feat: add utxo_digest RPC call

The utxo_digest rpc enables client to lookup a UTXO digest from a
leaf index.

src/rpc_server.rs

@@ -50,17 +50,19 @@ pub struct DashBoardOverviewDataFromClient {
     pub confirmations: Option<BlockHeight>,
 }
 
+/// Provides summary information about a Block
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq, Eq)]
 pub struct BlockInfo {
-    height: BlockHeight,
-    digest: Digest,
-    timestamp: Timestamp,
-    difficulty: U32s<TARGET_DIFFICULTY_U32_SIZE>,
-    num_inputs: usize,
-    num_outputs: usize,
-    fee: NeptuneCoins,
+    pub height: BlockHeight,
+    pub digest: Digest,
+    pub timestamp: Timestamp,
+    pub difficulty: U32s<TARGET_DIFFICULTY_U32_SIZE>,
+    pub num_inputs: usize,
+    pub num_outputs: usize,
+    pub fee: NeptuneCoins,
 }
 
+/// Provides alternatives for looking up a block.
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq, Eq)]
 pub enum BlockSelector {
     Digest(Digest),      // Identifies block by Digest (hash)
@@ -125,6 +127,9 @@ pub trait RPC {
     /// Return the digest for the specified block selector (genesis, tip, or height)
     async fn block_digest(block_selector: BlockSelector) -> Option<Digest>;
 
+    /// Return the digest for the specified UTXO index
+    async fn utxo_digest(index: u64) -> Option<Digest>;
+
     /// Return the block header for the specified block
     async fn header(block_selector: BlockSelector) -> Option<BlockHeader>;
 
@@ -253,6 +258,16 @@ impl RPC for NeptuneRPCServer {
         self.confirmations_internal().await
     }
 
+    async fn utxo_digest(self, _: context::Context, index: u64) -> Option<Digest> {
+        let state = self.state.lock_guard().await;
+        let aocl = &state.chain.archival_state().archival_mutator_set.ams().aocl;
+
+        match index > 0 && index < aocl.len().await {
+            true => Some(aocl.get_leaf_async(index).await),
+            false => None,
+        }
+    }
+
     async fn block_digest(
         self,
         _: context::Context,

src/util_types/mutator_set/archival_mmr.rs

@@ -52,6 +52,12 @@ where
         self.digests.len().await == 1
     }
 
+    /// Returns len of Digests.  Note that elem 0
+    /// is always a dummy digest.
+    pub async fn len(&self) -> u64 {
+        self.digests.len().await
+    }
+
     /// Return the number of leaves in the tree
     pub async fn count_leaves(&self) -> u64 {
         node_index_to_leaf_index(self.digests.len().await).unwrap()