smt: change SimpleSmt::open() to return a sparse path

This is in preparation for the next commit, where we change
`SimpleSmt::open()` to return a `SparseMerklePath`, which cannot
dereference to a Vec.

CHANGELOG.md

@@ -1,6 +1,5 @@
 ## 0.15.0 (TBD)
 
-- Added default constructors to `MmrPeaks` and `PartialMmr` (#409).
 
 ## 0.14.0 (2025-03-15)
 

Cargo.lock

@@ -254,7 +254,7 @@ dependencies = [
  "clap",
  "criterion-plot",
  "is-terminal",
- "itertools 0.10.5",
+ "itertools",
  "num-traits",
  "once_cell",
  "oorandom",
@@ -275,7 +275,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6b50826342786a51a89e2da3a28f1c32b06e387201bc2d19791f622c673706b1"
 dependencies = [
  "cast",
- "itertools 0.10.5",
+ "itertools",
 ]
 
 [[package]]
@@ -453,15 +453,6 @@ dependencies = [
  "either",
 ]
 
-[[package]]
-name = "itertools"
-version = "0.14.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2b192c782037fadd9cfa75548310488aabdbf3d2da73885b31bd0abd03351285"
-dependencies = [
- "either",
-]
-
 [[package]]
 name = "itoa"
 version = "1.0.15"
@@ -533,7 +524,6 @@ dependencies = [
  "glob",
  "hashbrown",
  "hex",
- "itertools 0.14.0",
  "num",
  "num-complex",
  "proptest",

miden-crypto/Cargo.toml

@@ -83,7 +83,6 @@ assert_matches = { version = "1.5", default-features = false }
 criterion = { version = "0.5", features = ["html_reports"] }
 getrandom = { version = "0.3", default-features = false }
 hex = { version = "0.4", default-features = false, features = ["alloc"] }
-itertools = { version = "0.14" }
 proptest = { version = "1.6", default-features = false, features = ["alloc"]}
 rand_chacha = { version = "0.9", default-features = false }
 rand-utils = { version = "0.12", package = "winter-rand-utils" }

miden-crypto/src/merkle/mmr/partial.rs

@@ -70,18 +70,6 @@ pub struct PartialMmr {
     pub(crate) track_latest: bool,
 }
 
-impl Default for PartialMmr {
-    /// Creates a new [PartialMmr] with default values.
-    fn default() -> Self {
-        let forest = 0;
-        let peaks = Vec::new();
-        let nodes = BTreeMap::new();
-        let track_latest = false;
-
-        Self { forest, peaks, nodes, track_latest }
-    }
-}
-
 impl PartialMmr {
     // CONSTRUCTORS
     // --------------------------------------------------------------------------------------------

miden-crypto/src/merkle/mmr/peaks.rs

@@ -34,13 +34,6 @@ pub struct MmrPeaks {
     peaks: Vec<RpoDigest>,
 }
 
-impl Default for MmrPeaks {
-    /// Returns new [`MmrPeaks`] instantiated from an empty vector of peaks and 0 leaves.
-    fn default() -> Self {
-        Self { num_leaves: 0, peaks: Vec::new() }
-    }
-}
-
 impl MmrPeaks {
     // CONSTRUCTOR
     // --------------------------------------------------------------------------------------------

miden-crypto/src/merkle/mmr/tests.rs

@@ -12,23 +12,6 @@ use crate::{
     merkle::{InOrderIndex, MerklePath, MerkleTree, MmrProof, NodeIndex, int_to_node},
 };
 
-#[test]
-fn tests_empty_mmr_peaks() {
-    let peaks = MmrPeaks::default();
-    assert_eq!(peaks.num_peaks(), 0);
-    assert_eq!(peaks.num_leaves(), 0);
-}
-
-#[test]
-fn test_empty_partial_mmr() {
-    let mmr = PartialMmr::default();
-    assert_eq!(mmr.num_leaves(), 0);
-    assert_eq!(mmr.forest(), 0);
-    assert_eq!(mmr.peaks(), MmrPeaks::default());
-    assert!(mmr.nodes.is_empty());
-    assert!(!mmr.track_latest);
-}
-
 #[test]
 fn test_position_equal_or_higher_than_leafs_is_never_contained() {
     let empty_forest = 0;

