From 201f113b5096978c2f2afca3f293870b2398f81a Mon Sep 17 00:00:00 2001
From: Roberto Saltini <saltini.roberto@gmail.com>
Date: Tue, 28 Mar 2023 15:27:13 +1100
Subject: [PATCH 01/12] Introduce get_epoch_boundary_block

---
 specs/phase0/fork-choice.md | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/specs/phase0/fork-choice.md b/specs/phase0/fork-choice.md
index 6e281d5c3..044d85053 100644
--- a/specs/phase0/fork-choice.md
+++ b/specs/phase0/fork-choice.md
@@ -192,6 +192,17 @@ def get_ancestor(store: Store, root: Root, slot: Slot) -> Root:
     return root
 ```
 
+#### `get_epoch_boundary_block`
+
+```python
+def get_epoch_boundary_block(store: Store, root: Root, epoch: Epoch) -> Root:
+    """
+    Compute the epoch boundary block for epoch ``epoch`` in the chain of block ``root``
+    """
+    epoch_first_slot = compute_start_slot_at_epoch(epoch)
+    return get_ancestor(store, root, epoch_first_slot)
+```
+
 #### `get_weight`
 
 ```python
@@ -278,10 +289,9 @@ def filter_block_tree(store: Store, block_root: Root, blocks: Dict[Root, BeaconB
             voting_source.epoch + 2 >= current_epoch
         )
 
-    finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     correct_finalized = (
         store.finalized_checkpoint.epoch == GENESIS_EPOCH
-        or store.finalized_checkpoint.root == get_ancestor(store, block_root, finalized_slot)
+        or store.finalized_checkpoint.root == get_epoch_boundary_block(store, block_root, store.finalized_checkpoint.epoch)
     )
 
     # If expected finalized/justified, add to viable block-tree and signal viability to parent.
@@ -442,8 +452,7 @@ def validate_on_attestation(store: Store, attestation: Attestation, is_from_bloc
     assert store.blocks[attestation.data.beacon_block_root].slot <= attestation.data.slot
 
     # LMD vote must be consistent with FFG vote target
-    target_slot = compute_start_slot_at_epoch(target.epoch)
-    assert target.root == get_ancestor(store, attestation.data.beacon_block_root, target_slot)
+    assert target.root == get_epoch_boundary_block(store, attestation.data.beacon_block_root, target.epoch)
 
     # Attestations can only affect the fork choice of subsequent slots.
     # Delay consideration in the fork choice until their slot is in the past.
@@ -506,7 +515,7 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert get_ancestor(store, block.parent_root, finalized_slot) == store.finalized_checkpoint.root
+    assert get_epoch_boundary_block(store, block.parent_root, store.finalized_checkpoint.epoch) == store.finalized_checkpoint.root
 
     # Check the block is valid and compute the post-state
     state = pre_state.copy()

From ddbd82e1be7fc38bcdcf672865bb036ab80837a7 Mon Sep 17 00:00:00 2001
From: Roberto Saltini <saltini.roberto@gmail.com>
Date: Tue, 28 Mar 2023 15:51:34 +1100
Subject: [PATCH 02/12] Add toc

---
 specs/phase0/fork-choice.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/specs/phase0/fork-choice.md b/specs/phase0/fork-choice.md
index 044d85053..058089a62 100644
--- a/specs/phase0/fork-choice.md
+++ b/specs/phase0/fork-choice.md
@@ -18,6 +18,7 @@
     - [`get_current_slot`](#get_current_slot)
     - [`compute_slots_since_epoch_start`](#compute_slots_since_epoch_start)
     - [`get_ancestor`](#get_ancestor)
+    - [`get_epoch_boundary_block`](#get_epoch_boundary_block)
     - [`get_weight`](#get_weight)
     - [`get_voting_source`](#get_voting_source)
     - [`filter_block_tree`](#filter_block_tree)

From c7029ce19e3f25d2d0e3ea301650561d1ac67d12 Mon Sep 17 00:00:00 2001
From: Roberto Saltini <saltini.roberto@gmail.com>
Date: Wed, 29 Mar 2023 12:40:58 +1100
Subject: [PATCH 03/12] Rename get_epoch_boundary_block to
 get_ancestor_at_epoch_boundary

---
 specs/phase0/fork-choice.md | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/specs/phase0/fork-choice.md b/specs/phase0/fork-choice.md
index 058089a62..2ee6ecb92 100644
--- a/specs/phase0/fork-choice.md
+++ b/specs/phase0/fork-choice.md
@@ -18,7 +18,7 @@
     - [`get_current_slot`](#get_current_slot)
     - [`compute_slots_since_epoch_start`](#compute_slots_since_epoch_start)
     - [`get_ancestor`](#get_ancestor)
-    - [`get_epoch_boundary_block`](#get_epoch_boundary_block)
+    - [`get_ancestor_at_epoch_boundary`](#get_ancestor_at_epoch_boundary)
     - [`get_weight`](#get_weight)
     - [`get_voting_source`](#get_voting_source)
     - [`filter_block_tree`](#filter_block_tree)
@@ -193,10 +193,10 @@ def get_ancestor(store: Store, root: Root, slot: Slot) -> Root:
     return root
 ```
 
-#### `get_epoch_boundary_block`
+#### `get_ancestor_at_epoch_boundary`
 
