From c1939cd56218ac7afe053ea062052f79468b0cea Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Grzegorz=20=C5=9Awirski?= <grzegorz@swirski.name>
Date: Wed, 16 Apr 2025 17:47:33 +0200
Subject: [PATCH] use Cow instead of the additional IntoIter struct

---
 miden-crypto/src/merkle/sparse_path.rs | 71 +++++++-------------------
 1 file changed, 18 insertions(+), 53 deletions(-)

diff --git a/miden-crypto/src/merkle/sparse_path.rs b/miden-crypto/src/merkle/sparse_path.rs
index a918510..32ba2d6 100644
--- a/miden-crypto/src/merkle/sparse_path.rs
+++ b/miden-crypto/src/merkle/sparse_path.rs
@@ -1,4 +1,4 @@
-use alloc::vec::Vec;
+use alloc::{borrow::Cow, vec::Vec};
 use core::{
     iter::{self, FusedIterator},
     num::NonZero,
@@ -252,20 +252,10 @@ impl From<SparseMerklePath> for Vec<RpoDigest> {
 // ITERATORS
 // ================================================================================================
 
-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`].
+/// Iterator for [`SparseMerklePath`].
 pub struct SparseMerklePathIter<'p> {
     /// The "inner" value we're iterating over.
-    path: &'p SparseMerklePath,
+    path: Cow<'p, SparseMerklePath>,
 
     /// The depth a `next()` call will get. `next_depth == 0` indicates that the iterator has been
     /// exhausted.
@@ -306,57 +296,32 @@ 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 = IntoIter;
+    type IntoIter = SparseMerklePathIter<'static>;
     type Item = <Self::IntoIter as Iterator>::Item;
 
-    fn into_iter(self) -> IntoIter {
+    fn into_iter(self) -> SparseMerklePathIter<'static> {
         let tree_depth = self.depth();
-        IntoIter { path: self, next_depth: tree_depth }
+        SparseMerklePathIter {
+            path: Cow::Owned(self),
+            next_depth: tree_depth,
+        }
     }
 }
 
-impl Iterator for IntoIter {
-    type Item = RpoDigest;
+impl<'p> IntoIterator for &'p SparseMerklePath {
+    type Item = <SparseMerklePathIter<'p> as Iterator>::Item;
+    type IntoIter = SparseMerklePathIter<'p>;
 
-    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))
+    fn into_iter(self) -> SparseMerklePathIter<'p> {
+        let tree_depth = self.depth();
+        SparseMerklePathIter {
+            path: Cow::Borrowed(self),
+            next_depth: tree_depth,
+        }
     }
 }
 
-impl ExactSizeIterator for IntoIter {
-    fn len(&self) -> usize {
-        self.next_depth as usize
-    }
-}
-
-impl FusedIterator for IntoIter {}
-
-// TODO: impl DoubleEndedIterator.
-
 // COMPARISONS
 // ================================================================================================
 impl PartialEq<MerklePath> for SparseMerklePath {