Merge #223

223: adds stream::skip_while combinator r=stjepang a=montekki

Ref: #129 
Stdlib: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.skip_while

Co-authored-by: Fedor Sakharov <fedor.sakharov@gmail.com>

src/stream/stream/mod.rs

@@ -35,6 +35,7 @@ mod next;
 mod nth;
 mod scan;
 mod skip;
+mod skip_while;
 mod take;
 mod zip;
 
@@ -42,6 +43,7 @@ pub use filter::Filter;
 pub use fuse::Fuse;
 pub use scan::Scan;
 pub use skip::Skip;
+pub use skip_while::SkipWhile;
 pub use take::Take;
 pub use zip::Zip;
 
@@ -693,7 +695,13 @@ pub trait Stream {
         Scan::new(self, initial_state, f)
     }
 
-    /// Creates a combinator that skips the first `n` elements.
+    /// 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
     ///
@@ -703,6 +711,27 @@ pub trait Stream {
     /// 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)
+    }
+
+    /// Creates a combinator that skips the first `n` elements.
+    ///
+    /// ## Examples
+    ///
     /// let s: VecDeque<usize> = vec![1, 2, 3].into_iter().collect();
     /// let mut skipped = s.skip(2);
     ///

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};
+
+/// A stream to skip elements of another stream based on a predicate.
+#[derive(Debug)]
+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),
+            }
+        }
+    }
+}