 ```python
-def get_epoch_boundary_block(store: Store, root: Root, epoch: Epoch) -> Root:
+def get_ancestor_at_epoch_boundary(store: Store, root: Root, epoch: Epoch) -> Root:
     """
     Compute the epoch boundary block for epoch ``epoch`` in the chain of block ``root``
     """
@@ -292,7 +292,7 @@ def filter_block_tree(store: Store, block_root: Root, blocks: Dict[Root, BeaconB
 
     correct_finalized = (
         store.finalized_checkpoint.epoch == GENESIS_EPOCH
-        or store.finalized_checkpoint.root == get_epoch_boundary_block(store, block_root, store.finalized_checkpoint.epoch)
+        or store.finalized_checkpoint.root == get_ancestor_at_epoch_boundary(store, block_root, store.finalized_checkpoint.epoch)
     )
 
     # If expected finalized/justified, add to viable block-tree and signal viability to parent.
@@ -453,7 +453,7 @@ def validate_on_attestation(store: Store, attestation: Attestation, is_from_bloc
     assert store.blocks[attestation.data.beacon_block_root].slot <= attestation.data.slot
 
     # LMD vote must be consistent with FFG vote target
-    assert target.root == get_epoch_boundary_block(store, attestation.data.beacon_block_root, target.epoch)
+    assert target.root == get_ancestor_at_epoch_boundary(store, attestation.data.beacon_block_root, target.epoch)
 
     # Attestations can only affect the fork choice of subsequent slots.
     # Delay consideration in the fork choice until their slot is in the past.
@@ -516,7 +516,7 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert get_epoch_boundary_block(store, block.parent_root, store.finalized_checkpoint.epoch) == store.finalized_checkpoint.root
+    assert get_ancestor_at_epoch_boundary(store, block.parent_root, store.finalized_checkpoint.epoch) == store.finalized_checkpoint.root
 
     # Check the block is valid and compute the post-state
     state = pre_state.copy()

From e255d09840855e16e62f0e531f2ca50fa45e2724 Mon Sep 17 00:00:00 2001
From: Roberto Saltini <saltini.roberto@gmail.com>
Date: Fri, 31 Mar 2023 10:52:52 +1100
Subject: [PATCH 04/12] Apply changes to Bellatrix and Daneb

---
 specs/bellatrix/fork-choice.md | 2 +-
 specs/deneb/fork-choice.md     | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/specs/bellatrix/fork-choice.md b/specs/bellatrix/fork-choice.md
index ed7d60a93..aac26f3dd 100644
--- a/specs/bellatrix/fork-choice.md
+++ b/specs/bellatrix/fork-choice.md
@@ -170,7 +170,7 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert get_ancestor(store, block.parent_root, finalized_slot) == store.finalized_checkpoint.root
+    assert get_ancestor_at_epoch_boundary(store, block.parent_root, store.finalized_checkpoint.epoch) == store.finalized_checkpoint.root
 
     # Check the block is valid and compute the post-state
     state = pre_state.copy()
diff --git a/specs/deneb/fork-choice.md b/specs/deneb/fork-choice.md
index 61714cf1a..83cdb9972 100644
--- a/specs/deneb/fork-choice.md
+++ b/specs/deneb/fork-choice.md
@@ -82,7 +82,7 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert get_ancestor(store, block.parent_root, finalized_slot) == store.finalized_checkpoint.root
+    assert get_ancestor_at_epoch_boundary(store, block.parent_root, store.finalized_checkpoint.epoch) == store.finalized_checkpoint.root
 
     # [New in Deneb]
     # Check if blob data is available

From 4cac76181827562a010c9c45dba1bb9f80f4f6cd Mon Sep 17 00:00:00 2001
From: Hsiao-Wei Wang <hsiaowei.eth@gmail.com>
Date: Wed, 5 Apr 2023 11:38:20 +0800
Subject: [PATCH 05/12] make linter happy

---
 specs/bellatrix/fork-choice.md |  6 +++++-
 specs/deneb/fork-choice.md     |  6 +++++-
 specs/phase0/fork-choice.md    | 12 ++++++++++--
 3 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/specs/bellatrix/fork-choice.md b/specs/bellatrix/fork-choice.md
index aac26f3dd..d22436c9d 100644
--- a/specs/bellatrix/fork-choice.md
+++ b/specs/bellatrix/fork-choice.md
@@ -170,7 +170,11 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert get_ancestor_at_epoch_boundary(store, block.parent_root, store.finalized_checkpoint.epoch) == store.finalized_checkpoint.root
+    assert store.finalized_checkpoint.root == get_ancestor_at_epoch_boundary(
+        store,
+        block.parent_root,
+        store.finalized_checkpoint.epoch,
+    )
 
