Replace Vec with VecDeque for proper BFT queue behavior in NodeIter

Author: WilsonKuo

algorithms/linfa-trees/src/decision_trees/algorithm.rs

@@ -1,7 +1,7 @@
 //! Linear decision trees
 //!
 use std::cmp::Ordering;
-use std::collections::{HashMap, HashSet};
+use std::collections::{HashMap, HashSet, VecDeque};
 use std::hash::{Hash, Hasher};
 
 use linfa::dataset::AsSingleTargets;
@@ -553,7 +553,7 @@ impl<F: Float, L: Label> DecisionTree<F, L> {
         // queue of nodes yet to explore
         let queue = vec![&self.root_node];
 
-        NodeIter::new(queue)
+        NodeIter::new(VecDeque::from(queue))
     }
 
     /// Return features_idx of this tree (BFT)

algorithms/linfa-trees/src/decision_trees/iter.rs

@@ -1,17 +1,18 @@
 use std::fmt::Debug;
 use std::iter::Iterator;
+use std::collections::VecDeque;
 
 use super::TreeNode;
 use linfa::{Float, Label};
 
 /// Level-order (BFT) iterator of nodes in a decision tree
 #[derive(Debug, Clone, PartialEq)]
 pub struct NodeIter<'a, F, L> {
-    queue: Vec<&'a TreeNode<F, L>>,
+    queue: VecDeque<&'a TreeNode<F, L>>,
 }
 
 impl<'a, F, L> NodeIter<'a, F, L> {
-    pub fn new(queue: Vec<&'a TreeNode<F, L>>) -> Self {
+    pub fn new(queue: VecDeque<&'a TreeNode<F, L>>) -> Self {
         NodeIter { queue }
     }
 }
@@ -21,11 +22,11 @@ impl<'a, F: Float, L: Debug + Label> Iterator for NodeIter<'a, F, L> {
 
     fn next(&mut self) -> Option<Self::Item> {
         #[allow(clippy::manual_inspect)]
-        self.queue.pop().map(|node| {
+        self.queue.pop_front().map(|node| {
             node.children()
                 .into_iter()
                 .filter_map(|x| x.as_ref())
-                .for_each(|child| self.queue.push(child));
+                .for_each(|child| self.queue.push_back(child));
             node
         })
     }