add stream::all method

Author: Shady Khalifa

src/stream/stream.rs

@@ -27,6 +27,7 @@ use cfg_if::cfg_if;
 
 use crate::future::Future;
 use crate::task::{Context, Poll};
+use std::marker::PhantomData;
 
 cfg_if! {
     if #[cfg(feature = "docs")] {
@@ -111,6 +112,71 @@ pub trait Stream {
             remaining: n,
         }
     }
+
+    /// Tests if every element of the stream matches a predicate.
+    ///
+    /// `all()` takes a closure that returns `true` or `false`. It applies
+    /// this closure to each element of the stream, and if they all return
+    /// `true`, then so does `all()`. If any of them return `false`, it
+    /// returns `false`.
+    ///
+    /// `all()` is short-circuiting; in other words, it will stop processing
+    /// as soon as it finds a `false`, given that no matter what else happens,
+    /// the result will also be `false`.
+    ///
+    /// An empty stream returns `true`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// # fn main() { async_std::task::block_on(async {
+    /// #
+    /// use async_std::prelude::*;
+    /// use async_std::stream;
+    ///
+    /// let mut s = stream::repeat(9).take(3);
+    ///
+    /// while let Some(v) = s.next().await {
+    ///     assert_eq!(v, 9);
+    /// }
+    /// #
+    /// # }) }
+    /// ```
+    /// ```
+    /// let a = [1, 2, 3];
+    ///
+    /// assert!(a.iter().all(|&x| x > 0));
+    ///
+    /// assert!(!a.iter().all(|&x| x > 2));
+    /// ```
+    ///
+    /// Stopping at the first `false`:
+    ///
+    /// ```
+    /// let a = [1, 2, 3];
+    ///
+    /// let mut iter = a.iter();
+    ///
+    /// assert!(!iter.all(|&x| x != 2));
+    ///
+    /// // we can still use `iter`, as there are more elements.
+    /// assert_eq!(iter.next(), Some(&3));
+    /// ```
+    #[inline]
+    fn all<F>(&mut self, f: F) -> AllFuture<'_, Self, F, Self::Item>
+    where
+        Self: Sized,
+        F: FnMut(Self::Item) -> bool,
+    {
+        AllFuture {
+            stream: self,
+            result: true,
+            __item: PhantomData,
+            f,
+        }
+    }
 }
 
 impl<T: futures::Stream + Unpin + ?Sized> Stream for T {
@@ -168,3 +234,45 @@ impl<S: futures::Stream> futures::Stream for Take<S> {
         }
     }
 }
+
+pub struct AllFuture<'a, S, F, T>
+where
+    S: ?Sized,
+    F: FnMut(T) -> bool,
+{
+    stream: &'a mut S,
+    f: F,
+    result: bool,
+    __item: PhantomData<T>,
+}
+
+impl<'a, S, F, T> AllFuture<'a, S, F, T>
+where
+    S: ?Sized,
+    F: FnMut(T) -> bool,
+{
+    pin_utils::unsafe_pinned!(stream: &'a mut S);
+    pin_utils::unsafe_unpinned!(result: bool);
+    pin_utils::unsafe_unpinned!(f: F);
+}
+
+impl<S, F> Future for AllFuture<'_, S, F, S::Item>
+where
+    S: futures::Stream + Unpin + ?Sized,
+    F: FnMut(S::Item) -> bool,
+{
+    type Output = bool;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        use futures::Stream;
+        let next = futures::ready!(self.as_mut().stream().poll_next(cx));
+        match next {
+            Some(v) => {
+                // me: *screams*
+                *self.as_mut().result() = (self.as_mut().f())(v);
+                Poll::Pending
+            }
+            None => Poll::Ready(self.result),
+        }
+    }
+}