     # Check the block is valid and compute the post-state
     state = pre_state.copy()
diff --git a/specs/deneb/fork-choice.md b/specs/deneb/fork-choice.md
index 83cdb9972..e76e159c4 100644
--- a/specs/deneb/fork-choice.md
+++ b/specs/deneb/fork-choice.md
@@ -82,7 +82,11 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert get_ancestor_at_epoch_boundary(store, block.parent_root, store.finalized_checkpoint.epoch) == store.finalized_checkpoint.root
+    assert store.finalized_checkpoint.root == get_ancestor_at_epoch_boundary(
+        store,
+        block.parent_root,
+        store.finalized_checkpoint.epoch,
+    )
 
     # [New in Deneb]
     # Check if blob data is available
diff --git a/specs/phase0/fork-choice.md b/specs/phase0/fork-choice.md
index 2ee6ecb92..478dd2142 100644
--- a/specs/phase0/fork-choice.md
+++ b/specs/phase0/fork-choice.md
@@ -292,7 +292,11 @@ def filter_block_tree(store: Store, block_root: Root, blocks: Dict[Root, BeaconB
 
     correct_finalized = (
         store.finalized_checkpoint.epoch == GENESIS_EPOCH
-        or store.finalized_checkpoint.root == get_ancestor_at_epoch_boundary(store, block_root, store.finalized_checkpoint.epoch)
+        or store.finalized_checkpoint.root == get_ancestor_at_epoch_boundary(
+            store,
+            block_root,
+            store.finalized_checkpoint.epoch,
+        )
     )
 
     # If expected finalized/justified, add to viable block-tree and signal viability to parent.
@@ -516,7 +520,11 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert get_ancestor_at_epoch_boundary(store, block.parent_root, store.finalized_checkpoint.epoch) == store.finalized_checkpoint.root
+    assert store.finalized_checkpoint.root == get_ancestor_at_epoch_boundary(
+        store,
+        block.parent_root,
+        store.finalized_checkpoint.epoch,
+    )
 
     # Check the block is valid and compute the post-state
     state = pre_state.copy()

From 41386092b761d61792c2ad0d2c07a76c4511a823 Mon Sep 17 00:00:00 2001
From: Roberto Saltini <saltini.roberto@gmail.com>
Date: Sat, 8 Apr 2023 14:00:01 +1000
Subject: [PATCH 06/12] Apply changes to p2p-interface.md

---
 specs/phase0/p2p-interface.md | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/specs/phase0/p2p-interface.md b/specs/phase0/p2p-interface.md
index f52752931..56c1b8cfb 100644
--- a/specs/phase0/p2p-interface.md
+++ b/specs/phase0/p2p-interface.md
@@ -317,7 +317,7 @@ The following validations MUST pass before forwarding the `signed_beacon_block`
 - _[REJECT]_ The block's parent (defined by `block.parent_root`) passes validation.
 - _[REJECT]_ The block is from a higher slot than its parent.
 - _[REJECT]_ The current `finalized_checkpoint` is an ancestor of `block` -- i.e.
-  `get_ancestor(store, block.parent_root, compute_start_slot_at_epoch(store.finalized_checkpoint.epoch))
+  `get_ancestor_at_epoch_boundary(store, block.parent_root, store.finalized_checkpoint.epoch)
   == store.finalized_checkpoint.root`
 - _[REJECT]_ The block is proposed by the expected `proposer_index` for the block's slot
   in the context of the current shuffling (defined by `parent_root`/`slot`).
@@ -356,7 +356,7 @@ The following validations MUST pass before forwarding the `signed_aggregate_and_
   (a client MAY queue aggregates for processing once block is retrieved).
 - _[REJECT]_ The block being voted for (`aggregate.data.beacon_block_root`) passes validation.
 - _[IGNORE]_ The current `finalized_checkpoint` is an ancestor of the `block` defined by `aggregate.data.beacon_block_root` -- i.e.
-  `get_ancestor(store, aggregate.data.beacon_block_root, compute_start_slot_at_epoch(store.finalized_checkpoint.epoch))
+  `get_ancestor_at_epoch_boundary(store, aggregate.data.beacon_block_root, finalized_checkpoint.epoch)
   == store.finalized_checkpoint.root`
 
 
@@ -425,9 +425,9 @@ The following validations MUST pass before forwarding the `attestation` on the s
   (a client MAY queue attestations for processing once block is retrieved).
 - _[REJECT]_ The block being voted for (`attestation.data.beacon_block_root`) passes validation.
 - _[REJECT]_ The attestation's target block is an ancestor of the block named in the LMD vote -- i.e.
-  `get_ancestor(store, attestation.data.beacon_block_root, compute_start_slot_at_epoch(attestation.data.target.epoch)) == attestation.data.target.root`
+  `get_ancestor_at_epoch_boundary(store, attestation.data.beacon_block_root, attestation.data.target.epoch) == attestation.data.target.root`
 - _[IGNORE]_ The current `finalized_checkpoint` is an ancestor of the `block` defined by `attestation.data.beacon_block_root` -- i.e.
-  `get_ancestor(store, attestation.data.beacon_block_root, compute_start_slot_at_epoch(store.finalized_checkpoint.epoch))
+  `get_ancestor_at_epoch_boundary(store, attestation.data.beacon_block_root, store.finalized_checkpoint.epoch)
   == store.finalized_checkpoint.root`
 
