From ed29a27e598a1b1c816ea811e8d2d8dffaee9b7b Mon Sep 17 00:00:00 2001
From: Igor Malovitsa <igor.malovitsa@gmail.com>
Date: Thu, 17 Sep 2026 05:03:56 +0000
Subject: [PATCH 1/2] Fix exclusive zippers rooted at an emptied link

prepare_cf returned an existing link even when it was the empty sentinel,
left by remove_branches, take_map or grafting nothing. A write zipper
rooted there couldn't move or read. Give it a fresh node, as for a
missing link.
---
 src/dense_byte_node.rs | 13 +++++--------
 src/zipper_head.rs     | 30 ++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/src/dense_byte_node.rs b/src/dense_byte_node.rs
index ab6bf2db..9cf736ae 100644
--- a/src/dense_byte_node.rs
+++ b/src/dense_byte_node.rs
@@ -636,14 +636,11 @@ impl<V: Clone + Send + Sync, A: Allocator> CellByteNode<V, A> {
         }
         let cf = self.get_mut(k).unwrap();
         let (rec, val) = cf.both_mut_refs();
-        let rec = match rec {
-            Some(rec) => rec,
-            None => {
-                *rec = Some(TrieNodeODRc::new_allocated_in(0, 0, alloc));
-                rec.as_mut().unwrap()
-            }
-        };
-        (rec, val)
+        //A zipper can't be rooted at the empty sentinel, so an emptied link gets a real node too
+        if rec.as_ref().map_or(true, |rec| rec.is_empty()) {
+            *rec = Some(TrieNodeODRc::new_allocated_in(0, 0, alloc));
+        }
+        (rec.as_mut().unwrap(), val)
     }
 }
 
diff --git a/src/zipper_head.rs b/src/zipper_head.rs
index 20357a74..a2d46bad 100644
--- a/src/zipper_head.rs
+++ b/src/zipper_head.rs
@@ -1637,4 +1637,34 @@ mod tests {
             }
         }
     }
+
+    /// An exclusive zipper rooted at a link emptied by `remove_branches`, `take_map` or a graft of nothing
+    #[test]
+    fn exclusive_zipper_at_emptied_link() {
+        let preps: [fn(&mut WriteZipperUntracked<u64>); 4] = [
+            |z| { z.remove_branches(false); },
+            |z| { z.take_map(false); },
+            |z| { let e = PathMap::<u64>::new(); z.graft(&e.read_zipper()); },
+            |z| { let e = PathMap::<u64>::new(); z.restrict(&e.read_zipper()); },
+        ];
+        for (i, prep) in preps.iter().enumerate() {
+            let mut map = PathMap::<u64>::new();
+            for p in [&[0u8, 0, 1][..], &[0, 0, 1, 2], &[0]] { map.set_val_at(p, 1); }
+            {
+                let mut wz = map.write_zipper_at_path(&[0u8, 0]);
+                prep(&mut wz);
+                let zh = wz.zipper_head();
+                let mut w = zh.write_zipper_at_exclusive_path(&[]).unwrap();
+                assert!(!w.descend_to_existing_byte(1), "prep {i}");
+                w.descend_to(&[7u8, 7]);
+                assert!(!w.path_exists(), "prep {i}");
+                assert_eq!(w.ascend_until(), 2, "prep {i}");
+                w.descend_to(&[7u8, 7]);
+                w.set_val(5);
+            }
+            assert_eq!(map.get(&[0u8, 0, 7, 7]), Some(&5), "prep {i}");
+            assert_eq!(map.get(&[0u8]), Some(&1), "prep {i}");
+            assert_eq!(map.val_count(), 2, "prep {i}");
+        }
+    }
 }

From 889382076fed16210e873dfbfb2560419e7cadd0 Mon Sep 17 00:00:00 2001
From: Luke Peterson <luke@infoplatter.com>
Date: Thu, 1 Oct 2026 22:07:05 -0600
Subject: [PATCH 2/2] Adding additional test to be sure converted
 zipper-root-node gets pruned appropriately

---
 src/zipper_head.rs | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/src/zipper_head.rs b/src/zipper_head.rs
index a2d46bad..a7fb8624 100644
--- a/src/zipper_head.rs
+++ b/src/zipper_head.rs
@@ -1503,6 +1503,22 @@ mod tests {
         drop(rz);
     }
 
+    #[test]
+    fn cleanup_write_zipper_at_emptied_link() {
+        let mut map = PathMap::<u64>::new();
+        map.set_val_at(&[0u8], 1);
+        map.set_val_at(&[0u8, 0, 1], 2);
+        map.write_zipper_at_path(&[0u8, 0]).remove_branches(false);
+        assert!(map.read_zipper_at_path(&[0u8, 0]).path_exists());
+
+        let zh = map.zipper_head();
+        let wz = zh.write_zipper_at_exclusive_path(&[0u8, 0]).unwrap();
+        zh.cleanup_write_zipper(wz);
+
+        assert!(!zh.read_zipper_at_path(&[0u8, 0]).unwrap().path_exists());
+        assert_eq!(zh.read_zipper_at_path(&[0u8]).unwrap().val(), Some(&1));
+    }
+
     /// A ZipperHead turns the parent node into a CellByteNode; joining a list node with it must not
     /// treat the cell node as a DenseByteNode
     #[test]