Fix stale slot_1 availability bit in LineListNode

src/line_list_node.rs

@@ -339,6 +339,9 @@ impl<V: Clone + Send + Sync, A: Allocator> LineListNode<V, A> {
                 const LEN_MASK: u16 = 0xfc0; //bits 11 to 6, inclusive
                 self.header &= !LEN_MASK;
                 self.header |= (new_len << 6) as u16;
+                if !self.is_used::<1>() && new_len < KEY_BYTES_CNT {
+                    self.header &= !(1u16 << 12);
+                }
                 if self.is_used::<1>() {
                     let key_len_1 = self.key_len_1();
                     unsafe {
@@ -2541,8 +2544,7 @@ impl<V: Clone + Send + Sync, A: Allocator> TrieNode<V, A> for LineListNode<V, A>
                     let dst_ptr = base_ptr;
                     core::ptr::copy(src_ptr, dst_ptr, new_key_len);
                 }
-                temp_node.header &= 0xf03f; //Zero out the old length, and reset it
-                temp_node.header |= (new_key_len << 6) as u16;
+                temp_node.shorten_key_len::<0>(new_key_len);
                 debug_assert!(validate_node(&temp_node));
                 return Some(TrieNodeODRc::new_in(temp_node, self.alloc.clone()))
             } else {
@@ -2771,6 +2773,15 @@ pub(crate) fn validate_node<V: Clone + Send + Sync, A: Allocator>(node: &LineLis
         panic!()
     }
 
+    if !node.is_used::<1>() {
+        let slot_1_unavailable = node.header & (1 << 12) > 0;
+        let expected_unavailable = node.is_used::<0>() && key0.len() == KEY_BYTES_CNT;
+        if slot_1_unavailable != expected_unavailable {
+            println!("Invalid node - stale slot_1 availability bit. {node:?}");
+            panic!()
+        }
+    }
+
     //key0 must always be alphabetically before key1, if slot_1 is filled
     if node.is_used::<1>() && key0 > key1 {
         println!("Invalid node - keys not sorted {node:?}");
@@ -2904,6 +2915,51 @@ mod tests {
         assert_eq!(new_node.node_get_val("hello".as_bytes()), Some(&42));
     }
 
+    /// Regression for a stale "slot_1 unavailable" bit after shortening a full-width slot_0 key.
+    #[test]
+    fn test_line_list_shorten_key_releases_slot_1_availability() {
+        let full_key = vec![b'a'; KEY_BYTES_CNT];
+        let prefix_len = (KEY_BYTES_CNT / 3).max(1);
+        debug_assert!(prefix_len < KEY_BYTES_CNT);
+        let prefix = vec![b'a'; prefix_len];
+        let suffix = &full_key[prefix.len()..];
+
+        let mut new_node = LineListNode::<usize, GlobalAlloc>::new_in(global_alloc());
+        assert_eq!(new_node.node_set_val(&full_key, 24).map_err(|_| 0), Ok((None, false)));
+
+        let detached = new_node.take_node_at_key(&prefix, false).unwrap();
+        assert_eq!(detached.as_tagged().node_get_val(suffix), Some(&24));
+
+        assert_eq!(new_node.key_len_0(), prefix.len());
+        assert!(!new_node.is_used::<1>());
+        assert!(
+            new_node.is_available_1(),
+            "slot_1 should become available after shortening slot_0 below KEY_BYTES_CNT"
+        );
+
+        assert_eq!(new_node.node_set_val(b"z", 42).map_err(|_| 0), Ok((None, false)));
+        assert_eq!(new_node.node_get_val(b"z"), Some(&42));
+    }
+
+    /// Regression for the single-slot drop_head_dyn path preserving a stale slot_1-unavailable bit.
+    #[test]
+    fn test_line_list_drop_head_releases_slot_1_availability() {
+        let full_key = vec![b'a'; KEY_BYTES_CNT];
+        let drop_bytes = (KEY_BYTES_CNT / 3).max(1);
+        let expected_key_len = KEY_BYTES_CNT - drop_bytes;
+
+        let mut new_node = LineListNode::<usize, GlobalAlloc>::new_in(global_alloc());
+        assert_eq!(new_node.node_set_val(&full_key, 24).map_err(|_| 0), Ok((None, false)));
+
+        let mut shortened = new_node.drop_head_dyn(drop_bytes).unwrap().as_tagged().as_list().unwrap().clone();
+        assert_eq!(shortened.key_len_0(), expected_key_len);
+        assert!(!shortened.is_used::<1>());
+        assert!(shortened.is_available_1());
+
+        assert_eq!(shortened.node_set_val(b"z", 42).map_err(|_| 0), Ok((None, false)));
+        assert_eq!(shortened.node_get_val(b"z"), Some(&42));
+    }
+
     /// This tests that a common prefix is found with the entry in slot_0, when slot_1 is already full
     #[test]
     fn test_line_list_node_shared_prefixes_slot_0() {
@@ -3337,4 +3393,4 @@ mod tests {
 //          and ZipperMoving
 // 2. implement a val_count convenience on top of 1.
 
-//GOAT, Paths in caching Cata:  https://github.com/Adam-Vandervorst/PathMap/pull/8#discussion_r2004828957
+//GOAT, Paths in caching Cata:  https://github.com/Adam-Vandervorst/PathMap/pull/8#discussion_r2004828957