miden-crypto/src/merkle/path.rs

@@ -29,7 +29,7 @@ impl MerklePath {
 
     /// Creates a new Merkle path from a list of nodes.
     ///
-    /// The list must be in order of deepest to shallowest.
+    /// The list is assumed to be in order of deepest to shallowest.
     pub fn new(nodes: Vec<RpoDigest>) -> Self {
         assert!(nodes.len() <= u8::MAX.into(), "MerklePath may have at most 256 items");
         Self { nodes }
@@ -43,9 +43,19 @@ impl MerklePath {
     /// The `depth` parameter is defined in terms of `self.depth()`. Merkle paths conventionally do
     /// not include the root, so the shallowest depth is `1`, and the deepest depth is
     /// `self.depth()`.
-    pub fn at_depth(&self, depth: NonZero<u8>) -> Option<RpoDigest> {
+    pub fn at_depth(&self, depth: NonZero<u8>) -> Option<&RpoDigest> {
         let index = u8::checked_sub(self.depth(), depth.get())?;
-        self.nodes.get(index as usize).copied()
+        self.nodes.get(index as usize)
+    }
+
+    /// Returns a reference to the path node at the specified index, or [None] if the index is out
+    /// of bounds.
+    ///
+    /// The node at index 0 is the deepest part of the path.
+    ///
+    /// This is a checked version of using the indexing operator `[]`.
+    pub fn at_idx(&self, index: usize) -> Option<&RpoDigest> {
+        self.nodes.get(index)
     }
 
     /// Returns the depth in which this Merkle path proof is valid.

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

@@ -84,14 +84,14 @@ pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
     /// 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_node_hash(index)).collect()
+        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_node_hash(&self, index: NodeIndex) -> RpoDigest {
+    fn get_hash(&self, index: NodeIndex) -> RpoDigest {
         if index.is_root() {
             return self.root();
         }

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

@@ -1,11 +1,12 @@
 use alloc::collections::BTreeSet;
 
+use crate::merkle::{SparseMerklePath, SparseValuePath};
+
 use super::{
     super::ValuePath, EMPTY_WORD, EmptySubtreeRoots, InnerNode, InnerNodeInfo, InnerNodes,
     LeafIndex, MerkleError, MerklePath, MutationSet, NodeIndex, RpoDigest, SMT_MAX_DEPTH,
     SMT_MIN_DEPTH, SparseMerkleTree, Word,
 };
-use crate::merkle::{SparseMerklePath, SparseValuePath};
 
 #[cfg(test)]
 mod tests;
@@ -172,7 +173,7 @@ impl<const DEPTH: u8> SimpleSmt<DEPTH> {
     /// path to the leaf, as well as the leaf itself.
     pub fn open(&self, key: &LeafIndex<DEPTH>) -> SparseValuePath {
         let value = RpoDigest::new(self.get_value(key));
-        let nodes = key.index.proof_indices().map(|index| self.get_node_hash(index));
+        let nodes = key.index.proof_indices().map(|index| self.get_hash(index));
         // `from_sized_iter()` returns an error if there are more nodes than `SMT_MAX_DEPTH`, but
         // this could only happen if we have more levels than `SMT_MAX_DEPTH` ourselves, which is
         // guarded against in `SimpleSmt::new()`.

