types: use untagged serialization for Parents

Summary:
Tell serde to use an [untagged representation](https://serde.rs/enum-representations.html#untagged) of this enum. This means that `Parents::None` will map to a null value, `Parents::One` will map to an array representing a `Node`, and a `Parents::Two` will map to an array of 2 arrays representing `Node`.

Using CBOR serialization, this means that these variants are 1 byte, 21 bytes, and 43 bytes respectively.

Differential Revision: D14174309

fbshipit-source-id: 0217a23c4ee5409ab293525d7b6e5ae969b5504d

lib/types/Cargo.toml

@@ -10,11 +10,13 @@ for-tests = ["rand", "quickcheck"]
 [dependencies]
 failure = "0.1.3"
 failure_derive = "0.1.3"
+quickcheck = { version = "0.6.2", optional = true }
 rand = { version = "0.5.5", optional = true }
 serde = "1.0.84"
 serde_derive = "1.0.84"
-quickcheck = { version = "0.6.2", optional = true }
 
 [dev-dependencies]
-rand = "0.5.5"
 quickcheck = "0.6.2"
+rand = "0.5.5"
+serde_cbor = "0.9.0"
+serde_json = "1.0.38"

lib/types/src/parents.rs

@@ -25,6 +25,7 @@ use crate::node::{Node, NULL_ID};
     Serialize,
     Deserialize
 )]
+#[serde(untagged)]
 pub enum Parents {
     None,
     One(Node),
@@ -143,6 +144,9 @@ impl Arbitrary for Parents {
 mod tests {
     use super::*;
 
+    use serde_cbor;
+    use serde_json::{self, json};
+
     #[test]
     fn from_iter() {
         let none = Parents::from_iter(vec![NULL_ID, NULL_ID]);
@@ -180,4 +184,41 @@ mod tests {
         let two = parents.into_iter().collect::<Vec<_>>();
         assert_eq!(two, vec![p1, p2]);
     }
+
+    #[test]
+    fn untagged_serializtaion() {
+        let parents = Parents::None;
+        let none = serde_json::to_value(&parents).unwrap();
+        assert_eq!(none, json!(null));
+
+        let p1 = Node::from_byte_array([0x1; 20]);
+        let parents = Parents::One(p1);
+        let one = serde_json::to_value(&parents).unwrap();
+        let expected = json!([1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]);
+        assert_eq!(one, expected);
+
+        let p2 = Node::from_byte_array([0x2; 20]);
+        let parents = Parents::Two(p1, p2);
+        let two = serde_json::to_value(&parents).unwrap();
+        let p1_json = json!([1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]);
+        let p2_json = json!([2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2]);
+        assert_eq!(two, json!([p1_json, p2_json]));
+    }
+
+    #[test]
+    fn serialized_size_cbor() {
+        let parents = Parents::None;
+        let none = serde_cbor::to_vec(&parents).unwrap();
+        assert_eq!(none.len(), 1);
+
+        let p1 = Node::from_byte_array([0x1; 20]);
+        let parents = Parents::One(p1);
+        let one = serde_cbor::to_vec(&parents).unwrap();
+        assert_eq!(one.len(), 21);
+
+        let p2 = Node::from_byte_array([0x2; 20]);
+        let parents = Parents::Two(p1, p2);
+        let two = serde_cbor::to_vec(&parents).unwrap();
+        assert_eq!(two.len(), 43);
+    }
 }