Fix preorder stack population for when root changes within a tree.

Fixes #187

Author: molpopgen

src/trees.rs

@@ -668,6 +668,11 @@ impl NodeIterator for PreorderNodeIterator<'_> {
             None => {
                 if let Some(r) = self.root_stack.pop() {
                     self.current_node_ = Some(r);
+                    let mut c = self.tree.right_child(r).unwrap();
+                    while c != NodeId::NULL {
+                        self.node_stack.push(c);
+                        c = self.tree.left_sib(c).unwrap();
+                    }
                 }
             }
         };
@@ -1428,8 +1433,30 @@ pub(crate) mod test_trees {
                 assert_eq!(nsamples, tree.num_tracked_samples(n).unwrap());
             }
             assert_eq!(preoder_nodes.len(), postoder_nodes.len());
+
+            // Test our preorder against the tskit functions in 0.99.15
+            {
+                let mut nodes: Vec<NodeId> = vec![
+                    NodeId::NULL;
+                    unsafe { ll_bindings::tsk_tree_get_size_bound(tree.as_ptr()) }
+                        as usize
+                ];
+                let mut num_nodes: tsk_size_t = 0;
+                let ptr = std::ptr::addr_of_mut!(num_nodes);
+                unsafe {
+                    ll_bindings::tsk_tree_preorder(
+                        tree.as_ptr(),
+                        -1,
+                        nodes.as_mut_ptr() as *mut tsk_id_t,
+                        ptr,
+                    );
+                }
+                assert_eq!(num_nodes as usize, preoder_nodes.len());
+                for i in 0..num_nodes as usize {
+                    assert_eq!(preoder_nodes[i], nodes[i]);
+                }
+            }
         }
-        assert!(false);
     }
 
     #[test]