feat(rln): get_leaf ffi and public api

rln/src/ffi.rs

@@ -222,6 +222,12 @@ pub extern "C" fn set_leaf(ctx: *mut RLN, index: usize, input_buffer: *const Buf
     call!(ctx, set_leaf, index, input_buffer)
 }
 
+#[allow(clippy::not_unsafe_ptr_arg_deref)]
+#[no_mangle]
+pub extern "C" fn get_leaf(ctx: *mut RLN, index: usize, output_buffer: *mut Buffer) -> bool {
+    call_with_output_arg!(ctx, get_leaf, output_buffer, index)
+}
+
 #[allow(clippy::not_unsafe_ptr_arg_deref)]
 #[no_mangle]
 pub extern "C" fn set_next_leaf(ctx: *mut RLN, input_buffer: *const Buffer) -> bool {

rln/src/public.rs

@@ -217,6 +217,31 @@ impl RLN<'_> {
         Ok(())
     }
 
+    /// Gets a leaf value at position index in the internal Merkle tree.
+    /// The leaf value is written to output_data.
+    /// Input values are:
+    /// - `index`: the index of the leaf
+    ///
+    /// Example:
+    /// ```
+    /// use crate::protocol::*;
+    /// use std::io::Cursor;
+    ///
+    /// let id_index = 10;
+    /// let mut buffer = Cursor::new(Vec::<u8>::new());
+    /// rln.get_leaf(id_index, &mut buffer).unwrap();
+    /// let id_commitment = deserialize_field_element(&buffer.into_inner()).unwrap();
+    pub fn get_leaf<W: Write>(&self, index: usize, mut output_data: W) -> Result<()> {
+        // We get the leaf at input index
+        let leaf = self.tree.get(index)?;
+
+        // We serialize the leaf and write it to output
+        let leaf_byte = fr_to_bytes_le(&leaf);
+        output_data.write_all(&leaf_byte)?;
+
+        Ok(())
+    }
+
     /// Sets multiple leaves starting from position index in the internal Merkle tree.
     ///
     /// If n leaves are passed as input, these will be set at positions `index`, `index+1`, ..., `index+n-1` respectively.
@@ -1829,4 +1854,32 @@ mod test {
         // We ensure that an empty value was written to output_buffer, i.e. no secret is recovered
         assert!(serialized_identity_secret_hash.is_empty());
     }
+
+    #[test]
+    fn test_get_leaf() {
+        // We generate a random tree
+        let tree_height = 10;
+        let mut rng = thread_rng();
+        let input_buffer =
+            Cursor::new(json!({ "resources_folder": TEST_RESOURCES_FOLDER }).to_string());
+        let mut rln = RLN::new(tree_height, input_buffer).unwrap();
+
+        // We generate a random leaf
+        let leaf = Fr::rand(&mut rng);
+
+        // We generate a random index
+        let index = rng.gen_range(0..rln.tree.capacity());
+
+        // We add the leaf to the tree
+        let mut buffer = Cursor::new(fr_to_bytes_le(&leaf));
+        rln.set_leaf(index, &mut buffer).unwrap();
+
+        // We get the leaf
+        let mut output_buffer = Cursor::new(Vec::<u8>::new());
+        rln.get_leaf(index, &mut output_buffer).unwrap();
+
+        // We ensure that the leaf is the same as the one we added
+        let (received_leaf, _) = bytes_le_to_fr(output_buffer.into_inner().as_ref());
+        assert_eq!(received_leaf, leaf);
+    }
 }

rln/tests/ffi.rs

@@ -1156,4 +1156,49 @@ mod test {
 
         assert_eq!(received_hash, expected_hash);
     }
+
+    #[test]
+    fn test_get_leaf() {
+        // We create a RLN instance
+        let tree_height = TEST_TREE_HEIGHT;
+        let no_of_leaves = 1 << TEST_TREE_HEIGHT;
+
+        let mut rln_pointer = MaybeUninit::<*mut RLN>::uninit();
+        let input_config = json!({ "resources_folder": TEST_RESOURCES_FOLDER }).to_string();
+        let input_buffer = &Buffer::from(input_config.as_bytes());
+        let success = new(tree_height, input_buffer, rln_pointer.as_mut_ptr());
+        assert!(success, "RLN object creation failed");
+        let rln_pointer = unsafe { &mut *rln_pointer.assume_init() };
+
+        // We generate a new identity tuple from an input seed
+        let seed_bytes: &[u8] = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
+        let input_buffer = &Buffer::from(seed_bytes);
+        let mut output_buffer = MaybeUninit::<Buffer>::uninit();
+        let success =
+            seeded_extended_key_gen(rln_pointer, input_buffer, output_buffer.as_mut_ptr());
+        assert!(success, "seeded key gen call failed");
+
+        let output_buffer = unsafe { output_buffer.assume_init() };
+        let result_data = <&[u8]>::from(&output_buffer).to_vec();
+        let (_, _, _, id_commitment) = deserialize_identity_tuple(result_data);
+
+        // We insert the id_commitment into the tree at a random index
+        let mut rng = thread_rng();
+        let index = rng.gen_range(0..no_of_leaves) as usize;
+        let leaf = fr_to_bytes_le(&id_commitment);
+        let input_buffer = &Buffer::from(leaf.as_ref());
+        let success = set_leaf(rln_pointer, index, input_buffer);
+        assert!(success, "set leaf call failed");
+
+        // We get the leaf at the same index
+        let mut output_buffer = MaybeUninit::<Buffer>::uninit();
+        let success = get_leaf(rln_pointer, index, output_buffer.as_mut_ptr());
+        assert!(success, "get leaf call failed");
+        let output_buffer = unsafe { output_buffer.assume_init() };
+        let result_data = <&[u8]>::from(&output_buffer).to_vec();
+        let (received_id_commitment, _) = bytes_le_to_fr(&result_data);
+
+        // We check that the received id_commitment is the same as the one we inserted
+        assert_eq!(received_id_commitment, id_commitment);
+    }
 }