miden-crypto/src/merkle/sparse_path.rs

@@ -1,4 +1,4 @@
-use alloc::{borrow::Cow, vec::Vec};
+use alloc::vec::Vec;
 use core::{
     iter::{self, FusedIterator},
     num::NonZero,
@@ -14,10 +14,10 @@ use super::{
 /// with empty nodes.
 ///
 /// Empty nodes in the path are stored only as their position, represented with a bitmask. A
-/// maximum of 64 nodes (`SMT_MAX_DEPTH`) can be stored (empty and non-empty). The more nodes in a
-/// path are empty, the less memory this struct will use. This type calculates empty nodes on-demand
-/// when iterated through, converted to a [MerklePath], or an empty node is retrieved with
-/// [`SparseMerklePath::at_depth()`], which will incur overhead.
+/// maximum of 64 nodes in the path can be empty. The more nodes in a path are empty, the less
+/// memory this struct will use. This type calculates empty nodes on-demand when iterated through,
+/// converted to a [MerklePath], or an empty node is retrieved with [`SparseMerklePath::at_idx()`]
+/// or [`SparseMerklePath::at_depth()`], which will incur overhead.
 ///
 /// NOTE: This type assumes that Merkle paths always span from the root of the tree to a leaf.
 /// Partial paths are not supported.
@@ -25,8 +25,6 @@ use super::{
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct SparseMerklePath {
     /// A bitmask representing empty nodes. The set bit corresponds to the depth of an empty node.
-    /// The least significant bit (bit 0) describes depth 1 node (root's children).
-    /// The `bit index + 1` is equal to node's depth.
     empty_nodes_mask: u64,
     /// The non-empty nodes, stored in depth-order, but not contiguous across depth.
     nodes: Vec<RpoDigest>,
@@ -38,7 +36,8 @@ impl SparseMerklePath {
     /// of deepest to shallowest.
     ///
     /// Knowing the size is necessary to calculate the depth of the tree, which is needed to detect
-    /// which nodes are empty nodes.
+    /// which nodes are empty nodes. If you know the size but your iterator type is not
+    /// [ExactSizeIterator], use [`SparseMerklePath::from_iter_with_depth()`].
     ///
     /// # Errors
     /// Returns [MerkleError::DepthTooBig] if `tree_depth` is greater than [SMT_MAX_DEPTH].
@@ -47,27 +46,38 @@ impl SparseMerklePath {
         I: IntoIterator<IntoIter: ExactSizeIterator, Item = RpoDigest>,
     {
         let iterator = iterator.into_iter();
-        let tree_depth = iterator.len() as u8;
+        // `iterator.len() as u8` will truncate, but not below `SMT_MAX_DEPTH`, which
+        // `from_iter_with_depth` checks for.
+        Self::from_iter_with_depth(iterator.len() as u8, iterator)
+    }
 
+    /// Constructs a sparse Merkle path from a manually specified tree depth, and an iterator over
+    /// Merkle nodes from deepest to shallowest.
+    ///
+    /// Knowing the size is necessary to calculate the depth of the tree, which is needed to detect
+    /// which nodes are empty nodes.
+    ///
+    /// # Errors
+    /// Returns [MerkleError::DepthTooBig] if `tree_depth` is greater than [SMT_MAX_DEPTH].
+    pub fn from_iter_with_depth(
+        tree_depth: u8,
+        iter: impl IntoIterator<Item = RpoDigest>,
+    ) -> Result<Self, MerkleError> {
         if tree_depth > SMT_MAX_DEPTH {
             return Err(MerkleError::DepthTooBig(tree_depth as u64));
         }
 
-        let mut empty_nodes_mask: u64 = 0;
-        let mut nodes: Vec<RpoDigest> = Default::default();
-
-        for (depth, node) in iter::zip(path_depth_iter(tree_depth), iterator) {
+        let path: Self = iter::zip(path_depth_iter(tree_depth), iter)
+            .map(|(depth, node)| {
                 let &equivalent_empty_node = EmptySubtreeRoots::entry(tree_depth, depth.get());
                 let is_empty = node == equivalent_empty_node;
                 let node = if is_empty { None } else { Some(node) };
 
-            match node {
-                Some(node) => nodes.push(node),
-                None => empty_nodes_mask |= Self::bitmask_for_depth(depth),
-            }
-        }
+                (depth, node)
+            })
+            .collect();
 
-        Ok(SparseMerklePath { nodes, empty_nodes_mask })
+        Ok(path)
     }
 
     /// Returns the total depth of this path, i.e., the number of nodes this path represents.
@@ -85,24 +95,63 @@ impl SparseMerklePath {
     /// Returns [MerkleError::DepthTooBig] if `node_depth` is greater than the total depth of this
     /// path.
     pub fn at_depth(&self, node_depth: NonZero<u8>) -> Result<RpoDigest, MerkleError> {
+        let node = self
+            .at_depth_nonempty(node_depth)?
+            .unwrap_or_else(|| *EmptySubtreeRoots::entry(self.depth(), node_depth.get()));
+
+        Ok(node)
+    }
+
+    /// Get a specific non-empty node in this path at a given depth, or `None` if the specified
+    /// node is an empty node.
+    ///
+    /// # Errors
+    /// Returns [MerkleError::DepthTooBig] if `node_depth` is greater than the total depth of this
+    /// path.
+    pub fn at_depth_nonempty(
+        &self,
+        node_depth: NonZero<u8>,
+    ) -> Result<Option<RpoDigest>, MerkleError> {
         if node_depth.get() > self.depth() {
             return Err(MerkleError::DepthTooBig(node_depth.get().into()));
         }
 
