feat(rln): Add IncrementalMerkleTree to RLN

- Sapling-based version
- Also Update ffi with get_root

rln/Cargo.toml

@@ -61,3 +61,4 @@ blake2 = "0.8.1"
 
 # TODO Remove this and use arkworks instead
 sapling-crypto = { package = "sapling-crypto_ce", version = "0.1.3", default-features = false }
+bellman = { package = "bellman_ce", version = "0.3.4", default-features = false }

rln/src/ffi.rs

@@ -28,6 +28,20 @@ impl<'a> From<&Buffer> for &'a [u8] {
     }
 }
 
+#[allow(clippy::not_unsafe_ptr_arg_deref)]
+#[no_mangle]
+pub extern "C" fn get_root(ctx: *const RLN, output_buffer: *mut Buffer) -> bool {
+    let rln = unsafe { &*ctx };
+    let mut output_data: Vec<u8> = Vec::new();
+    match rln.get_root(&mut output_data) {
+        Ok(_) => true,
+        Err(_) => false,
+    };
+    unsafe { *output_buffer = Buffer::from(&output_data[..]) };
+    std::mem::forget(output_data);
+    true
+}
+
 #[allow(clippy::not_unsafe_ptr_arg_deref)]
 #[no_mangle]
 pub extern "C" fn new_circuit(ctx: *mut *mut RLN) -> bool {

rln/src/public.rs

@@ -1,3 +1,6 @@
+use crate::merkle::IncrementalMerkleTree;
+use crate::poseidon::{Poseidon as PoseidonHasher, PoseidonParams};
+
 use ark_circom::{CircomBuilder, CircomCircuit, CircomConfig};
 use ark_std::rand::thread_rng;
 
@@ -17,9 +20,18 @@ use num_bigint::BigInt;
 use serde::Deserialize;
 use serde_json;
 
+// XXX
+use bellman::pairing::ff::{Field, PrimeField, PrimeFieldRepr, ScalarEngine};
+use sapling_crypto::bellman::pairing::bn256::Bn256;
+
+// TODO Add Engine here? i.e. <E: Engine> not <Bn254>
+// NOTE Bn254 vs Bn256 mismatch! Tree is originally Bn256
+// TODO Figure out Bn254 vs Bn256 mismatch
 pub struct RLN {
     circom: CircomCircuit<Bn254>,
     params: ProvingKey<Bn254>,
+    // TODO Replace Bn256 with Bn254 here
+    tree: IncrementalMerkleTree<Bn256>,
 }
 
 #[derive(Debug, Deserialize)]
@@ -141,7 +153,26 @@ impl RLN {
 
         println!("Public inputs {:#?} ", inputs);
 
-        RLN { circom, params }
+        // TODO Add as parameter(s)
+        let merkle_depth: usize = 3;
+        // XXX
+        let poseidon_params = PoseidonParams::<Bn256>::new(8, 55, 3, None, None, None);
+        let hasher = PoseidonHasher::new(poseidon_params.clone());
+        let tree = IncrementalMerkleTree::empty(hasher, merkle_depth);
+
+        RLN {
+            circom,
+            params,
+            tree,
+        }
+    }
+
+    /// returns current membership root
+    /// * `root` is a scalar field element in 32 bytes
+    pub fn get_root<W: Write>(&self, mut result_data: W) -> io::Result<()> {
+        let root = self.tree.get_root();
+        root.into_repr().write_le(&mut result_data)?;
+        Ok(())
     }
 
     // TODO Input Read