diff --git a/eth2/lmd_ghost/src/proto_array.rs b/eth2/lmd_ghost/src/proto_array.rs
index 0b458afbba..a12ce67d46 100644
--- a/eth2/lmd_ghost/src/proto_array.rs
+++ b/eth2/lmd_ghost/src/proto_array.rs
@@ -14,6 +14,15 @@ pub enum Error {
     FinalizedNodeUnknown(Hash256),
     JustifiedNodeUnknown(Hash256),
     InvalidFinalizedRootChange,
+    InvalidNodeIndex(usize),
+    InvalidParentIndex(usize),
+    InvalidBestChildIndex(usize),
+    InvalidJustifiedIndex(usize),
+    InvalidBestDescendant(usize),
+    InvalidParentDelta(usize),
+    InvalidNodeDelta(usize),
+    DeltaOverflow(usize),
+    IndexOverflow(&'static str),
     RevertedFinalizedEpoch,
     StartOutOfBounds,
     IndexOutOfBounds,
@@ -61,20 +70,21 @@ impl ProtoArrayForkChoice {
         justified_epoch: Epoch,
         parent_root: Hash256,
     ) -> Result<(), Error> {
-        let node = DagNode {
+        let block = DagNode {
             root,
             justified_epoch,
             finalized_epoch,
             parent: Some(parent_root),
         };
 
-        self.proto_array.write().on_new_node(node)
+        self.proto_array.write().on_new_block(block)
     }
 
     pub fn find_head<F>(
         &self,
         start_block_root: Hash256,
         justified_epoch: Epoch,
+        justified_root: Hash256,
         finalized_epoch: Epoch,
         finalized_root: Hash256,
         latest_balances: BalanceSnapshot,
@@ -90,13 +100,13 @@ impl ProtoArrayForkChoice {
                     target,
                     score_delta,
                 })
-                .collect();
+                .collect::<Vec<_>>();
 
         let mut proto_array = self.proto_array.write();
 
-        proto_array.update_ffg(justified_epoch, finalized_epoch, finalized_root)?;
-        proto_array.apply_score_changes(score_changes)?;
-        proto_array.head_fn(&start_block_root)
+        proto_array.maybe_prune(finalized_epoch, finalized_root)?;
+        proto_array.apply_score_changes(&score_changes, justified_epoch)?;
+        proto_array.find_head(&justified_root)
     }
 }
 
