future_move: improve pruning logic

src/logic/future_moves.rs

@@ -480,9 +480,16 @@ impl FutureMoves {
             }
 
             for idx in indexes {
-                // TODO! add check here to make sure that this index is
-                // still connected to the root from the previous trimming
-                // steps
+                // any nodes that have no parent node and aren't
+                // the root node, is orphaned
+                if self.arena[idx].parent.is_none() && self.current_root != Some(idx) {
+                    // propegate orphan-ness to the children
+                    let children: Vec<usize> = self.arena[idx].children.drain(..).collect();
+                    for idx in children {
+                        self.arena[idx].parent = None;
+                    }
+                    continue;
+                }
 
                 let mut m = self.arena[idx].clone();
                 // don't attempt and trim moves that have already been trimmed