-        let node = if let Some(nonempty_index) = self.get_nonempty_index(node_depth) {
-            self.nodes[nonempty_index]
-        } else {
-            *EmptySubtreeRoots::entry(self.depth(), node_depth.get())
-        };
+        if self.is_depth_empty(node_depth) {
+            return Ok(None);
+        }
 
-        Ok(node)
+        // Our index needs to account for all the empty nodes that aren't in `self.nodes`.
+        let nonempty_index = self.get_nonempty_index(node_depth);
+
+        Ok(Some(self.nodes[nonempty_index]))
+    }
+
+    /// Returns the path node at the specified index, or [None] if the index is out of bounds.
+    ///
+    /// The node at index 0 is the deepest part of the path.
+    ///
+    /// ```
+    /// # use core::num::NonZero;
+    /// # use miden_crypto::{ZERO, ONE, hash::rpo::RpoDigest, merkle::SparseMerklePath};
+    /// # let zero = RpoDigest::new([ZERO; 4]);
+    /// # let one = RpoDigest::new([ONE; 4]);
+    /// # let sparse_path = SparseMerklePath::from_sized_iter(vec![zero, one, one, zero]).unwrap();
+    /// let depth = NonZero::new(sparse_path.depth()).unwrap();
+    /// assert_eq!(
+    ///     sparse_path.at_idx(0).unwrap(),
+    ///     sparse_path.at_depth(depth).unwrap(),
+    /// );
+    /// ```
+    pub fn at_idx(&self, index: usize) -> Option<RpoDigest> {
+        // If this overflows *or* if the depth is zero then the index was out of bounds.
+        let depth = NonZero::new(u8::checked_sub(self.depth(), index as u8)?)?;
+        self.at_depth(depth).ok()
     }
 
     // PROVIDERS
     // ============================================================================================
 
     /// Constructs a borrowing iterator over the nodes in this path.
-    /// Starts from the leaf and iterates toward the root (excluding the root).
     pub fn iter(&self) -> impl ExactSizeIterator<Item = RpoDigest> {
         self.into_iter()
     }
@@ -119,20 +168,14 @@ impl SparseMerklePath {
         (self.empty_nodes_mask & Self::bitmask_for_depth(node_depth)) != 0
     }
 
-    /// Index of the non-empty node in the `self.nodes` vector. If the specified depth is
-    /// empty, None is returned.
-    fn get_nonempty_index(&self, node_depth: NonZero<u8>) -> Option<usize> {
-        if self.is_depth_empty(node_depth) {
-            return None;
-        }
-
+    fn get_nonempty_index(&self, node_depth: NonZero<u8>) -> usize {
         let bit_index = node_depth.get() - 1;
         let without_shallower = self.empty_nodes_mask >> bit_index;
         let empty_deeper = without_shallower.count_ones() as usize;
         // The vec index we would use if we didn't have any empty nodes to account for...
         let normal_index = (self.depth() - node_depth.get()) as usize;
         // subtracted by the number of empty nodes that are deeper than us.
-        Some(normal_index - empty_deeper)
+        normal_index - empty_deeper
     }
 }
 
@@ -152,21 +195,8 @@ impl Deserializable for SparseMerklePath {
         source: &mut R,
     ) -> Result<Self, DeserializationError> {
         let depth = source.read_u8()?;
-        if depth > SMT_MAX_DEPTH {
-            return Err(DeserializationError::InvalidValue(format!(
-                "SparseMerklePath max depth exceeded ({} > {})",
-                depth, SMT_MAX_DEPTH
-            )));
-        }
         let empty_nodes_mask = source.read_u64()?;
-        let empty_nodes_count = empty_nodes_mask.count_ones();
-        if empty_nodes_count > depth as u32 {
-            return Err(DeserializationError::InvalidValue(format!(
-                "SparseMerklePath has more empty nodes ({}) than its full length ({})",
-                empty_nodes_count, depth
-            )));
-        }
-        let count = depth as u32 - empty_nodes_count;
+        let count = depth as u32 - empty_nodes_mask.count_ones();
         let nodes = source.read_many::<RpoDigest>(count as usize)?;
         Ok(Self { empty_nodes_mask, nodes })
     }
@@ -181,13 +211,13 @@ impl From<SparseMerklePath> for MerklePath {
     }
 }
 
