From 18de104d385d5d91ea7c1dc7dbdf22bebad0e642 Mon Sep 17 00:00:00 2001
From: Bronson Philippa <bronson.philippa@gmail.com>
Date: Mon, 28 Mar 2022 20:19:33 +1000
Subject: [PATCH] Extended Encode and Decode for HashMap and HashSet to support
 custom hashers (#529)

---
 src/features/impl_std.rs | 16 ++++++++++------
 tests/std.rs             | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 6 deletions(-)

diff --git a/src/features/impl_std.rs b/src/features/impl_std.rs
index c923001c..0f282982 100644
--- a/src/features/impl_std.rs
+++ b/src/features/impl_std.rs
@@ -361,7 +361,7 @@ impl Decode for SocketAddrV6 {
 impl std::error::Error for EncodeError {}
 impl std::error::Error for DecodeError {}
 
-impl<K, V> Encode for HashMap<K, V>
+impl<K, V, S> Encode for HashMap<K, V, S>
 where
     K: Encode,
     V: Encode,
@@ -376,16 +376,18 @@ where
     }
 }
 
-impl<K, V> Decode for HashMap<K, V>
+impl<K, V, S> Decode for HashMap<K, V, S>
 where
     K: Decode + Eq + std::hash::Hash,
     V: Decode,
+    S: std::hash::BuildHasher + Default,
 {
     fn decode<D: Decoder>(decoder: &mut D) -> Result<Self, DecodeError> {
         let len = crate::de::decode_slice_len(decoder)?;
         decoder.claim_container_read::<(K, V)>(len)?;
 
-        let mut map = HashMap::with_capacity(len);
+        let hash_builder: S = Default::default();
+        let mut map = HashMap::with_capacity_and_hasher(len, hash_builder);
         for _ in 0..len {
             // See the documentation on `unclaim_bytes_read` as to why we're doing this here
             decoder.unclaim_bytes_read(core::mem::size_of::<(K, V)>());
@@ -398,15 +400,17 @@ where
     }
 }
 
-impl<T> Decode for HashSet<T>
+impl<T, S> Decode for HashSet<T, S>
 where
     T: Decode + Eq + Hash,
+    S: std::hash::BuildHasher + Default,
 {
     fn decode<D: Decoder>(decoder: &mut D) -> Result<Self, DecodeError> {
         let len = crate::de::decode_slice_len(decoder)?;
         decoder.claim_container_read::<T>(len)?;
 
-        let mut map = HashSet::new();
+        let hash_builder: S = Default::default();
+        let mut map: HashSet<T, S> = HashSet::with_capacity_and_hasher(len, hash_builder);
         for _ in 0..len {
             // See the documentation on `unclaim_bytes_read` as to why we're doing this here
             decoder.unclaim_bytes_read(core::mem::size_of::<T>());
@@ -418,7 +422,7 @@ where
     }
 }
 
-impl<T> Encode for HashSet<T>
+impl<T, S> Encode for HashSet<T, S>
 where
     T: Encode,
 {
diff --git a/tests/std.rs b/tests/std.rs
index 658a4211..e4a392d0 100644
--- a/tests/std.rs
+++ b/tests/std.rs
@@ -106,6 +106,20 @@ fn test_std_commons() {
     set.insert("World".to_string());
     the_same(set);
 
+    // HashMap and HashSet with custom hash algorithm
+    type MyBuildHasher = std::hash::BuildHasherDefault<ExampleCustomHasher>;
+    let mut custom_map: std::collections::HashMap<String, String, MyBuildHasher> =
+        Default::default();
+    custom_map.insert("Hello".to_owned(), "world".to_owned());
+    custom_map.insert("How".to_owned(), "are".to_owned());
+    custom_map.insert("you".to_owned(), "doing?".to_owned());
+    the_same(custom_map);
+
+    let mut custom_set: std::collections::HashSet<String, MyBuildHasher> = Default::default();
+    custom_set.insert("Hello".to_string());
+    custom_set.insert("World".to_string());
+    the_same(custom_set);
+
     // Borrowed values
     let config = bincode::config::standard();
     let mut buffer = [0u8; 1024];
@@ -141,3 +155,21 @@ fn test_system_time_out_of_range() {
         }
     );
 }
+
+/// Simple example of user-defined hasher to test encoding/decoding HashMap and HashSet with custom hash algorithms.
+#[derive(Copy, Clone, Default)]
+pub struct ExampleCustomHasher {
+    pub hash: u64,
+}
+
+impl std::hash::Hasher for ExampleCustomHasher {
+    fn write(&mut self, value: &[u8]) {
+        for (index, &item) in value.iter().enumerate() {
+            self.hash ^= u64::from(item) << ((index % 8) * 8);
+        }
+    }
+
+    fn finish(&self) -> u64 {
+        self.hash
+    }
+}