 

From 334114d9d373d3a71ab49720ba4831d0b1fce6dd Mon Sep 17 00:00:00 2001
From: Roberto Saltini <saltini.roberto@gmail.com>
Date: Tue, 18 Apr 2023 13:14:53 +1000
Subject: [PATCH 07/12] Rename get_ancestor_at_epoch_boundary to
 get_checkpoint_block

---
 specs/bellatrix/fork-choice.md |  2 +-
 specs/deneb/fork-choice.md     |  2 +-
 specs/phase0/fork-choice.md    | 12 ++++++------
 specs/phase0/p2p-interface.md  |  8 ++++----
 4 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/specs/bellatrix/fork-choice.md b/specs/bellatrix/fork-choice.md
index d22436c9d..6c7a31508 100644
--- a/specs/bellatrix/fork-choice.md
+++ b/specs/bellatrix/fork-choice.md
@@ -170,7 +170,7 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert store.finalized_checkpoint.root == get_ancestor_at_epoch_boundary(
+    assert store.finalized_checkpoint.root == get_checkpoint_block(
         store,
         block.parent_root,
         store.finalized_checkpoint.epoch,
diff --git a/specs/deneb/fork-choice.md b/specs/deneb/fork-choice.md
index e76e159c4..8a33fecc5 100644
--- a/specs/deneb/fork-choice.md
+++ b/specs/deneb/fork-choice.md
@@ -82,7 +82,7 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert store.finalized_checkpoint.root == get_ancestor_at_epoch_boundary(
+    assert store.finalized_checkpoint.root == get_checkpoint_block(
         store,
         block.parent_root,
         store.finalized_checkpoint.epoch,
diff --git a/specs/phase0/fork-choice.md b/specs/phase0/fork-choice.md
index 478dd2142..a2bbb8f62 100644
--- a/specs/phase0/fork-choice.md
+++ b/specs/phase0/fork-choice.md
@@ -18,7 +18,7 @@
     - [`get_current_slot`](#get_current_slot)
     - [`compute_slots_since_epoch_start`](#compute_slots_since_epoch_start)
     - [`get_ancestor`](#get_ancestor)
-    - [`get_ancestor_at_epoch_boundary`](#get_ancestor_at_epoch_boundary)
+    - [`get_checkpoint_block`](#get_checkpoint_block)
     - [`get_weight`](#get_weight)
     - [`get_voting_source`](#get_voting_source)
     - [`filter_block_tree`](#filter_block_tree)
@@ -193,10 +193,10 @@ def get_ancestor(store: Store, root: Root, slot: Slot) -> Root:
     return root
 ```
 
-#### `get_ancestor_at_epoch_boundary`
+#### `get_checkpoint_block`
 
 ```python
-def get_ancestor_at_epoch_boundary(store: Store, root: Root, epoch: Epoch) -> Root:
+def get_checkpoint_block(store: Store, root: Root, epoch: Epoch) -> Root:
     """
     Compute the epoch boundary block for epoch ``epoch`` in the chain of block ``root``
     """
@@ -292,7 +292,7 @@ def filter_block_tree(store: Store, block_root: Root, blocks: Dict[Root, BeaconB
 
     correct_finalized = (
         store.finalized_checkpoint.epoch == GENESIS_EPOCH
-        or store.finalized_checkpoint.root == get_ancestor_at_epoch_boundary(
+        or store.finalized_checkpoint.root == get_checkpoint_block(
             store,
             block_root,
             store.finalized_checkpoint.epoch,
@@ -457,7 +457,7 @@ def validate_on_attestation(store: Store, attestation: Attestation, is_from_bloc
     assert store.blocks[attestation.data.beacon_block_root].slot <= attestation.data.slot
 
     # LMD vote must be consistent with FFG vote target
-    assert target.root == get_ancestor_at_epoch_boundary(store, attestation.data.beacon_block_root, target.epoch)
+    assert target.root == get_checkpoint_block(store, attestation.data.beacon_block_root, target.epoch)
 
