base.complete()
+    }
+
+    fn full(&self) -> bool {
+        self.base.full()
+    }
+}