Adds Stream::partial_cmp

src/stream/stream/mod.rs

@@ -37,6 +37,7 @@ mod map;
 mod min_by;
 mod next;
 mod nth;
+mod partial_cmp;
 mod scan;
 mod skip;
 mod skip_while;
@@ -56,6 +57,7 @@ use for_each::ForEachFuture;
 use min_by::MinByFuture;
 use next::NextFuture;
 use nth::NthFuture;
+use partial_cmp::PartialCmpFuture;
 use try_for_each::TryForEeachFuture;
 
 pub use chain::Chain;
@@ -1187,6 +1189,42 @@ extension_trait! {
         {
             Merge::new(self, other)
         }
+
+        #[doc = r#"
+            Lexicographically compares the elements of this `Stream` with those
+            of another.
+
+            # Examples
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::prelude::*;
+            use std::collections::VecDeque;
+            use std::cmp::Ordering;
+            let s1 = VecDeque::from(vec![1]);
+            let s2 = VecDeque::from(vec![1, 2]);
+            let s3 = VecDeque::from(vec![1, 2, 3]);
+            let s4 = VecDeque::from(vec![1, 2, 4]);
+            assert_eq!(s1.clone().partial_cmp(s1.clone()).await, Some(Ordering::Equal));
+            assert_eq!(s1.clone().partial_cmp(s2.clone()).await, Some(Ordering::Less));
+            assert_eq!(s2.clone().partial_cmp(s1.clone()).await, Some(Ordering::Greater));       
+            assert_eq!(s3.clone().partial_cmp(s4.clone()).await, Some(Ordering::Less));
+            assert_eq!(s4.clone().partial_cmp(s3.clone()).await, Some(Ordering::Greater));                             
+            #
+            # }) }
+            ```
+        "#]
+        fn partial_cmp<S>(
+           self,
+           other: S
+        ) -> impl Future<Output = Option<Ordering>>  [PartialCmpFuture<Self, S>]
+        where
+            Self: Sized + Stream,
+            S: Stream,
+            <Self as Stream>::Item: PartialOrd<S::Item>,
+        {
+            PartialCmpFuture::new(self, other)
+        }
     }
 
     impl<S: Stream + Unpin + ?Sized> Stream for Box<S> {

src/stream/stream/partial_cmp.rs

@@ -0,0 +1,92 @@
+use std::cmp::Ordering;
+use std::pin::Pin;
+
+use super::fuse::Fuse;
+use crate::future::Future;
+use crate::prelude::*;
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+// Lexicographically compares the elements of this `Stream` with those
+// of another.
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+pub struct PartialCmpFuture<L: Stream, R: Stream> {
+    l: Fuse<L>,
+    r: Fuse<R>,
+    l_cache: Option<L::Item>,
+    r_cache: Option<R::Item>,
+}
+
+impl<L: Stream, R: Stream> PartialCmpFuture<L, R> {
+    pin_utils::unsafe_pinned!(l: Fuse<L>);
+    pin_utils::unsafe_pinned!(r: Fuse<R>);
+    pin_utils::unsafe_unpinned!(l_cache: Option<L::Item>);
+    pin_utils::unsafe_unpinned!(r_cache: Option<R::Item>);
+
+    pub(super) fn new(l: L, r: R) -> Self {
+        PartialCmpFuture {
+            l: l.fuse(),
+            r: r.fuse(),
+            l_cache: None,
+            r_cache: None,
+        }
+    }
+}
+
+impl<L: Stream, R: Stream> Future for PartialCmpFuture<L, R>
+where
+    L: Stream + Sized,
+    R: Stream + Sized,
+    L::Item: PartialOrd<R::Item>,
+{
+    type Output = Option<Ordering>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        loop {
+            // Short circuit logic
+            // Stream that completes earliest can be considered Less, etc
+            let l_complete = self.l.done && self.as_mut().l_cache.is_none();
+            let r_complete = self.r.done && self.as_mut().r_cache.is_none();
+
+            if l_complete && r_complete {
+                return Poll::Ready(Some(Ordering::Equal));
+            } else if l_complete {
+                return Poll::Ready(Some(Ordering::Less));
+            } else if r_complete {
+                return Poll::Ready(Some(Ordering::Greater));
+            }
+
+            // Get next value if possible and necesary
+            if !self.l.done && self.as_mut().l_cache.is_none() {
+                let l_next = futures_core::ready!(self.as_mut().l().poll_next(cx));
+                if let Some(item) = l_next {
+                    *self.as_mut().l_cache() = Some(item);
+                }
+            }
+
+            if !self.r.done && self.as_mut().r_cache.is_none() {
+                let r_next = futures_core::ready!(self.as_mut().r().poll_next(cx));
+                if let Some(item) = r_next {
+                    *self.as_mut().r_cache() = Some(item);
+                }
+            }
+
+            // Compare if both values are available.
+            if self.as_mut().l_cache.is_some() && self.as_mut().r_cache.is_some() {
+                let l_value = self.as_mut().l_cache().take().unwrap();
+                let r_value = self.as_mut().r_cache().take().unwrap();
+                let result = l_value.partial_cmp(&r_value);
+
+                if let Some(Ordering::Equal) = result {
+                    // Reset cache to prepare for next comparison
+                    *self.as_mut().l_cache() = None;
+                    *self.as_mut().r_cache() = None;
+                } else {
+                    // Return non equal value
+                    return Poll::Ready(result);
+                }
+            }
+        }
+    }
+}