Calculate incremental note commitment trees (#2407)

* Make sapling::tree::NoteCommitmentTree incrementally updateable

* Make orchard::tree::NoteCommitmentTree incrementally updateable

* Apply suggestions from code review

Co-authored-by: teor <teor@riseup.net>

* Changed to incrementalmerkletree implementation; update MerkleCRH^Orchard

* Improvements from review; organize file names (vectors.rs -> tree.rs)

Co-authored-by: Conrado Gouvea <conrado@zfnd.org>
Co-authored-by: teor <teor@riseup.net>
Co-authored-by: Conrado P. L. Gouvea <conradoplg@gmail.com>

zebra-chain/Cargo.toml

@@ -46,6 +46,7 @@ zcash_primitives = { git = "https://github.com/zcash/librustzcash.git", rev = "0
 bigint = "4"
 uint = "0.9.1"
 bls12_381 = "0.5.0"
+incrementalmerkletree = "0.1.0"
 
 proptest = { version = "0.10", optional = true }
 proptest-derive = { version = "0.3.0", optional = true }

zebra-chain/src/orchard/tests.rs

@@ -1,3 +1,4 @@
 mod preallocate;
 mod prop;
-pub mod test_vectors;
+mod test_vectors;
+mod tree;

zebra-chain/src/orchard/tests/tree.rs

@@ -0,0 +1,47 @@
+use halo2::arithmetic::FieldExt;
+use halo2::pasta::pallas;
+
+use crate::orchard::tests::test_vectors;
+use crate::orchard::tree::*;
+
+#[test]
+fn empty_roots() {
+    zebra_test::init();
+
+    for i in 0..EMPTY_ROOTS.len() {
+        assert_eq!(
+            EMPTY_ROOTS[i].to_bytes(),
+            // The test vector is in reversed order.
+            test_vectors::EMPTY_ROOTS[MERKLE_DEPTH - i]
+        );
+    }
+}
+
+#[test]
+fn incremental_roots() {
+    zebra_test::init();
+
+    let mut leaves = vec![];
+
+    let mut incremental_tree = NoteCommitmentTree::default();
+
+    for (i, commitment_set) in test_vectors::COMMITMENTS.iter().enumerate() {
+        for cm_x_bytes in commitment_set.iter() {
+            let cm_x = pallas::Base::from_bytes(&cm_x_bytes).unwrap();
+
+            leaves.push(cm_x);
+
+            let _ = incremental_tree.append(cm_x);
+        }
+
+        assert_eq!(
+            hex::encode(incremental_tree.hash()),
+            hex::encode(test_vectors::ROOTS[i].anchor)
+        );
+
+        assert_eq!(
+            hex::encode((NoteCommitmentTree::from(leaves.clone())).hash()),
+            hex::encode(test_vectors::ROOTS[i].anchor)
+        );
+    }
+}