feat: borsh serde

Cargo.toml

@@ -36,6 +36,9 @@ allocator-api2 = { version = "0.2.9", optional = true, default-features = false,
 # Equivalent trait which can be shared with other hash table implementations.
 equivalent = { version = "1.0", optional = true, default-features = false }
 
+# borsh serde
+borsh = { version = "1.5.0", default-features = false, optional = true, features = ["derive"]}
+
 [dev-dependencies]
 lazy_static = "1.4"
 rand = { version = "0.8.3", features = ["small_rng"] }
@@ -66,6 +69,8 @@ raw = []
 # time cost.
 inline-more = []
 
+borsh = ["dep:borsh"]
+
 [package.metadata.docs.rs]
 features = ["nightly", "rayon", "serde", "raw"]
 rustdoc-args = ["--generate-link-to-definition"]

src/external_trait_impls/borsh/hash_map.rs

@@ -0,0 +1,78 @@
+use crate::HashMap;
+
+use borsh::{
+    io::{Read, Result, Write},
+    BorshDeserialize, BorshSerialize,
+};
+
+impl<K: BorshSerialize, V: BorshSerialize, S: BorshSerialize> BorshSerialize for HashMap<K, V, S> {
+    fn serialize<W: Write>(&self, writer: &mut W) -> Result<()> {
+        // assuming hash may have some seed,
+        // as borsh is supposed by default to be deterministic, need to write it down
+        // if allocator is compile time, than one can just impl wrapper with zero bytes serde of it
+        self.hash_builder.serialize(writer)?;
+        // considering A stateless
+        self.len().serialize(writer)?;
+        for kv in self.iter() {
+            kv.serialize(writer)?;
+        }
+        Ok(())
+    }
+}
+
+impl<
+        K: BorshDeserialize + core::hash::Hash + Eq,
+        V: BorshDeserialize,
+        S: BorshDeserialize + core::hash::BuildHasher,
+    > BorshDeserialize for HashMap<K, V, S>
+{
+    fn deserialize_reader<R: Read>(reader: &mut R) -> Result<Self> {
+        let hash_builder = S::deserialize_reader(reader)?;
+        let len = usize::deserialize_reader(reader)?;
+        let mut map = HashMap::with_capacity_and_hasher(len, hash_builder);
+        for _ in 0..len {
+            let (k, v) = <(K, V)>::deserialize_reader(reader)?;
+            // can use raw api here to init from memory, so can do it other time
+            map.insert(k, v);
+        }
+        Ok(map)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use borsh::{BorshDeserialize, BorshSerialize};
+    use std::vec::Vec;
+
+    #[derive(Default, BorshDeserialize, BorshSerialize, Clone)]
+    struct NoHash;
+
+    impl core::hash::BuildHasher for NoHash {
+        type Hasher = NoHash;
+        fn build_hasher(&self) -> NoHash {
+            Self
+        }
+    }
+
+    impl core::hash::Hasher for NoHash {
+        fn finish(&self) -> u64 {
+            42
+        }
+
+        fn write(&mut self, _bytes: &[u8]) {}
+    }
+
+    #[test]
+    fn encdec() {
+        let mut map = crate::HashMap::<_, _, NoHash>::default();
+        map.insert(1, 2);
+        map.insert(3, 4);
+        let mut buf = Vec::new();
+        map.serialize(&mut buf).unwrap();
+        let original = map.clone();
+        map = crate::HashMap::<_, _, NoHash>::deserialize_reader(&mut &buf[..]).unwrap();
+        assert_eq!(original[&1], map[&1]);
+        assert_eq!(original[&3], map[&3]);
+        assert_eq!(original.len(), map.len());
+    }
+}

src/external_trait_impls/borsh/mod.rs

@@ -0,0 +1 @@
+mod hash_map;

src/external_trait_impls/mod.rs

@@ -1,3 +1,5 @@
+#[cfg(feature = "borsh")]
+mod borsh;
 #[cfg(feature = "rayon")]
 pub(crate) mod rayon;
 #[cfg(feature = "rkyv")]