Accept an Amount<NonNegative> for the value arg of a ValueCommitment constructor

zebra-chain/src/commitments/sapling.rs

@@ -316,13 +316,12 @@ impl ValueCommitment {
     /// Generate a new _ValueCommitment_.
     ///
     /// https://zips.z.cash/protocol/protocol.pdf#concretehomomorphiccommit
-    // TODO: accept an Amount instead?
     #[allow(non_snake_case)]
-    pub fn new<T>(csprng: &mut T, value_bytes: [u8; 32]) -> Self
+    pub fn new<T>(csprng: &mut T, value: Amount<NonNegative>) -> Self
     where
         T: RngCore + CryptoRng,
     {
-        let v = jubjub::Fr::from_bytes(&value_bytes).unwrap();
+        let v = jubjub::Fr::from(value);
         let rcv = generate_trapdoor(csprng);
 
         let V = find_group_hash(*b"Zcash_cv", b"v");

zebra-chain/src/types/amount.rs

@@ -132,6 +132,16 @@ impl From<Amount<NonNegative>> for u64 {
     }
 }
 
+impl<C> From<Amount<C>> for jubjub::Fr {
+    fn from(a: Amount<C>) -> jubjub::Fr {
+        if a.0 < 0 {
+            jubjub::Fr::from(a.0.abs() as u64).neg()
+        } else {
+            jubjub::Fr::from(a.0 as u64)
+        }
+    }
+}
+
 impl<C> TryFrom<i64> for Amount<C>
 where
     C: AmountConstraint,