Added FromStream and Extend impls for HashMap

Author: sunjay

src/collections/hash_map/extend.rs

@@ -0,0 +1,36 @@
+use std::pin::Pin;
+use std::hash::{Hash, BuildHasher};
+use std::collections::HashMap;
+
+use crate::prelude::*;
+use crate::stream::{Extend, IntoStream};
+
+impl<K, V, H> Extend<(K, V)> for HashMap<K, V, H>
+where K: Eq + Hash,
+      H: BuildHasher + Default {
+    fn stream_extend<'a, S: IntoStream<Item = (K, V)> + 'a>(
+        &'a mut self,
+        stream: S,
+    ) -> Pin<Box<dyn Future<Output = ()> + 'a>> {
+        let stream = stream.into_stream();
+
+        // The following is adapted from the hashbrown source code:
+        // https://github.com/rust-lang/hashbrown/blob/d1ad4fc3aae2ade446738eea512e50b9e863dd0c/src/map.rs#L2470-L2491
+        //
+        // Keys may be already present or show multiple times in the stream. Reserve the entire
+        // hint lower bound if the map is empty. Otherwise reserve half the hint (rounded up), so
+        // the map will only resize twice in the worst case.
+
+        //TODO: Add this back in when size_hint is added to Stream/StreamExt
+        //let reserve = if self.is_empty() {
+        //    stream.size_hint().0
+        //} else {
+        //    (stream.size_hint().0 + 1) / 2
+        //};
+        //self.reserve(reserve);
+
+        Box::pin(stream.for_each(move |(k, v)| {
+            self.insert(k, v);
+        }))
+    }
+}

src/collections/hash_map/from_stream.rs

@@ -0,0 +1,27 @@
+use std::pin::Pin;
+use std::hash::{Hash, BuildHasher};
+use std::collections::HashMap;
+
+use crate::stream::{Extend, FromStream, IntoStream};
+
+impl<K, V, H> FromStream<(K, V)> for HashMap<K, V, H>
+where K: Eq + Hash,
+      H: BuildHasher + Default {
+    #[inline]
+    fn from_stream<'a, S: IntoStream<Item = (K, V)>>(
+        stream: S,
+    ) -> Pin<Box<dyn core::future::Future<Output = Self> + 'a>>
+    where
+        <S as IntoStream>::IntoStream: 'a,
+    {
+        let stream = stream.into_stream();
+
+        Box::pin(async move {
+            pin_utils::pin_mut!(stream);
+
+            let mut out = HashMap::with_hasher(Default::default());
+            out.stream_extend(stream).await;
+            out
+        })
+    }
+}

src/collections/hash_map/mod.rs

@@ -0,0 +1,7 @@
+//! The Rust hash map, implemented with quadratic probing and SIMD lookup.
+
+mod extend;
+mod from_stream;
+
+#[doc(inline)]
+pub use std::collections::HashMap;

src/collections/mod.rs

@@ -4,7 +4,9 @@
 //! data structures.
 
 pub mod vec_deque;
+pub mod hash_map;
 pub mod btree_map;
 
 pub use vec_deque::VecDeque;
+pub use hash_map::HashMap;
 pub use btree_map::BTreeMap;