     # Attestations can only affect the fork choice of subsequent slots.
     # Delay consideration in the fork choice until their slot is in the past.
@@ -520,7 +520,7 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert store.finalized_checkpoint.root == get_ancestor_at_epoch_boundary(
+    assert store.finalized_checkpoint.root == get_checkpoint_block(
         store,
         block.parent_root,
         store.finalized_checkpoint.epoch,
diff --git a/specs/phase0/p2p-interface.md b/specs/phase0/p2p-interface.md
index 56c1b8cfb..5401a15da 100644
--- a/specs/phase0/p2p-interface.md
+++ b/specs/phase0/p2p-interface.md
@@ -317,7 +317,7 @@ The following validations MUST pass before forwarding the `signed_beacon_block`
 - _[REJECT]_ The block's parent (defined by `block.parent_root`) passes validation.
 - _[REJECT]_ The block is from a higher slot than its parent.
 - _[REJECT]_ The current `finalized_checkpoint` is an ancestor of `block` -- i.e.
-  `get_ancestor_at_epoch_boundary(store, block.parent_root, store.finalized_checkpoint.epoch)
+  `get_checkpoint_block(store, block.parent_root, store.finalized_checkpoint.epoch)
   == store.finalized_checkpoint.root`
 - _[REJECT]_ The block is proposed by the expected `proposer_index` for the block's slot
   in the context of the current shuffling (defined by `parent_root`/`slot`).
@@ -356,7 +356,7 @@ The following validations MUST pass before forwarding the `signed_aggregate_and_
   (a client MAY queue aggregates for processing once block is retrieved).
 - _[REJECT]_ The block being voted for (`aggregate.data.beacon_block_root`) passes validation.
 - _[IGNORE]_ The current `finalized_checkpoint` is an ancestor of the `block` defined by `aggregate.data.beacon_block_root` -- i.e.
-  `get_ancestor_at_epoch_boundary(store, aggregate.data.beacon_block_root, finalized_checkpoint.epoch)
+  `get_checkpoint_block(store, aggregate.data.beacon_block_root, finalized_checkpoint.epoch)
   == store.finalized_checkpoint.root`
 
 
@@ -425,9 +425,9 @@ The following validations MUST pass before forwarding the `attestation` on the s
   (a client MAY queue attestations for processing once block is retrieved).
 - _[REJECT]_ The block being voted for (`attestation.data.beacon_block_root`) passes validation.
 - _[REJECT]_ The attestation's target block is an ancestor of the block named in the LMD vote -- i.e.
-  `get_ancestor_at_epoch_boundary(store, attestation.data.beacon_block_root, attestation.data.target.epoch) == attestation.data.target.root`
+  `get_checkpoint_block(store, attestation.data.beacon_block_root, attestation.data.target.epoch) == attestation.data.target.root`
 - _[IGNORE]_ The current `finalized_checkpoint` is an ancestor of the `block` defined by `attestation.data.beacon_block_root` -- i.e.
-  `get_ancestor_at_epoch_boundary(store, attestation.data.beacon_block_root, store.finalized_checkpoint.epoch)
+  `get_checkpoint_block(store, attestation.data.beacon_block_root, store.finalized_checkpoint.epoch)
   == store.finalized_checkpoint.root`
 
 

From 36fcb81b88c87f279cd4b46d094eb58514e9be8b Mon Sep 17 00:00:00 2001
From: Roberto Saltini <saltini.roberto@gmail.com>
Date: Tue, 18 Apr 2023 13:26:16 +1000
Subject: [PATCH 08/12] Break long statement into two statements

---
 specs/bellatrix/fork-choice.md |  3 ++-
 specs/deneb/fork-choice.md     |  3 ++-
 specs/phase0/fork-choice.md    | 15 +++++++++------
 3 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/specs/bellatrix/fork-choice.md b/specs/bellatrix/fork-choice.md
index 6c7a31508..68519ff90 100644
--- a/specs/bellatrix/fork-choice.md
+++ b/specs/bellatrix/fork-choice.md
@@ -170,11 +170,12 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert store.finalized_checkpoint.root == get_checkpoint_block(
+    finalized_checkpoint_block = get_checkpoint_block(
         store,
         block.parent_root,
         store.finalized_checkpoint.epoch,
     )
+    assert store.finalized_checkpoint.root == finalized_checkpoint_block
 
     # Check the block is valid and compute the post-state
     state = pre_state.copy()
diff --git a/specs/deneb/fork-choice.md b/specs/deneb/fork-choice.md
index 8a33fecc5..9faa11077 100644
--- a/specs/deneb/fork-choice.md
+++ b/specs/deneb/fork-choice.md
@@ -82,11 +82,12 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert store.finalized_checkpoint.root == get_checkpoint_block(
+    finalized_checkpoint_block = get_checkpoint_block(
         store,
         block.parent_root,
         store.finalized_checkpoint.epoch,
     )
+    assert store.finalized_checkpoint.root == finalized_checkpoint_block
 
     # [New in Deneb]
     # Check if blob data is available
diff --git a/specs/phase0/fork-choice.md b/specs/phase0/fork-choice.md
index a2bbb8f62..8582547fd 100644
--- a/specs/phase0/fork-choice.md
+++ b/specs/phase0/fork-choice.md
@@ -290,13 +290,15 @@ def filter_block_tree(store: Store, block_root: Root, blocks: Dict[Root, BeaconB
             voting_source.epoch + 2 >= current_epoch
         )
 