@@ -161,6 +171,324 @@ pub struct Epochs {
     finalized: Epoch,
 }
 
+pub struct ProtoNode {
+    root: Hash256,
+    parent: Option<usize>,
+    justified_epoch: Epoch,
+    finalized_epoch: Epoch,
+    weight: u64,
+    best_child: Option<usize>,
+    best_descendant: Option<usize>,
+}
+
+impl ProtoNode {
+    pub fn is_better_then(&self, other: &Self) -> bool {
+        if self.weight == other.weight {
+            self.root > other.root
+        } else {
+            self.weight > other.weight
+        }
+    }
+}
+
+pub struct ProtoArray {
+    ffg_update_required: bool,
+    justified_epoch: Epoch,
+    finalized_epoch: Epoch,
+    finalized_root: Hash256,
+    nodes: Vec<ProtoNode>,
+    indices: HashMap<Hash256, usize>,
+}
+
+impl ProtoArray {
+    pub fn apply_score_changes(
+        &mut self,
+        changes: &[ScoreChange],
+        justified_epoch: Epoch,
+    ) -> Result<(), Error> {
+        let mut deltas = produce_deltas(&self.indices, changes, self.nodes.len())?;
+
+        self.ffg_update_required = justified_epoch != self.justified_epoch;
+        if self.ffg_update_required {
+            self.justified_epoch = self.justified_epoch;
+        }
+
+        for node_index in (0..self.nodes.len()).rev() {
+            let node_delta = deltas
+                .get(node_index)
+                .copied()
+                .ok_or_else(|| Error::InvalidNodeDelta(node_index))?;
+
+            self.nodes
+                .get_mut(node_index)
+                .ok_or_else(|| Error::InvalidNodeIndex(node_index))?
+                .weight
+                .checked_add(node_delta as u64)
+                .ok_or_else(|| Error::DeltaOverflow(node_index))?;
+
+            if let Some(parent_index) = self
+                .nodes
+                .get(node_index)
+                .ok_or_else(|| Error::InvalidNodeIndex(node_index))?
+                .parent
+            {
+                if parent_index > 0 {
+                    let parent_delta = deltas
+                        .get_mut(parent_index)
+                        .ok_or_else(|| Error::InvalidParentDelta(parent_index))?;
+
+                    *parent_delta += node_delta;
+                }
+
+                let is_viable_for_head = self
+                    .nodes
+                    .get(node_index)
+                    .map(|node| self.node_is_viable_for_head(node))
+                    .ok_or_else(|| Error::InvalidNodeIndex(parent_index))?;
+
+                if !is_viable_for_head {
+                    // If the given node is not viable for the head and we are required to check
+                    // for FFG changes, then check to see if the child is the best child for the
+                    // parent. If so, remove the best-child link.
+                    if self.ffg_update_required {
+                        let parent_best_child = self
+                            .nodes
+                            .get(parent_index)
+                            .ok_or_else(|| Error::InvalidParentIndex(parent_index))?
+                            .best_child;
+
+                        if parent_best_child == Some(node_index) {
+                            let parent_node = self
+                                .nodes
+                                .get_mut(parent_index)
+                                .ok_or_else(|| Error::InvalidParentIndex(parent_index))?;
+
+                            parent_node.best_child = None;
+                            parent_node.best_descendant = None;
+                        }
+                    }
+
+                    continue;
+                }
+
+                let node_is_new_best_child = if let Some(parent_best_child_index) = self
+                    .nodes
+                    .get(parent_index)
+                    .ok_or_else(|| Error::InvalidParentIndex(parent_index))?
+                    .best_child
+                {
+                    let parent_best_child = self
+                        .nodes
+                        .get(parent_best_child_index)
+                        .ok_or_else(|| Error::InvalidBestChildIndex(parent_best_child_index))?;
+
+                    self.nodes
+                        .get(node_index)
+                        .ok_or_else(|| Error::InvalidNodeIndex(node_index))?
+                        .is_better_then(parent_best_child)
+                } else {
+                    true
+                };
+
+                if node_is_new_best_child {
+                    self.set_best_child(parent_index, node_index)?;
+                }
+            }
+        }
+
+        self.ffg_update_required = false;
+
+        Ok(())
+    }
+
+    pub fn on_new_block(&mut self, block: DagNode) -> Result<(), Error> {
+        let node_index = self.nodes.len();
+
+        let node = ProtoNode {
+            root: block.root,
+            parent: block
+                .parent
+                .and_then(|parent_root| self.indices.get(&parent_root).copied()),
+            justified_epoch: block.justified_epoch,
+            finalized_epoch: block.finalized_epoch,
+            weight: 0,
+            best_child: None,
+            best_descendant: None,
+        };
+
+        if let Some(parent_index) = node.parent {
+            let parent = self
+                .nodes
+                .get(parent_index)
+                .ok_or_else(|| Error::InvalidParentIndex(parent_index))?;
+
+            let node_is_best_child = if let Some(parent_best_child_index) = parent.best_child {
+                let parent_best_child = self
+                    .nodes
+                    .get(parent_best_child_index)
