diff --git a/src/logic/future_moves.rs b/src/logic/future_moves.rs
index 0b784e3..11b2750 100644
--- a/src/logic/future_moves.rs
+++ b/src/logic/future_moves.rs
@@ -528,7 +528,11 @@ impl FutureMoves {
             stack.extend(self.arena[idx].children.iter());
         }
 
-        let mut index_map = vec![None; self.arena.len()];
+        let mut index_map: HashMap<
+            usize,
+            usize,
+            BuildHasherDefault<nohash_hasher::NoHashHasher<usize>>,
+        > = HashMap::with_capacity_and_hasher(self.arena.len(), BuildHasherDefault::default());
 
         let (indexes, moves): (Vec<(usize, usize)>, Vec<Move>) = retain
             .into_iter()
@@ -541,7 +545,7 @@ impl FutureMoves {
             .unzip();
 
         for (new_idx, old_idx) in indexes {
-            index_map[old_idx] = Some(new_idx);
+            index_map.insert(old_idx, new_idx);
         }
 
         self.arena = moves
@@ -549,13 +553,12 @@ impl FutureMoves {
             .map(|mut node| {
                 node.parent = node
                     .parent
-                    .and_then(|parent| index_map.get(parent))
-                    .and_then(|&x| x);
+                    .and_then(|parent| index_map.get(&parent).copied());
 
                 for c in node.children.as_mut_slice() {
                     *c = index_map
-                        .get(*c)
-                        .and_then(|&x| x)
+                        .get(c)
+                        .copied()
                         .expect("index_map should contain the child's index");
                 }
 
@@ -563,7 +566,7 @@ impl FutureMoves {
             })
             .collect();
 
-        self.current_root = index_map[root];
+        self.current_root = index_map.get(&root).copied();
     }
 }