+    finalized_checkpoint_block = get_checkpoint_block(
+        store,
+        block.parent_root,
+        store.finalized_checkpoint.epoch,
+    )
+
     correct_finalized = (
         store.finalized_checkpoint.epoch == GENESIS_EPOCH
-        or store.finalized_checkpoint.root == get_checkpoint_block(
-            store,
-            block_root,
-            store.finalized_checkpoint.epoch,
-        )
+        or store.finalized_checkpoint.root == finalized_checkpoint_block
     )
 
     # If expected finalized/justified, add to viable block-tree and signal viability to parent.
@@ -520,11 +522,12 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
     assert block.slot > finalized_slot
     # Check block is a descendant of the finalized block at the checkpoint finalized slot
-    assert store.finalized_checkpoint.root == get_checkpoint_block(
+    finalized_checkpoint_block = get_checkpoint_block(
         store,
         block.parent_root,
         store.finalized_checkpoint.epoch,
     )
+    assert store.finalized_checkpoint.root == finalized_checkpoint_block    
 
     # Check the block is valid and compute the post-state
     state = pre_state.copy()

From c98560597351529f6f782fd434a2937d0f6c296e Mon Sep 17 00:00:00 2001
From: Roberto Saltini <saltini.roberto@gmail.com>
Date: Tue, 18 Apr 2023 13:49:08 +1000
Subject: [PATCH 09/12] Fix copy and past error

---
 specs/phase0/fork-choice.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/specs/phase0/fork-choice.md b/specs/phase0/fork-choice.md
index 8582547fd..0d5bfb4d7 100644
--- a/specs/phase0/fork-choice.md
+++ b/specs/phase0/fork-choice.md
@@ -292,7 +292,7 @@ def filter_block_tree(store: Store, block_root: Root, blocks: Dict[Root, BeaconB
 
     finalized_checkpoint_block = get_checkpoint_block(
         store,
-        block.parent_root,
+        block_root,
         store.finalized_checkpoint.epoch,
     )
 

From b5bd90dd5f6028d59de1fb5c97c5da95ac53b3aa Mon Sep 17 00:00:00 2001
From: Roberto Saltini <saltini.roberto@gmail.com>
Date: Tue, 18 Apr 2023 13:51:13 +1000
Subject: [PATCH 10/12] Applied changes to tests

---
 .../test/phase0/fork_choice/test_get_head.py  | 22 +++++++----
 .../test/phase0/fork_choice/test_on_block.py  | 38 +++++++++++++------
 2 files changed, 40 insertions(+), 20 deletions(-)

diff --git a/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py b/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py
index f5960ff70..f5c3aae15 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py
@@ -479,7 +479,7 @@ def test_voting_source_within_two_epoch(spec, state):
     - store.voting_source[block_root].epoch != store.justified_checkpoint.epoch, and
     - store.unrealized_justifications[block_root].epoch >= store.justified_checkpoint.epoch, and
     - store.voting_source[block_root].epoch + 2 >= current_epoch, and
-    - store.finalized_checkpoint.root == get_ancestor(store, block_root, finalized_slot)
+    - store.finalized_checkpoint.root == get_checkpoint_block(store, block_root, store.finalized_checkpoint.epoch)
     """
     test_steps = []
     # Initialization
@@ -536,8 +536,11 @@ def test_voting_source_within_two_epoch(spec, state):
     assert store.unrealized_justifications[last_fork_block_root].epoch >= store.justified_checkpoint.epoch
     # assert store.voting_source[last_fork_block_root].epoch + 2 >= \
     #     spec.compute_epoch_at_slot(spec.get_current_slot(store))
-    finalized_slot = spec.compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
-    assert store.finalized_checkpoint.root == spec.get_ancestor(store, last_fork_block_root, finalized_slot)
+    assert store.finalized_checkpoint.root == spec.get_checkpoint_block(
+        store,
+        last_fork_block_root,
+        store.finalized_checkpoint.epoch
+    )
     assert spec.get_head(store) == last_fork_block_root
 
     yield 'steps', test_steps
@@ -552,7 +555,7 @@ def test_voting_source_beyond_two_epoch(spec, state):
     - store.voting_source[block_root].epoch != store.justified_checkpoint.epoch, and
     - store.unrealized_justifications[block_root].epoch >= store.justified_checkpoint.epoch, and
     - store.voting_source[block_root].epoch + 2 < current_epoch, and
-    - store.finalized_checkpoint.root == get_ancestor(store, block_root, finalized_slot)
+    - store.finalized_checkpoint.root == get_checkpoint_block(store, block_root, store.finalized_checkpoint.epoch)
     """
     test_steps = []
     # Initialization
@@ -617,8 +620,11 @@ def test_voting_source_beyond_two_epoch(spec, state):
     assert store.unrealized_justifications[last_fork_block_root].epoch >= store.justified_checkpoint.epoch
     # assert store.voting_source[last_fork_block_root].epoch + 2 < \
     #     spec.compute_epoch_at_slot(spec.get_current_slot(store))
-    finalized_slot = spec.compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
-    assert store.finalized_checkpoint.root == spec.get_ancestor(store, last_fork_block_root, finalized_slot)
+    assert store.finalized_checkpoint.root == spec.get_checkpoint_block(
+        store,
+        last_fork_block_root,
+        store.finalized_checkpoint.epoch
+    )
     assert spec.get_head(store) == correct_head
 
