tests: add test suite for jay_diff; fix two bugs found

Move the inline assertions from jay_diff.py into tests/test_jay_diff.py
as proper pytest functions, grouped by function under test.  Add corner
cases for numeric/bool/None/list values, completely different structures,
empty containers, deep nesting, type changes (dict↔scalar), and
parametrised round-trip tests (diff→merge produces new).

Two bugs fixed during the exercise:
- jay_diff_rev: guard against _old being a non-dict (type change from
  dict to scalar caused TypeError on item assignment)
- jay_merge_delete / jay_merge_update: already fixed in previous commit;
  covered by new tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

requirements.txt

@@ -1,4 +1,5 @@
 carconnectivity
 carconnectivity-connector-volkswagen
 PyQt5
-pyqtgraph
+pyqtgraph
+pytest

tests/test_jay_diff.py

@@ -0,0 +1,522 @@
+"""Tests for utils/jay_diff.py."""
+import pytest
+from copy import deepcopy
+
+from utils.jay_diff import (
+    jay_diff_add, jay_diff_del, jay_diff_upd, jay_diff_upd_add,
+    jay_diff_full, jay_merge_add, jay_merge_delete, jay_merge_update, jay_merge_full,
+)
+
+
+# ── jay_diff_add ──────────────────────────────────────────────────────────────
+
+class TestJayDiffAdd:
+    def test_add_to_empty(self):
+        assert jay_diff_add({}, {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}}
+
+    def test_no_new_keys(self):
+        assert jay_diff_add({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'},
+                            {'a': 'hamburger', 'b': 'fries', 'c': 'coke'}) == {}
+
+    def test_old_has_extra_keys(self):
+        assert jay_diff_add({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi', 'd': 'icecream'},
+                            {'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'}) == {}
+
+    def test_one_new_key(self):
+        assert jay_diff_add({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'},
+                            {'a': 'hamburger', 'd': 'icecream', 'c': 'coke'}) == {'d': 'icecream'}
+
+    def test_new_key_among_overlap(self):
+        assert jay_diff_add({'a': 'hamburger', 'c': 'pepsi', 'd': 'icecream'},
+                            {'a': 'hamburger', 'b': 'fries', 'c': 'coke'}) == {'b': 'fries'}
+
+    def test_nested_new_key(self):
+        assert jay_diff_add({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke'}},
+                            {'c': {'f': 'coffee'}}) == {'c': {'f': 'coffee'}}
+
+    def test_nested_no_new_keys(self):
+        assert jay_diff_add({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                            {'a': 'hamburger', 'b': 'fries', 'c': {'f': 'coffee'}}) == {}
+
+
+# ── jay_diff_del ──────────────────────────────────────────────────────────────
+
+class TestJayDiffDel:
+    def test_one_deleted_key(self):
+        assert jay_diff_del({'a': 'hamburger', 'c': 'pepsi', 'd': 'icecream'},
+                            {'a': 'hamburger', 'b': 'fries', 'c': 'coke'}) == {'d': 'icecream'}
+
+    def test_no_deleted_keys_same_values(self):
+        assert jay_diff_del({'a': 'hamburger', 'b': 'fries', 'c': 'coke'},
+                            {'a': 'hamburger', 'b': 'fries', 'c': 'coke'}) == {}
+
+    def test_no_deleted_keys_different_values(self):
+        assert jay_diff_del({'a': 'hamburger', 'b': 'fries', 'c': 'coke'},
+                            {'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'}) == {}
+
+    def test_key_absent_from_new(self):
+        assert jay_diff_del({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'},
+                            {'a': 'hamburger', 'b': 'fries'}) == {'c': 'pepsi'}
+
+    def test_nested_deleted_key(self):
+        assert jay_diff_del({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                            {'a': 'hamburger', 'b': 'fries'}) == {'c': {'e': 'coke', 'f': 'coffee'}}
+
+    def test_nested_partial_delete(self):
+        assert jay_diff_del({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                            {'c': {'e': 'coke'}}) == {'a': 'hamburger', 'b': 'fries', 'c': {'f': 'coffee'}}
+
+    def test_nested_no_delete_different_value(self):
+        assert jay_diff_del({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                            {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'pepsi', 'f': 'coffee'}}) == {}
+
+
+# ── jay_diff_upd ──────────────────────────────────────────────────────────────
+
+class TestJayDiffUpd:
+    def test_multiple_updates(self):
