feat: add merkle node index

This commit introduces a wrapper structure to encapsulate the merkle
tree traversal.

related issue: #36

src/merkle/mod.rs

@@ -5,6 +5,9 @@ use super::{
 };
 use core::fmt;
 
+mod index;
+pub use index::NodeIndex;
+
 mod merkle_tree;
 pub use merkle_tree::MerkleTree;
 
@@ -22,11 +25,11 @@ pub use simple_smt::SimpleSmt;
 
 #[derive(Clone, Debug)]
 pub enum MerkleError {
-    DepthTooSmall(u32),
-    DepthTooBig(u32),
+    DepthTooSmall(u8),
+    DepthTooBig(u8),
     NumLeavesNotPowerOfTwo(usize),
-    InvalidIndex(u32, u64),
-    InvalidDepth(u32, u32),
+    InvalidIndex(NodeIndex),
+    InvalidDepth { expected: u8, provided: u8 },
     InvalidPath(MerklePath),
     InvalidEntriesCount(usize, usize),
     NodeNotInSet(u64),
@@ -41,11 +44,11 @@ impl fmt::Display for MerkleError {
             NumLeavesNotPowerOfTwo(leaves) => {
                 write!(f, "the leaves count {leaves} is not a power of 2")
             }
-            InvalidIndex(depth, index) => write!(
+            InvalidIndex(index) => write!(
                 f,
-                "the leaf index {index} is not valid for the depth {depth}"
+                "the index value {} is not valid for the depth {}", index.value(), index.depth()
             ),
-            InvalidDepth(expected, provided) => write!(
+            InvalidDepth { expected, provided } => write!(
                 f,
                 "the provided depth {provided} is not valid for {expected}"
             ),