     yield 'steps', test_steps
@@ -641,7 +647,7 @@ def test_incorrect_finalized(spec, state):
     # Check that the store doesn't allow for a head block that has:
     # - store.voting_source[block_root].epoch == store.justified_checkpoint.epoch, and
     # - store.finalized_checkpoint.epoch != GENESIS_EPOCH, and
-    # - store.finalized_checkpoint.root != get_ancestor(store, block_root, finalized_slot)
+    # - store.finalized_checkpoint.root != get_checkpoint_block(store, block_root, store.finalized_checkpoint.epoch)
     test_steps = []
     # Initialization
     store, anchor_block = get_genesis_forkchoice_store_and_block(spec, state)
@@ -718,7 +724,7 @@ def test_incorrect_finalized(spec, state):
     assert store.voting_source[last_fork_block_root].epoch == store.justified_checkpoint.epoch
     assert store.finalized_checkpoint.epoch != spec.GENESIS_EPOCH
     finalized_slot = spec.compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
-    assert store.finalized_checkpoint.root != spec.get_ancestor(store, last_fork_block_root, finalized_slot)
+    assert store.finalized_checkpoint.root != spec.get_checkpoint_block(store, block_root, store.finalized_checkpoint.epoch)
     assert spec.get_head(store) != last_fork_block_root
     assert spec.get_head(store) == head_root
 
diff --git a/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_on_block.py b/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_on_block.py
index 0af775339..a3f09c7c9 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_on_block.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_on_block.py
@@ -352,8 +352,7 @@ def test_new_finalized_slot_is_not_justified_checkpoint_ancestor(spec, state):
         # NOTE: Do not call `on_tick` here
         yield from add_block(spec, store, block, test_steps)
 
-    finalized_slot = spec.compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
-    ancestor_at_finalized_slot = spec.get_ancestor(store, pre_store_justified_checkpoint_root, finalized_slot)
+    ancestor_at_finalized_slot = spec.get_checkpoint_block(store, pre_store_justified_checkpoint_root, store.finalized_checkpoint.epoch)
     assert ancestor_at_finalized_slot != store.finalized_checkpoint.root
 
     assert store.finalized_checkpoint == another_state.finalized_checkpoint
@@ -428,8 +427,7 @@ def test_new_finalized_slot_is_justified_checkpoint_ancestor(spec, state):
     for block in all_blocks:
         yield from tick_and_add_block(spec, store, block, test_steps)
 
-    finalized_slot = spec.compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
-    ancestor_at_finalized_slot = spec.get_ancestor(store, pre_store_justified_checkpoint_root, finalized_slot)
+    ancestor_at_finalized_slot = spec.get_checkpoint_block(store, pre_store_justified_checkpoint_root, store.finalized_checkpoint.epoch)
     assert ancestor_at_finalized_slot == store.finalized_checkpoint.root
 
     assert store.finalized_checkpoint == another_state.finalized_checkpoint
@@ -857,10 +855,18 @@ def test_incompatible_justification_update_start_of_epoch(spec, state):
     # Now add the blocks & check that justification update was triggered
     for signed_block in signed_blocks:
         yield from tick_and_add_block(spec, store, signed_block, test_steps)
-    finalized_slot = spec.compute_start_slot_at_epoch(state.finalized_checkpoint.epoch)
-    assert spec.get_ancestor(store, last_block_root, finalized_slot) == state.finalized_checkpoint.root
-    justified_slot = spec.compute_start_slot_at_epoch(state.current_justified_checkpoint.epoch)
-    assert spec.get_ancestor(store, last_block_root, justified_slot) != state.current_justified_checkpoint.root
+    finalized_checkpoint_block = spec.get_checkpoint_block(
+        store,
+        last_block_root,
+        state.finalized_checkpoint.epoch,
+    )
+    assert finalized_checkpoint_block == state.finalized_checkpoint.root
+    justified_checkpoint_block = spec.get_checkpoint_block(
+        store,
+        last_block_root,
+        state.current_justified_checkpoint.epoch,
+    )
+    assert justified_checkpoint_block != state.current_justified_checkpoint.root
     assert store.finalized_checkpoint.epoch == 4
     assert store.justified_checkpoint.epoch == 6
 
