adds stream::skip_while combinator

src/stream/stream/mod.rs

@@ -33,6 +33,7 @@ mod min_by;
 mod next;
 mod nth;
 mod scan;
+mod skip_while;
 mod take;
 mod zip;
 
@@ -51,6 +52,7 @@ use fold::FoldFuture;
 use min_by::MinByFuture;
 use next::NextFuture;
 use nth::NthFuture;
+use skip_while::SkipWhile;
 
 use std::cmp::Ordering;
 use std::marker::PhantomData;
@@ -661,6 +663,38 @@ pub trait Stream {
         Scan::new(self, initial_state, f)
     }
 
+    /// Combinator that `skip`s elements based on a predicate.
+    ///
+    /// Takes a closure argument. It will call this closure on every element in
+    /// the stream and ignore elements until it returns `false`.
+    ///
+    /// After `false` is returned, `SkipWhile`'s job is over and all further
+    /// elements in the strem are yeilded.
+    ///
+    /// ## Examples
+    /// ```
+    /// # fn main() { async_std::task::block_on(async {
+    /// #
+    /// use std::collections::VecDeque;
+    /// use async_std::stream::Stream;
+    ///
+    /// let a: VecDeque<_> = vec![-1i32, 0, 1].into_iter().collect();
+    /// let mut s = a.skip_while(|x| x.is_negative());
+    ///
+    /// assert_eq!(s.next().await, Some(0));
+    /// assert_eq!(s.next().await, Some(1));
+    /// assert_eq!(s.next().await, None);
+    /// #
+    /// # }) }
+    /// ```
+    fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P, Self::Item>
+    where
+        Self: Sized,
+        P: FnMut(&Self::Item) -> bool,
+    {
+        SkipWhile::new(self, predicate)
+    }
+
     /// 'Zips up' two streams into a single stream of pairs.
     ///
     /// `zip()` returns a new stream that will iterate over two other streams, returning a tuple

src/stream/stream/skip_while.rs

@@ -0,0 +1,54 @@
+use std::marker::PhantomData;
+use std::pin::Pin;
+
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+pub struct SkipWhile<S, P, T> {
+    stream: S,
+    predicate: Option<P>,
+    __t: PhantomData<T>,
+}
+
+impl<S, P, T> SkipWhile<S, P, T> {
+    pin_utils::unsafe_pinned!(stream: S);
+    pin_utils::unsafe_unpinned!(predicate: Option<P>);
+
+    pub(crate) fn new(stream: S, predicate: P) -> Self {
+        SkipWhile {
+            stream,
+            predicate: Some(predicate),
+            __t: PhantomData,
+        }
+    }
+}
+
+impl<S, P> Stream for SkipWhile<S, P, S::Item>
+where
+    S: Stream,
+    P: FnMut(&S::Item) -> bool,
+{
+    type Item = S::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        loop {
+            let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+
+            match next {
+                Some(v) => match self.as_mut().predicate() {
+                    Some(p) => match p(&v) {
+                        true => (),
+                        false => {
+                            *self.as_mut().predicate() = None;
+                            return Poll::Ready(Some(v));
+                        }
+                    },
+                    None => return Poll::Ready(Some(v)),
+                },
+                None => return Poll::Ready(None),
+            }
+        }
+    }
+}