From 512ade7f5827ba14d6fc66ad97ab1ec8dd071fe2 Mon Sep 17 00:00:00 2001
From: Youngjoon Lee <5462944+youngjoon-lee@users.noreply.github.com>
Date: Fri, 28 Feb 2025 17:47:57 +0900
Subject: [PATCH] wip: checkpoint sync

---
 cryptarchia/cryptarchia.py          |  4 ++++
 cryptarchia/error.py                |  2 ++
 cryptarchia/sync/checkpoint_sync.py | 23 +++++++++++++++++++++++
 cryptarchia/sync/full_sync.py       | 25 ++++++++++++++++++++-----
 4 files changed, 49 insertions(+), 5 deletions(-)
 create mode 100644 cryptarchia/error.py
 create mode 100644 cryptarchia/sync/checkpoint_sync.py

diff --git a/cryptarchia/cryptarchia.py b/cryptarchia/cryptarchia.py
index 623fd42..9048a01 100644
--- a/cryptarchia/cryptarchia.py
+++ b/cryptarchia/cryptarchia.py
@@ -11,6 +11,8 @@ from typing import Dict, Generator, List, TypeAlias
 import numpy as np
 from sortedcontainers import SortedDict
 
+from cryptarchia.error import ParentNotFound
+
 logger = logging.getLogger(__name__)
 
 
@@ -454,6 +456,8 @@ class Follower:
             logger.warning("invalid header")
             return
 
+        if block.parent not in self.ledger_state:
+            raise ParentNotFound
         new_state = self.ledger_state[block.parent].copy()
         new_state.apply(block)
         self.ledger_state[block.id()] = new_state
diff --git a/cryptarchia/error.py b/cryptarchia/error.py
new file mode 100644
index 0000000..4f63922
--- /dev/null
+++ b/cryptarchia/error.py
@@ -0,0 +1,2 @@
+class ParentNotFound(Exception):
+    pass
diff --git a/cryptarchia/sync/checkpoint_sync.py b/cryptarchia/sync/checkpoint_sync.py
new file mode 100644
index 0000000..e1a1b1f
--- /dev/null
+++ b/cryptarchia/sync/checkpoint_sync.py
@@ -0,0 +1,23 @@
+from cryptarchia.cryptarchia import Follower, LedgerState, iter_chain
+from cryptarchia.sync.full_sync import full_sync
+
+
+def get_checkpoint(follower: Follower) -> LedgerState:
+    iter = iter_chain(follower.tip_id(), follower.ledger_state)
+    for _ in range(follower.config.k):
+        next(iter)
+    return next(iter)
+
+
+def checkpoint_sync(local: Follower, checkpoint: LedgerState, remotes: list[Follower]):
+    # apply the checkpoint to the local
+    checkpoint_block_id = checkpoint.block.id()
+    local.ledger_state[checkpoint_block_id] = checkpoint
+    local.local_chain = checkpoint_block_id
+    local.forks.remove(checkpoint_block_id)
+    local.block_storage.add_block(checkpoint.block)
+
+    # start forwards sync from the checkpoint
+    orphans = full_sync(local, remotes, local.tip().slot)
+    if orphans:
+        raise NotImplementedError("Orphaned blocks after checkpoint sync")
diff --git a/cryptarchia/sync/full_sync.py b/cryptarchia/sync/full_sync.py
index c7db7be..faa2611 100644
--- a/cryptarchia/sync/full_sync.py
+++ b/cryptarchia/sync/full_sync.py
@@ -1,31 +1,46 @@
 from collections import defaultdict
+from typing import Generator
 
-from cryptarchia.cryptarchia import Follower, Id, Slot
+from cryptarchia.cryptarchia import BlockHeader, Follower, Id, Slot
+from cryptarchia.error import ParentNotFound
 
 SLOT_TOLERANCE = 1
 
 
-def full_sync(local: Follower, remotes: list[Follower], start_slot: Slot):
+def full_sync(
+    local: Follower, remotes: list[Follower], start_slot: Slot
+) -> list[BlockHeader]:
     # Start a full sync from the remotes to the local, starting from the given slot.
+    # Return orphaned blocks that could not be applied to the local.
 
     # Sync only with remotes that are at least SLOT_TOLERANCE ahead of the local.
     # Continue until there is no target to sync with.
     #
     # Group the remotes by their tip slot, and sync only with one remote per group.
     # This is safe as long as the remote provides all blocks necessary for the future fork choice.
+    orphans: list[BlockHeader] = []
     while groups := group_targets(remotes, start_slot):
         for _tip_id, group in groups.items():
             remote = group[0]
-            range_sync(local, remote, start_slot, remote.tip().slot)
+            for orphan in range_sync(local, remote, start_slot, remote.tip().slot):
+                orphans.append(orphan)
         # Update the start_slot to check if the sync should continue.
         start_slot = Slot(local.tip().slot.absolute_slot + 1)
+    return orphans
 
 
-def range_sync(local: Follower, remote: Follower, from_slot: Slot, to_slot: Slot):
+def range_sync(
+    local: Follower, remote: Follower, from_slot: Slot, to_slot: Slot
+) -> Generator[BlockHeader, None, None]:
     # Fetch blocks in the given range of slots from the remote and apply them to the local.
     # Blocks should be fetched in order of slot.
     for block in remote.block_storage.blocks_by_range(from_slot, to_slot):
-        local.on_block(block)
+        try:
+            local.on_block(block)
+        except ParentNotFound:
+            yield block
+        except:
+            raise
 
 
 def group_targets(