From e8a346c852ef02e416d904cdc2cbd4e553f70231 Mon Sep 17 00:00:00 2001
From: Giacomo Pasini <g.pasini98@gmail.com>
Date: Tue, 3 Jun 2025 10:42:46 +0200
Subject: [PATCH] Fix LIB calculations with short chain

Fix LIB calculation where the chain is shorter than K
and add a regression test for this case
---
 cryptarchia/cryptarchia.py      |  2 +-
 cryptarchia/test_fork_choice.py | 25 ++++++++++++++++++++++++-
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/cryptarchia/cryptarchia.py b/cryptarchia/cryptarchia.py
index a46ce89..2e1d5bc 100644
--- a/cryptarchia/cryptarchia.py
+++ b/cryptarchia/cryptarchia.py
@@ -401,7 +401,7 @@ class Follower:
             return
         # prune forks that do not descend from the last immutable block, this is needed to avoid Genesis rule to roll back
         # past the LIB
-        self.lib = next(islice(iter_chain(self.local_chain, self.ledger_state), self.config.k, None), self.local_chain).block.id()
+        self.lib = next(islice(iter_chain(self.local_chain, self.ledger_state), self.config.k, None), self.genesis_state).block.id()
         self.forks = [
             f for f in self.forks if is_ancestor(self.lib, f, self.ledger_state)
         ]
diff --git a/cryptarchia/test_fork_choice.py b/cryptarchia/test_fork_choice.py
index c850f78..06e5c74 100644
--- a/cryptarchia/test_fork_choice.py
+++ b/cryptarchia/test_fork_choice.py
@@ -373,4 +373,27 @@ class TestForkChoice(TestCase):
         follower.on_block(b12)
 
         assert follower.tip_id() == b12.id()
-        assert follower.lib == b7.id(), follower.lib
\ No newline at end of file
+        assert follower.lib == b7.id(), follower.lib
+
+    def test_lib_calc_short_chain(self):
+        # Test that the LIB is correctly calculated for a short chain
+        n_a = Note(sk=0, value=10)
+        notes = [n_a]
+        config = mk_config(notes)
+        config.k = 10
+        genesis = mk_genesis_state(notes)
+        follower = Follower(genesis, config)
+        follower.to_online()
+
+        assert follower.lib == genesis.block.id(), follower.lib
+        blocks = [genesis.block]
+        for i in range(1, 11):
+            b = mk_block(blocks[-1], i, n_a)
+            blocks.append(b)
+            follower.on_block(b)
+            assert follower.lib == genesis.block.id(), follower.lib
+
+        b11 = mk_block(blocks[-1], 11, n_a)
+        follower.on_block(b11)
+
+        assert follower.lib == blocks[1].id(), follower.lib