WIP filter block tree

specs/core/0_fork-choice.md

@@ -136,17 +136,62 @@ def get_latest_attesting_balance(store: Store, root: Root) -> Gwei:
     ))
 ```
 
+#### `filter_block_tree`
+
+```python
+def filter_block_tree(store, block_root, blocks):
+    block = store.blocks[block_root]
+    children = [
+        root for root in store.blocks.keys()
+        if store.blocks[root].parent_root == block
+    ]
+
+    # If any children branches contain expected finalized/justified checkpoints,
+    # add to filtered block-tree and signal viability to parent.
+    if any(children):
+        if True in [filter_block_tree(child, blocks) for child in children]:
+            blocks[block_root] = block
+            return True
+        return False
+
+    # If leaf block, check finalized/justified checkpoints as matching latest.
+    # If matching, add to viable block-tree and signal viability to parent.
+    head_state = store.block_states[block_root]
+    is_viable_branch = (
+        head_state.finalized_checkpoint == store.finalized_checkpoint
+        and head_state.current_justified_checkpoint == store.justified_checkpoint
+    )
+    if is_viable_branch:
+        blocks[block_root] = block
+        return True
+
+    # Otherwise, not viable
+    return False
+```
+
+#### `get_filtered_block_tree`
+
+```python
+def get_filtered_block_tree(store: Store) -> Dict[Root, BeaconBlock]:
+    head = store.justified_checkpoint.root
+    blocks = {}
+    filter_block_tree(head, blocks)
+    return blocks
+```
+
 #### `get_head`
 
 ```python
 def get_head(store: Store) -> Root:
+    # Get filtered block tree that includes viable branches
+    blocks = get_filtered_block_tree(store)
     # Execute the LMD-GHOST fork choice
     head = store.justified_checkpoint.root
     justified_slot = compute_start_slot_at_epoch(store.justified_checkpoint.epoch)
     while True:
         children = [
-            root for root in store.blocks.keys()
-            if store.blocks[root].parent_root == head and store.blocks[root].slot > justified_slot
+            root for root in blocks.keys()
+            if blocks[root].parent_root == head and blocks[root].slot > justified_slot
         ]
         if len(children) == 0:
             return head