+                    .ok_or_else(|| Error::InvalidBestChildIndex(parent_best_child_index))?;
+
+                node.is_better_then(parent_best_child)
+            } else {
+                true
+            };
+
+            if node_is_best_child {
+                self.set_best_child(parent_index, node_index)?;
+            }
+        }
+
+        self.nodes.push(node);
+
+        Ok(())
+    }
+
+    pub fn find_head(&self, justified_root: &Hash256) -> Result<Hash256, Error> {
+        let justified_index = self
+            .indices
+            .get(justified_root)
+            .copied()
+            .ok_or_else(|| Error::JustifiedNodeUnknown(self.finalized_root))?;
+
+        let justified_node = self
+            .nodes
+            .get(justified_index)
+            .ok_or_else(|| Error::InvalidJustifiedIndex(justified_index))?;
+
+        let best_descendant_index = justified_node
+            .best_descendant
+            .unwrap_or_else(|| justified_index);
+
+        let best_node = self
+            .nodes
+            .get(best_descendant_index)
+            .ok_or_else(|| Error::InvalidBestDescendant(best_descendant_index))?;
+
+        Ok(best_node.root)
+    }
+
+    pub fn maybe_prune(
+        &mut self,
+        finalized_epoch: Epoch,
+        finalized_root: Hash256,
+    ) -> Result<(), Error> {
+        if finalized_epoch == self.finalized_epoch && finalized_root == self.finalized_root {
+            return Ok(());
+        } else if finalized_epoch == self.finalized_epoch && self.finalized_root != finalized_root {
+            return Err(Error::InvalidFinalizedRootChange);
+        } else if finalized_epoch < self.finalized_epoch {
+            return Err(Error::RevertedFinalizedEpoch);
+        } else {
+            self.finalized_epoch = finalized_epoch;
+            self.finalized_root = finalized_root;
+            self.ffg_update_required = true;
+        }
+
+        let finalized_index = *self
+            .indices
+            .get(&self.finalized_root)
+            .ok_or_else(|| Error::FinalizedNodeUnknown(self.finalized_root))?;
+
+        // Pruning at small numbers incurs more cost than benefit.
+        if finalized_index < PRUNE_THRESHOLD {
+            return Ok(());
+        }
+
+        for node_index in 0..finalized_index {
+            let root = &self
+                .nodes
+                .get(node_index)
+                .ok_or_else(|| Error::InvalidNodeIndex(node_index))?
+                .root;
+            self.indices.remove(root);
+        }
+
+        self.nodes = self.nodes.split_off(finalized_index);
+
+        self.nodes.iter_mut().try_for_each(|node| {
+            if let Some(parent) = node.parent {
+                // If `node.parent` is less than `finalized_index`, set it to `None`.
+                node.parent = parent.checked_sub(finalized_index);
+            }
+            if let Some(best_child) = node.best_child {
+                node.best_child = Some(
+                    best_child
+                        .checked_sub(finalized_index)
+                        .ok_or_else(|| Error::IndexOverflow("best_child"))?,
+                );
+            }
+            if let Some(best_descendant) = node.best_descendant {
+                node.best_descendant = Some(
+                    best_descendant
+                        .checked_sub(finalized_index)
+                        .ok_or_else(|| Error::IndexOverflow("best_descendant"))?,
+                );
+            }
+
+            Ok(())
+        })?;
+
+        Ok(())
+    }
+
+    fn set_best_child(&mut self, parent_index: usize, child_index: usize) -> Result<(), Error> {
+        let child_best_descendant = self
+            .nodes
+            .get(child_index)
+            .ok_or_else(|| Error::InvalidNodeIndex(child_index))?
+            .best_descendant;
+
+        let parent_node = self
+            .nodes
+            .get_mut(parent_index)
+            .ok_or_else(|| Error::InvalidParentIndex(parent_index))?;
+
+        parent_node.best_child = Some(child_index);
+        parent_node.best_descendant = if let Some(best_descendant) = child_best_descendant {
+            Some(best_descendant)
+        } else {
+            Some(child_index)
+        };
+
+        Ok(())
+    }
+
+    fn node_is_viable_for_head(&self, node: &ProtoNode) -> bool {
+        node.justified_epoch == self.justified_epoch && node.finalized_epoch == self.finalized_epoch
+    }
+}
+
+/// Takes a list of `ScoreChange`, returning a list of deltas.
+///
+/// The returned list has the length of `num_nodes`. Each `ScoreChanged` is mapped to a position in
+/// the returned list according to a lookup in `indices`.
+///
+/// ## Errors
+///
+/// It is an error to have a value in `indices` that is greater than or equal to `num_nodes`.
+fn produce_deltas(
+    indices: &HashMap<Hash256, usize>,
+    changes: &[ScoreChange],
+    num_nodes: usize,
+) -> Result<Vec<i64>, Error> {
+    let mut deltas: Vec<i64> = vec![0; num_nodes];
+
+    // Update `deltas` with the value of each `ScoreChange`.
+    changes.iter().try_for_each(|c| {
+        let i = indices
+            .get(&c.target)
+            .ok_or_else(|| Error::NodeUnknown(c.target))?;
+
+        let v = deltas.get_mut(*i).ok_or_else(|| Error::IndexOutOfBounds)?;
+        v.saturating_add(c.score_delta);
+
+        Ok(())
+    })?;
+
+    Ok(deltas)
+}
+
+/*
 pub struct ProtoArray {
     justified_epoch: Epoch,
     finalized_epoch: Epoch,
@@ -470,6 +798,7 @@ impl ProtoArray {
         Ok(())
     }
 }
+*/
 
 /// A Vec-wrapper which will grow to match any request.
 ///