-impl TryFrom<MerklePath> for SparseMerklePath {
-    type Error = MerkleError;
-
 /// # Errors
 ///
 /// This conversion returns [MerkleError::DepthTooBig] if the path length is greater than
 /// [`SMT_MAX_DEPTH`].
+impl TryFrom<MerklePath> for SparseMerklePath {
+    type Error = MerkleError;
+
     fn try_from(path: MerklePath) -> Result<Self, MerkleError> {
         SparseMerklePath::from_sized_iter(path)
     }
@@ -202,11 +232,41 @@ impl From<SparseMerklePath> for Vec<RpoDigest> {
 // ITERATORS
 // ================================================================================================
 
-/// Iterator for [`SparseMerklePath`]. Starts from the leaf and iterates toward the root (excluding
-/// the root).
+/// Contructs a [SparseMerklePath] out of an iterator of optional nodes, where `None` indicates an
+/// empty node.
+impl FromIterator<(NonZero<u8>, Option<RpoDigest>)> for SparseMerklePath {
+    fn from_iter<I>(iter: I) -> SparseMerklePath
+    where
+        I: IntoIterator<Item = (NonZero<u8>, Option<RpoDigest>)>,
+    {
+        let mut empty_nodes_mask: u64 = 0;
+        let mut nodes: Vec<RpoDigest> = Default::default();
+
+        for (depth, node) in iter {
+            match node {
+                Some(node) => nodes.push(node),
+                None => empty_nodes_mask |= Self::bitmask_for_depth(depth),
+            }
+        }
+
+        SparseMerklePath { nodes, empty_nodes_mask }
+    }
+}
+
+impl<'p> IntoIterator for &'p SparseMerklePath {
+    type Item = <SparseMerklePathIter<'p> as Iterator>::Item;
+    type IntoIter = SparseMerklePathIter<'p>;
+
+    fn into_iter(self) -> SparseMerklePathIter<'p> {
+        let tree_depth = self.depth();
+        SparseMerklePathIter { path: self, next_depth: tree_depth }
+    }
+}
+
+/// Borrowing iterator for [`SparseMerklePath`].
 pub struct SparseMerklePathIter<'p> {
     /// The "inner" value we're iterating over.
-    path: Cow<'p, SparseMerklePath>,
+    path: &'p SparseMerklePath,
 
     /// The depth a `next()` call will get. `next_depth == 0` indicates that the iterator has been
     /// exhausted.
@@ -223,10 +283,7 @@ impl Iterator for SparseMerklePathIter<'_> {
         self.next_depth = this_depth.get() - 1;
 
         // `this_depth` is only ever decreasing, so it can't ever exceed `self.path.depth()`.
-        let node = self
-            .path
-            .at_depth(this_depth)
-            .expect("current depth should never exceed the path depth");
+        let node = self.path.at_depth(this_depth).unwrap();
         Some(node)
     }
 
@@ -247,31 +304,56 @@ impl FusedIterator for SparseMerklePathIter<'_> {}
 
 // TODO: impl DoubleEndedIterator.
 
+/// Owning iterator for [SparseMerklePath].
+pub struct IntoIter {
+    /// The "inner" value we're iterating over.
+    path: SparseMerklePath,
+
+    /// The depth a `next()` call will get. `next_depth == 0` indicates that the iterator has been
+    /// exhausted.
+    next_depth: u8,
+}
+
 impl IntoIterator for SparseMerklePath {
-    type IntoIter = SparseMerklePathIter<'static>;
+    type IntoIter = IntoIter;
     type Item = <Self::IntoIter as Iterator>::Item;
 
-    fn into_iter(self) -> SparseMerklePathIter<'static> {
+    fn into_iter(self) -> IntoIter {
         let tree_depth = self.depth();
-        SparseMerklePathIter {
-            path: Cow::Owned(self),
-            next_depth: tree_depth,
-        }
+        IntoIter { path: self, next_depth: tree_depth }
     }
 }
 
-impl<'p> IntoIterator for &'p SparseMerklePath {
-    type Item = <SparseMerklePathIter<'p> as Iterator>::Item;
-    type IntoIter = SparseMerklePathIter<'p>;
+impl Iterator for IntoIter {
+    type Item = RpoDigest;
 