@@ -934,10 +940,18 @@ def test_incompatible_justification_update_end_of_epoch(spec, state):
     # Now add the blocks & check that justification update was triggered
     for signed_block in signed_blocks:
         yield from tick_and_add_block(spec, store, signed_block, test_steps)
-    finalized_slot = spec.compute_start_slot_at_epoch(state.finalized_checkpoint.epoch)
-    assert spec.get_ancestor(store, last_block_root, finalized_slot) == state.finalized_checkpoint.root
-    justified_slot = spec.compute_start_slot_at_epoch(state.current_justified_checkpoint.epoch)
-    assert spec.get_ancestor(store, last_block_root, justified_slot) != state.current_justified_checkpoint.root
+    finalized_checkpoint_block = spec.get_checkpoint_block(
+        store,
+        last_block_root,
+        state.finalized_checkpoint.epoch,
+    )
+    assert finalized_checkpoint_block == state.finalized_checkpoint.root
+    justified_checkpoint_block = spec.get_checkpoint_block(
+        store,
+        last_block_root,
+        state.current_justified_checkpoint.epoch,
+    )
+    assert justified_checkpoint_block != state.current_justified_checkpoint.root
     assert store.finalized_checkpoint.epoch == 4
     assert store.justified_checkpoint.epoch == 6
 

From 313439a04b121a1c53585396dbea6df02026404f Mon Sep 17 00:00:00 2001
From: Roberto Saltini <saltini.roberto@gmail.com>
Date: Tue, 18 Apr 2023 13:54:31 +1000
Subject: [PATCH 11/12] Fix lint erorrs

---
 .../test/phase0/fork_choice/test_get_head.py         |  6 +++++-
 .../test/phase0/fork_choice/test_on_block.py         | 12 ++++++++++--
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py b/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py
index f5c3aae15..30f94b854 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py
@@ -724,7 +724,11 @@ def test_incorrect_finalized(spec, state):
     assert store.voting_source[last_fork_block_root].epoch == store.justified_checkpoint.epoch
     assert store.finalized_checkpoint.epoch != spec.GENESIS_EPOCH
     finalized_slot = spec.compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
-    assert store.finalized_checkpoint.root != spec.get_checkpoint_block(store, block_root, store.finalized_checkpoint.epoch)
+    assert store.finalized_checkpoint.root != spec.get_checkpoint_block(
+        store,
+        block_root,
+        store.finalized_checkpoint.epoch
+    )
     assert spec.get_head(store) != last_fork_block_root
     assert spec.get_head(store) == head_root
 
diff --git a/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_on_block.py b/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_on_block.py
index a3f09c7c9..840413a36 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_on_block.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_on_block.py
@@ -352,7 +352,11 @@ def test_new_finalized_slot_is_not_justified_checkpoint_ancestor(spec, state):
         # NOTE: Do not call `on_tick` here
         yield from add_block(spec, store, block, test_steps)
 
-    ancestor_at_finalized_slot = spec.get_checkpoint_block(store, pre_store_justified_checkpoint_root, store.finalized_checkpoint.epoch)
+    ancestor_at_finalized_slot = spec.get_checkpoint_block(
+        store,
+        pre_store_justified_checkpoint_root,
+        store.finalized_checkpoint.epoch
+    )
     assert ancestor_at_finalized_slot != store.finalized_checkpoint.root
 
     assert store.finalized_checkpoint == another_state.finalized_checkpoint
@@ -427,7 +431,11 @@ def test_new_finalized_slot_is_justified_checkpoint_ancestor(spec, state):
     for block in all_blocks:
         yield from tick_and_add_block(spec, store, block, test_steps)
 
-    ancestor_at_finalized_slot = spec.get_checkpoint_block(store, pre_store_justified_checkpoint_root, store.finalized_checkpoint.epoch)
+    ancestor_at_finalized_slot = spec.get_checkpoint_block(
+        store,
+        pre_store_justified_checkpoint_root,
+        store.finalized_checkpoint.epoch
+    )
     assert ancestor_at_finalized_slot == store.finalized_checkpoint.root
 
     assert store.finalized_checkpoint == another_state.finalized_checkpoint

From ffb84598cf5f7d29bd6220e977f76d47c599bb4f Mon Sep 17 00:00:00 2001
From: Roberto Saltini <saltini.roberto@gmail.com>
Date: Tue, 18 Apr 2023 16:03:10 +1000
Subject: [PATCH 12/12] Fixed doc in get_checkpoint_block

---
 specs/phase0/fork-choice.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/specs/phase0/fork-choice.md b/specs/phase0/fork-choice.md
index 0d5bfb4d7..e25ae6e90 100644
--- a/specs/phase0/fork-choice.md
+++ b/specs/phase0/fork-choice.md
@@ -198,7 +198,7 @@ def get_ancestor(store: Store, root: Root, slot: Slot) -> Root:
 ```python
 def get_checkpoint_block(store: Store, root: Root, epoch: Epoch) -> Root:
     """
-    Compute the epoch boundary block for epoch ``epoch`` in the chain of block ``root``
+    Compute the checkpoint block for epoch ``epoch`` in the chain of block ``root``
     """
     epoch_first_slot = compute_start_slot_at_epoch(epoch)
     return get_ancestor(store, root, epoch_first_slot)