diff --git a/Cargo.toml b/Cargo.toml
index 2d375f3..b00ab1d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -3,6 +3,10 @@ name = "othello"
 version = "0.1.0"
 edition = "2021"
 
+[profile.release]
+# for profiling
+debug = true
+
 [dependencies]
 num = "0.4"
 rand = "0.9"
diff --git a/src/complexagent.rs b/src/complexagent.rs
index 80895fb..da17e60 100644
--- a/src/complexagent.rs
+++ b/src/complexagent.rs
@@ -223,15 +223,18 @@ impl FutureMoves {
             stack.extend(self.arena[idx].children.iter().copied());
         }
 
-        let mut new_arena = Vec::new();
+        let mut new_arena = Vec::with_capacity(retain.iter().filter(|x| **x).count());
         let mut index_map = vec![None; self.arena.len()];
 
-        for (old_idx, _) in retain.iter().enumerate().rev().filter(|(_, a)| **a) {
-            index_map[old_idx] = Some(new_arena.len());
+        for (old_idx, keep) in retain.iter().enumerate().rev() {
             let mut node = self.arena.remove(old_idx);
-            node.parent = node.parent.and_then(|p| index_map[p]);
-            node.children = node.children.iter().filter_map(|&c| index_map[c]).collect();
-            new_arena.push(node);
+            if *keep {
+                index_map[old_idx] = Some(new_arena.len());
+
+                node.parent = node.parent.and_then(|p| index_map[p]);
+                node.children = node.children.iter().filter_map(|&c| index_map[c]).collect();
+                new_arena.push(node);
+            }
         }
 
         self.arena = new_arena;