Fix a bug in the merkle algorithm

coordinator/tributary/src/merkle.rs

@@ -18,7 +18,7 @@ pub(crate) fn merkle(hash_args: &[[u8; 32]]) -> [u8; 32] {
           b"branch_hash".as_ref(),
           hashes[i].as_ref(),
           hashes
-            .get(i + i)
+            .get(i + 1)
             .map(|hash| {
               let res: &[u8] = hash.as_ref();
               res

coordinator/tributary/src/tests/merkle.rs

@@ -0,0 +1,34 @@
+use std::collections::HashSet;
+
+use rand_core::{RngCore, OsRng};
+
+#[test]
+fn merkle() {
+  let mut used = HashSet::new();
+  // Test this produces a unique root
+  let mut test = |hashes: &[[u8; 32]]| {
+    let hash = crate::merkle(hashes);
+    assert!(!used.contains(&hash));
+    used.insert(hash);
+  };
+
+  // Zero should be a special case which return 0
+  assert_eq!(crate::merkle(&[]), [0; 32]);
+  test(&[]);
+
+  let mut one = [0; 32];
+  OsRng.fill_bytes(&mut one);
+  let mut two = [0; 32];
+  OsRng.fill_bytes(&mut two);
+  let mut three = [0; 32];
+  OsRng.fill_bytes(&mut three);
+
+  // Make sure it's deterministic
+  assert_eq!(crate::merkle(&[one]), crate::merkle(&[one]));
+
+  // Test a few basic structures
+  test(&[one]);
+  test(&[one, two]);
+  test(&[one, two, three]);
+  test(&[one, three]);
+}