-    fn into_iter(self) -> SparseMerklePathIter<'p> {
-        let tree_depth = self.depth();
-        SparseMerklePathIter {
-            path: Cow::Borrowed(self),
-            next_depth: tree_depth,
+    fn next(&mut self) -> Option<RpoDigest> {
+        let this_depth = self.next_depth;
+        // Paths don't include the root, so if `this_depth` is 0 then we keep returning `None`.
+        let this_depth = NonZero::new(this_depth)?;
+        self.next_depth = this_depth.get() - 1;
+
+        // `this_depth` is only ever decreasing, so it can't ever exceed `self.path.depth()`.
+        let node = self.path.at_depth(this_depth).unwrap();
+        Some(node)
+    }
+
+    // IntoIter always knows its exact size.
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let remaining = ExactSizeIterator::len(self);
+        (remaining, Some(remaining))
     }
 }
+
+impl ExactSizeIterator for IntoIter {
+    fn len(&self) -> usize {
+        self.next_depth as usize
     }
+}
+
+impl FusedIterator for IntoIter {}
+
+// TODO: impl DoubleEndedIterator.
 
 // COMPARISONS
 // ================================================================================================
@@ -324,13 +406,13 @@ impl From<(SparseMerklePath, Word)> for SparseValuePath {
     }
 }
 
-impl TryFrom<ValuePath> for SparseValuePath {
-    type Error = MerkleError;
-
 /// # Errors
 ///
 /// This conversion returns [MerkleError::DepthTooBig] if the path length is greater than
 /// [`SMT_MAX_DEPTH`].
+impl TryFrom<ValuePath> for SparseValuePath {
+    type Error = MerkleError;
+
     fn try_from(other: ValuePath) -> Result<Self, MerkleError> {
         let ValuePath { value, path } = other;
         let path = SparseMerklePath::try_from(path)?;
@@ -374,12 +456,10 @@ fn path_depth_iter(tree_depth: u8) -> impl ExactSizeIterator<Item = NonZero<u8>>
     top_down_iter.rev()
 }
 
-// TESTS
-// ================================================================================================
 #[cfg(test)]
 mod tests {
     use alloc::vec::Vec;
-    use core::num::NonZero;
+    use core::{iter, num::NonZero};
 
     use assert_matches::assert_matches;
 
@@ -477,34 +557,145 @@ mod tests {
 
         assert_eq!(sparse_path.empty_nodes_mask, EMPTY_BITS);
 
-        // Keep track of how many non-empty nodes we have seen
-        let mut nonempty_idx = 0;
+        // Depth 8.
+        {
+            let depth: u8 = 8;
 
-        // Test starting from the deepest nodes (depth 8)
-        for depth in (1..=8).rev() {
+            // Check that the way we calculate these indices is correct.
             let idx = (sparse_path.depth() - depth) as usize;
-            let bit = 1 << (depth - 1);
+            assert_eq!(idx, 0);
 
-            // Check that the depth bit is set correctly...
+            // Check that the way we calculate these bitmasks is correct.
+            let bit = 0b1000_0000;
+            assert_eq!(bit, 1 << (depth - 1));
+
+            // Check that the depth-8 bit is not set...
             let is_set = (sparse_path.empty_nodes_mask & bit) != 0;
+            assert!(!is_set);
+            // ...which should match the status of the `sparse_nodes` element being `None`.
             assert_eq!(is_set, sparse_nodes.get(idx).unwrap().is_none());
 
