transfer-contract: add next_pos contract call

contracts/transfer/src/lib.rs

@@ -104,6 +104,11 @@ unsafe fn existing_nullifiers(arg_len: u32) -> u32 {
     })
 }
 
+#[no_mangle]
+unsafe fn num_notes(arg_len: u32) -> u32 {
+    rusk_abi::wrap_call(arg_len, |_: ()| STATE.num_notes())
+}
+
 // "Feeder" queries
 
 #[no_mangle]

contracts/transfer/src/state.rs

@@ -434,6 +434,11 @@ impl TransferState {
         self.tree.root()
     }
 
+    /// Get the count of the notes in the tree.
+    pub fn num_notes(&self) -> u64 {
+        self.tree.leaves_len()
+    }
+
     /// Get the opening
     pub fn opening(
         &self,

contracts/transfer/src/tree.rs

@@ -97,4 +97,8 @@ impl Tree {
     ) -> Option<PoseidonOpening<(), TRANSFER_TREE_DEPTH, A>> {
         self.tree.opening(pos)
     }
+
+    pub fn leaves_len(&self) -> u64 {
+        self.leaves.len() as u64
+    }
 }

contracts/transfer/tests/transfer.rs

@@ -149,6 +149,12 @@ fn leaves_from_pos(session: &mut Session, pos: u64) -> Result<Vec<TreeLeaf>> {
         .collect())
 }
 
+fn num_notes(session: &mut Session) -> Result<u64> {
+    session
+        .call(TRANSFER_CONTRACT, "num_notes", &(), u64::MAX)
+        .map(|r| r.data)
+}
+
 fn update_root(session: &mut Session) -> Result<()> {
     session
         .call(TRANSFER_CONTRACT, "update_root", &(), POINT_LIMIT)
@@ -254,6 +260,13 @@ fn transfer() {
 
     assert_eq!(leaves.len(), 1, "There should be one note in the state");
 
+    let pos = num_notes(session).expect("Getting num_notes should succeed");
+    assert_eq!(
+        pos,
+        leaves.last().expect("note to exists").note.pos() + 1,
+        "num_notes should match position of last note + 1"
+    );
+
     let leaves = leaves_from_pos(session, 0)
         .expect("Getting leaves in the given range should succeed");
 
@@ -364,7 +377,14 @@ fn transfer() {
         "There should be three notes in the tree at this block height"
     );
 
-    let leaves = leaves_from_height(session, input_note.pos() + 1)
+    let pos = num_notes(session).expect("Getting num_notes should succeed");
+    assert_eq!(
+        pos,
+        leaves.last().expect("note to exists").note.pos() + 1,
+        "num_notes should match position of last note + 1"
+    );
+
+    let leaves = leaves_from_pos(session, input_note.pos() + 1)
         .expect("Getting the notes should succeed");
     assert_eq!(
         leaves.len(),