feat: add function to test empty trees

Reviewed-on: https://git.vergara.tech///Vergara_Tech/zcash-haskell/pulls/98
Co-authored-by: Rene Vergara <rene@vergara.network>
Co-committed-by: Rene Vergara <rene@vergara.network>

librustzcash-wrapper/src/lib.rs

@@ -36,7 +36,8 @@ use incrementalmerkletree::{
     frontier::{
         CommitmentTree,
         Frontier,
-        NonEmptyFrontier
+        NonEmptyFrontier,
+        PathFiller
     },
     witness::IncrementalWitness
 };
@@ -1618,6 +1619,17 @@ pub extern "C" fn rust_wrapper_combine_orchard_nodes(
     }
 }
 
+#[no_mangle]
+pub extern "C" fn rust_wrapper_get_orchard_root(
+    level: u8,
+    out: *mut u8,
+    out_len: &mut usize
+    ){
+    let tree: CommitmentTree<MerkleHashOrchard, 32> = CommitmentTree::empty();
+    let root = tree.root_at_depth(level, PathFiller::empty());
+    let h = Hhex { bytes: root.to_bytes().to_vec() };
+    marshall_to_haskell_var(&h, out, out_len, RW);
+}
 
 #[no_mangle]
 pub extern "C" fn rust_wrapper_read_orchard_commitment_tree(

src/C/Zcash.chs

@@ -276,6 +276,13 @@ import ZcashHaskell.Types
   -> `()'
 #} 
 
+{# fun unsafe rust_wrapper_get_orchard_root as rustWrapperGetOrchardRootTest
+     { `Int8'
+     , getVarBuffer `Buffer HexString'&
+     }
+  -> `()'
+#}
+
 {# fun unsafe rust_wrapper_read_orchard_commitment_tree as rustWrapperReadOrchardCommitmentTree
      { toBorshVar* `OrchardFrontier'&
      , toBorshVar* `BS.ByteString'&

src/ZcashHaskell/Orchard.hs

@@ -218,6 +218,10 @@ getOrchardWitnessAnchor wit =
   withPureBorshVarBuffer $
   rustWrapperReadOrchardWitnessAnchor $ toBytes $ orchWit wit
 
+getOrchardRootTest :: Int -> HexString
+getOrchardRootTest level =
+  withPureBorshVarBuffer $ rustWrapperGetOrchardRootTest $ fromIntegral level
+
 -- | Update a Orchard commitment tree
 updateOrchardCommitmentTree ::
      OrchardFrontier -- ^ the base tree