-            if is_set {
-                // Check that we don't return digests for empty nodes
-                let &test_node = sparse_nodes.get(idx).unwrap();
-                assert_eq!(test_node, None);
-            } else {
-                // Check that we can calculate non-empty indices correctly.
+            // And finally, check that we can calculate non-empty indices correctly.
             let control_node = raw_nodes.get(idx).unwrap();
-                assert_eq!(
-                    sparse_path.get_nonempty_index(NonZero::new(depth).unwrap()).unwrap(),
-                    nonempty_idx
-                );
+            let nonempty_idx: usize = 0;
+            assert_eq!(sparse_path.get_nonempty_index(NonZero::new(depth).unwrap()), nonempty_idx);
             let test_node = sparse_path.nodes.get(nonempty_idx).unwrap();
             assert_eq!(test_node, control_node);
-
-                nonempty_idx += 1;
         }
+
+        // Rinse and repeat for each remaining depth.
+
+        // Depth 7.
+        {
+            let depth: u8 = 7;
+            let idx = (sparse_path.depth() - depth) as usize;
+            assert_eq!(idx, 1);
+            let bit = 0b0100_0000;
+            assert_eq!(bit, 1 << (depth - 1));
+            let is_set = (sparse_path.empty_nodes_mask & bit) != 0;
+            assert!(is_set);
+            assert_eq!(is_set, sparse_nodes.get(idx).unwrap().is_none());
+
+            let &test_node = sparse_nodes.get(idx).unwrap();
+            assert_eq!(test_node, None);
+        }
+
+        // Depth 6.
+        {
+            let depth: u8 = 6;
+            let idx = (sparse_path.depth() - depth) as usize;
+            assert_eq!(idx, 2);
+            let bit = 0b0010_0000;
+            assert_eq!(bit, 1 << (depth - 1));
+            let is_set = (sparse_path.empty_nodes_mask & bit) != 0;
+            assert_eq!(is_set, sparse_nodes.get(idx).unwrap().is_none());
+            assert!(is_set);
+
+            let &test_node = sparse_nodes.get(idx).unwrap();
+            assert_eq!(test_node, None);
+        }
+
+        // Depth 5.
+        {
+            let depth: u8 = 5;
+            let idx = (sparse_path.depth() - depth) as usize;
+            assert_eq!(idx, 3);
+            let bit = 0b0001_0000;
+            assert_eq!(bit, 1 << (depth - 1));
+            let is_set = (sparse_path.empty_nodes_mask & bit) != 0;
+            assert_eq!(is_set, sparse_nodes.get(idx).unwrap().is_none());
+            assert!(!is_set);
+
+            let control_node = raw_nodes.get(idx).unwrap();
+            let nonempty_idx: usize = 1;
+            assert_eq!(sparse_path.nodes.get(nonempty_idx).unwrap(), control_node);
+            assert_eq!(sparse_path.get_nonempty_index(NonZero::new(depth).unwrap()), nonempty_idx,);
+            let test_node = sparse_path.nodes.get(nonempty_idx).unwrap();
+            assert_eq!(test_node, control_node);
+        }
+
+        // Depth 4.
+        {
+            let depth: u8 = 4;
+            let idx = (sparse_path.depth() - depth) as usize;
+            assert_eq!(idx, 4);
+            let bit = 0b0000_1000;
+            assert_eq!(bit, 1 << (depth - 1));
+            let is_set = (sparse_path.empty_nodes_mask & bit) != 0;
+            assert_eq!(is_set, sparse_nodes.get(idx).unwrap().is_none());
+            assert!(!is_set);
+
+            let control_node = raw_nodes.get(idx).unwrap();
+            let nonempty_idx: usize = 2;
+            assert_eq!(sparse_path.nodes.get(nonempty_idx).unwrap(), control_node);
+            assert_eq!(sparse_path.get_nonempty_index(NonZero::new(depth).unwrap()), nonempty_idx,);
+            let test_node = sparse_path.nodes.get(nonempty_idx).unwrap();
+            assert_eq!(test_node, control_node);
+        }
+
+        // Depth 3.
+        {
+            let depth: u8 = 3;
+            let idx = (sparse_path.depth() - depth) as usize;
+            assert_eq!(idx, 5);
+            let bit = 0b0000_0100;
+            assert_eq!(bit, 1 << (depth - 1));
+            let is_set = (sparse_path.empty_nodes_mask & bit) != 0;
+            assert!(is_set);
+            assert_eq!(is_set, sparse_nodes.get(idx).unwrap().is_none());
+
+            let &test_node = sparse_nodes.get(idx).unwrap();
+            assert_eq!(test_node, None);
+        }
+
+        // Depth 2.
+        {
+            let depth: u8 = 2;
+            let idx = (sparse_path.depth() - depth) as usize;
+            assert_eq!(idx, 6);
+            let bit = 0b0000_0010;
+            assert_eq!(bit, 1 << (depth - 1));
+            let is_set = (sparse_path.empty_nodes_mask & bit) != 0;
+            assert!(is_set);
+            assert_eq!(is_set, sparse_nodes.get(idx).unwrap().is_none());
+
+            let &test_node = sparse_nodes.get(idx).unwrap();
+            assert_eq!(test_node, None);
+        }
+
+        // Depth 1.
+        {
+            let depth: u8 = 1;
+            let idx = (sparse_path.depth() - depth) as usize;
+            assert_eq!(idx, 7);
+            let bit = 0b0000_0001;
+            assert_eq!(bit, 1 << (depth - 1));
+            let is_set = (sparse_path.empty_nodes_mask & bit) != 0;
+            assert!(is_set);
+            assert_eq!(is_set, sparse_nodes.get(idx).unwrap().is_none());
+
+            let &test_node = sparse_nodes.get(idx).unwrap();
+            assert_eq!(test_node, None);
         }
     }
 
