ensure that we effectively generated trees and not graph

src/reader.rs

@@ -361,7 +361,8 @@ impl<'t, D: Distance> Reader<'t, D> {
     /// Verify that the whole reader is correctly formed:
     /// - We can access all the items.
     /// - All the tree nodes are part of a tree.
-    /// - No tree share a same tree node.
+    /// - No tree shares the same tree node.
+    /// - We're effectively working with trees and not graphs (i.e., an item or tree node cannot be linked twice in the tree)
     #[cfg(any(test, feature = "assert_reader_validity"))]
     pub fn assert_validity(&self, rtxn: &RoTxn) -> Result<()> {
         // First, get all the items
@@ -402,6 +403,7 @@ impl<'t, D: Distance> Reader<'t, D> {
     }
 
     /// Return first the number of tree nodes and second the items accessible from a node.
+    /// And ensure that an item or tree node is never linked twice in the tree
     #[cfg(any(test, feature = "assert_reader_validity"))]
     fn gather_items_and_tree_ids(
         &self,
@@ -421,9 +423,16 @@ impl<'t, D: Distance> Reader<'t, D> {
                 let left = self.gather_items_and_tree_ids(rtxn, left)?;
                 let right = self.gather_items_and_tree_ids(rtxn, right)?;
 
+                let total_trees_size = left.0.len() + right.0.len();
+                let total_items_size = left.1.len() + right.1.len();
+
                 let mut trees = left.0 | right.0;
                 let items = left.1 | right.1;
 
+                // We should never find the same tree node or item ID in a single tree.
+                assert_eq!(total_trees_size, trees.len());
+                assert_eq!(total_items_size, items.len());
+
                 trees.insert(node_id.item);
 
                 Ok((trees, items))