From 517d16f2fde8f6f6f47f1214943748d0a6c33fa8 Mon Sep 17 00:00:00 2001
From: Michael Sproul <michael@sigmaprime.io>
Date: Tue, 31 Mar 2026 15:47:50 +1100
Subject: [PATCH] Revert parent->child optimisation

---
 consensus/proto_array/src/proto_array.rs | 70 ++++++++----------------
 1 file changed, 24 insertions(+), 46 deletions(-)

diff --git a/consensus/proto_array/src/proto_array.rs b/consensus/proto_array/src/proto_array.rs
index 81211690ff..374190f9ed 100644
--- a/consensus/proto_array/src/proto_array.rs
+++ b/consensus/proto_array/src/proto_array.rs
@@ -1188,26 +1188,6 @@ impl ProtoArray {
         Ok((best_fc_node.root, best_fc_node.payload_status))
     }
 
-    /// Build a parent->children index. Invalid nodes are excluded
-    /// (they aren't in store.blocks in the spec).
-    fn build_children_index(&self) -> Vec<Vec<usize>> {
-        let mut children = vec![vec![]; self.nodes.len()];
-        for (i, node) in self.nodes.iter().enumerate() {
-            if node
-                .execution_status()
-                .is_ok_and(|status| status.is_invalid())
-            {
-                continue;
-            }
-            if let Some(parent) = node.parent()
-                && parent < children.len()
-            {
-                children[parent].push(i);
-            }
-        }
-        children
-    }
-
     /// Spec: `get_filtered_block_tree`.
     ///
     /// Returns the set of node indices on viable branches — those with at least
@@ -1218,7 +1198,6 @@ impl ProtoArray {
         current_slot: Slot,
         best_justified_checkpoint: Checkpoint,
         best_finalized_checkpoint: Checkpoint,
-        children_index: &[Vec<usize>],
     ) -> HashSet<usize> {
         let mut viable = HashSet::new();
         self.filter_block_tree::<E>(
@@ -1226,7 +1205,6 @@ impl ProtoArray {
             current_slot,
             best_justified_checkpoint,
             best_finalized_checkpoint,
-            children_index,
             &mut viable,
         );
         viable
@@ -1239,17 +1217,25 @@ impl ProtoArray {
         current_slot: Slot,
         best_justified_checkpoint: Checkpoint,
         best_finalized_checkpoint: Checkpoint,
-        children_index: &[Vec<usize>],
         viable: &mut HashSet<usize>,
     ) -> bool {
         let Some(node) = self.nodes.get(node_index) else {
             return false;
         };
 
-        let children = children_index
-            .get(node_index)
-            .map(|c| c.as_slice())
-            .unwrap_or(&[]);
+        // Skip invalid children — they aren't in store.blocks in the spec.
+        let children: Vec<usize> = self
+            .nodes
+            .iter()
+            .enumerate()
+            .filter(|(_, child)| {
+                child.parent() == Some(node_index)
+                    && !child
+                        .execution_status()
+                        .is_ok_and(|status| status.is_invalid())
+            })
+            .map(|(i, _)| i)
+            .collect();
 
         if !children.is_empty() {
             // Evaluate ALL children (no short-circuit) to mark all viable branches.
@@ -1261,7 +1247,6 @@ impl ProtoArray {
                         current_slot,
                         best_justified_checkpoint,
                         best_finalized_checkpoint,
-                        children_index,
                         viable,
                     )
                 })
@@ -1306,16 +1291,12 @@ impl ProtoArray {
             payload_status: PayloadStatus::Pending,
         };
 
-        // Build parent->children index once for O(1) lookups.
-        let children_index = self.build_children_index();
-
         // Spec: `get_filtered_block_tree`.
         let viable_nodes = self.get_filtered_block_tree::<E>(
             start_index,
             current_slot,
             best_justified_checkpoint,
             best_finalized_checkpoint,
-            &children_index,
         );
 
         // Compute once rather than per-child per-level.
@@ -1324,7 +1305,7 @@ impl ProtoArray {
 
         loop {
             let children: Vec<_> = self
-                .get_node_children(&head, &children_index)?
+                .get_node_children(&head)?
                 .into_iter()
                 .filter(|(fc_node, _)| viable_nodes.contains(&fc_node.proto_node_index))
                 .collect();
@@ -1487,7 +1468,6 @@ impl ProtoArray {
     fn get_node_children(
         &self,
         node: &IndexedForkChoiceNode,
-        children_index: &[Vec<usize>],
     ) -> Result<Vec<(IndexedForkChoiceNode, ProtoNode)>, Error> {
         if node.payload_status == PayloadStatus::Pending {
             let proto_node = self
@@ -1501,25 +1481,23 @@ impl ProtoArray {
             }
             Ok(children)
         } else {
-            let child_indices = children_index
-                .get(node.proto_node_index)
-                .map(|c| c.as_slice())
-                .unwrap_or(&[]);
-            Ok(child_indices
+            Ok(self
+                .nodes
                 .iter()
-                .filter_map(|&child_index| {
-                    let child_node = self.nodes.get(child_index)?;
-                    if child_node.get_parent_payload_status() != node.payload_status {
-                        return None;
-                    }
-                    Some((
+                .enumerate()
+                .filter(|(_, child_node)| {
+                    child_node.parent() == Some(node.proto_node_index)
+                        && child_node.get_parent_payload_status() == node.payload_status
+                })
+                .map(|(child_index, child_node)| {
+                    (
                         IndexedForkChoiceNode {
                             root: child_node.root(),
                             proto_node_index: child_index,
                             payload_status: PayloadStatus::Pending,
                         },
                         child_node.clone(),
-                    ))
+                    )
                 })
                 .collect())
         }