@@ -516,25 +707,21 @@ mod tests {
             let index = NodeIndex::from(Smt::key_to_leaf_index(key));
 
             let control_path = tree.get_path(key);
-            for (&control_node, proof_index) in
-                itertools::zip_eq(&*control_path, index.proof_indices())
-            {
-                let proof_node = tree.get_node_hash(proof_index);
-                assert_eq!(control_node, proof_node);
+            for (&control_node, proof_index) in iter::zip(&*control_path, index.proof_indices()) {
+                let proof_node = tree.get_hash(proof_index);
+                assert_eq!(control_node, proof_node, "WHat");
             }
 
             let sparse_path =
                 SparseMerklePath::from_sized_iter(control_path.clone().into_iter()).unwrap();
-            for (sparse_node, proof_idx) in
-                itertools::zip_eq(sparse_path.clone(), index.proof_indices())
-            {
-                let proof_node = tree.get_node_hash(proof_idx);
-                assert_eq!(sparse_node, proof_node);
+            for (sparse_node, proof_idx) in iter::zip(sparse_path.clone(), index.proof_indices()) {
+                let proof_node = tree.get_hash(proof_idx);
+                assert_eq!(sparse_node, proof_node, "WHat");
             }
 
             assert_eq!(control_path.depth(), sparse_path.depth());
-            for (control, sparse) in itertools::zip_eq(control_path, sparse_path) {
-                assert_eq!(control, sparse);
+            for (i, (control, sparse)) in iter::zip(control_path, sparse_path).enumerate() {
+                assert_eq!(control, sparse, "on iteration {i}");
             }
         }
     }
@@ -551,15 +738,24 @@ mod tests {
 
             // Test random access by depth.
             for depth in path_depth_iter(control_path.depth()) {
-                let control_node = control_path.at_depth(depth).unwrap();
+                let &control_node = control_path.at_depth(depth).unwrap();
                 let sparse_node = sparse_path.at_depth(depth).unwrap();
                 assert_eq!(control_node, sparse_node, "at depth {depth} for entry {i}");
             }
+
+            // Test random access by index.
+            // Letting index get to `control_path.len()` will test that both sides correctly return
+            // `None` for out of bounds access.
+            for index in 0..=(control_path.len()) {
+                let control_node = control_path.at_idx(index).copied();
+                let sparse_node = sparse_path.at_idx(index);
+                assert_eq!(control_node, sparse_node);
+            }
         }
     }
 
     #[test]
-    fn test_borrowing_iterator() {
+    fn test_owning_iterator() {
         let tree = make_smt(8192);
 
         for (key, _value) in tree.entries() {
@@ -570,9 +766,7 @@ mod tests {
 
             // Test that both iterators yield the same amount of the same values.
             let mut count: u64 = 0;
-            for (&control_node, sparse_node) in
-                itertools::zip_eq(control_path.iter(), sparse_path.iter())
-            {
+            for (&control_node, sparse_node) in iter::zip(control_path.iter(), sparse_path.iter()) {
                 count += 1;
                 assert_eq!(control_node, sparse_node);
             }
@@ -581,7 +775,7 @@ mod tests {
     }
 
     #[test]
-    fn test_owning_iterator() {
+    fn test_borrowing_iterator() {
         let tree = make_smt(8192);
 
         for (key, _value) in tree.entries() {
@@ -593,7 +787,7 @@ mod tests {
 
             // Test that both iterators yield the same amount of the same values.
             let mut count: u64 = 0;
-            for (control_node, sparse_node) in itertools::zip_eq(control_path, sparse_path) {
+            for (control_node, sparse_node) in iter::zip(control_path, sparse_path) {
                 count += 1;
                 assert_eq!(control_node, sparse_node);
             }
@@ -612,6 +806,7 @@ mod tests {
             sparse_path.at_depth(NonZero::new(1).unwrap()),
             Err(MerkleError::DepthTooBig(1))
         );
+        assert_eq!(sparse_path.at_idx(0), None);
         assert_eq!(sparse_path.iter().next(), None);
         assert_eq!(sparse_path.into_iter().next(), None);
     }