smt: refactor MerklePath logic

Computation of the correct node indices to get is moved to
`NodeIndex::proof_indices()`, and getting a node's index based on its
parent is generalized into `SparseMerkleTree::get_hash()`.

miden-crypto/src/merkle/index.rs

@@ -164,6 +164,18 @@ impl NodeIndex {
         self.depth = self.depth.saturating_sub(delta);
         self.value >>= delta as u32;
     }
+
+    // ITERATORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Return an iterator of the indices required for a Merkle proof of inclusion of a node at
+    /// `self`.
+    ///
+    /// This is *exclusive* on both ends: neither `self` nor the root index are included in the
+    /// returned iterator.
+    pub fn proof_indices(&self) -> impl ExactSizeIterator<Item = NodeIndex> + use<> {
+        ProofIter { next_index: self.sibling() }
+    }
 }
 
 impl Display for NodeIndex {
@@ -188,6 +200,39 @@ impl Deserializable for NodeIndex {
     }
 }
 
+/// Implementation for [`NodeIndex::proof_indices()`].
+#[derive(Debug, Default, Copy, Clone, Eq, PartialEq, Hash)]
+struct ProofIter {
+    next_index: NodeIndex,
+}
+
+impl Iterator for ProofIter {
+    type Item = NodeIndex;
+
+    fn next(&mut self) -> Option<NodeIndex> {
+        if self.next_index.is_root() {
+            return None;
+        }
+
+        let index = self.next_index;
+        self.next_index = index.parent().sibling();
+
+        Some(index)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let remaining = ExactSizeIterator::len(self);
+
+        (remaining, Some(remaining))
+    }
+}
+
+impl ExactSizeIterator for ProofIter {
+    fn len(&self) -> usize {
+        self.next_index.depth() as usize
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use assert_matches::assert_matches;

miden-crypto/src/merkle/smt/mod.rs

@@ -79,28 +79,34 @@ pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
     // PROVIDED METHODS
     // ---------------------------------------------------------------------------------------------
 
+    /// Returns a [MerklePath] to the specified key.
+    ///
+    /// Mostly this is an implementation detail of [`Self::open()`].
+    fn get_path(&self, key: &Self::Key) -> MerklePath {
+        let index = NodeIndex::from(Self::key_to_leaf_index(key));
+        index.proof_indices().map(|index| self.get_hash(index)).collect()
+    }
+
+    /// Get the hash of a node at an arbitrary index, including the root or leaf hashes.
+    ///
+    /// The root index simply returns [`Self::root()`]. Other hashes are retrieved by calling
+    /// [`Self::get_inner_node()`] on the parent, and returning the respective child hash.
+    fn get_hash(&self, index: NodeIndex) -> RpoDigest {
+        if index.is_root() {
+            return self.root();
+        }
+
+        let InnerNode { left, right } = self.get_inner_node(index.parent());
+
+        let index_is_right = index.is_value_odd();
+        if index_is_right { right } else { left }
+    }
+
     /// Returns an opening of the leaf associated with `key`. Conceptually, an opening is a Merkle
     /// path to the leaf, as well as the leaf itself.
     fn open(&self, key: &Self::Key) -> Self::Opening {
         let leaf = self.get_leaf(key);
-
-        let mut index: NodeIndex = {
-            let leaf_index: LeafIndex<DEPTH> = Self::key_to_leaf_index(key);
-            leaf_index.into()
-        };
-
-        let merkle_path = {
-            let mut path = Vec::with_capacity(index.depth() as usize);
-            for _ in 0..index.depth() {
-                let is_right = index.is_value_odd();
-                index.move_up();
-                let InnerNode { left, right } = self.get_inner_node(index);
-                let value = if is_right { left } else { right };
-                path.push(value);
-            }
-
-            MerklePath::new(path)
-        };
+        let merkle_path = self.get_path(key);
 
         Self::path_and_leaf_to_opening(merkle_path, leaf)
     }