+        assert jay_diff_upd({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                            {'a': 'hotdog', 'b': 'potatoes'}) == {'a': 'hotdog', 'b': 'potatoes'}
+
+    def test_nested_update(self):
+        assert jay_diff_upd({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                            {'c': {'e': 'pepsi'}}) == {'c': {'e': 'pepsi'}}
+
+    def test_one_update(self):
+        assert jay_diff_upd({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                            {'a': 'hotdog', 'b': 'fries'}) == {'a': 'hotdog'}
+
+    def test_no_update_new_key_ignored(self):
+        assert jay_diff_upd({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'},
+                            {'a': 'hamburger', 'd': 'icecream', 'c': 'coke'}) == {'c': 'coke'}
+
+    def test_nested_no_update_new_subkey_ignored(self):
+        assert jay_diff_upd({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke'}},
+                            {'c': {'f': 'coffee'}}) == {}
+
+    def test_nested_update_one_of_two(self):
+        assert jay_diff_upd({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                            {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'pepsi', 'f': 'coffee'}}) \
+            == {'c': {'e': 'pepsi'}}
+
+
+# ── jay_diff_upd_add ──────────────────────────────────────────────────────────
+
+class TestJayDiffUpdAdd:
+    def test_update_and_add(self):
+        assert jay_diff_upd_add({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                                {'a': 'hotdog', 'b': 'potatoes', 'd': 'icecream'}) \
+            == {'a': 'hotdog', 'b': 'potatoes', 'd': 'icecream'}
+
+    def test_nested_update_and_add(self):
+        assert jay_diff_upd_add({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                                {'c': {'e': 'pepsi', 'g': 'tea', 'h': 'cake'}, 'b': 'potatoes'}) \
+            == {'b': 'potatoes', 'c': {'e': 'pepsi', 'g': 'tea', 'h': 'cake'}}
+
+    def test_update_only(self):
+        assert jay_diff_upd_add({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                                {'a': 'hotdog', 'b': 'fries'}) == {'a': 'hotdog'}
+
+    def test_nested_update_and_new_subkey(self):
+        assert jay_diff_upd_add({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                                {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'pepsi', 'f': 'coffee', 'h': 'cake'}}) \
+            == {'c': {'e': 'pepsi', 'h': 'cake'}}
+
+    def test_add_new_top_and_update(self):
+        assert jay_diff_upd_add({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'},
+                                {'a': 'hamburger', 'd': 'icecream', 'c': 'coke'}) \
+            == {'c': 'coke', 'd': 'icecream'}
+
+    def test_identical(self):
+        assert jay_diff_upd_add({'a': 'hamburger', 'b': 'fries'},
+                                {'a': 'hamburger', 'b': 'fries'}) == {}
+
+
+# ── jay_diff_full (combine_upd_add=False) ─────────────────────────────────────
+
+class TestJayDiffFullSeparate:
+    F = False
+
+    def test_add_all(self):
+        assert jay_diff_full({}, {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}}, self.F) \
+            == {'add': {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}}}
+
+    def test_update_one(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'},
+                             {'a': 'hamburger', 'b': 'fries', 'c': 'coke'}, self.F) \
+            == {'update': {'c': 'coke'}}
+
+    def test_delete_one(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi', 'd': 'icecream'},
+                             {'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'}, self.F) \
+            == {'delete': {'d': 'icecream'}}
+
+    def test_add_delete_update(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'},
+                             {'a': 'hamburger', 'd': 'icecream', 'c': 'coke'}, self.F) \
+            == {'add': {'d': 'icecream'}, 'delete': {'b': 'fries'}, 'update': {'c': 'coke'}}
+
+    def test_identical(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': 'coke'},
+                             {'a': 'hamburger', 'b': 'fries', 'c': 'coke'}, self.F) == {}
+
+    def test_delete_nested(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                             {'a': 'hamburger', 'b': 'fries'}, self.F) \
+            == {'delete': {'c': {'e': 'coke', 'f': 'coffee'}}}
+
+    def test_nested_partial_delete(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                             {'a': 'hamburger', 'b': 'fries', 'c': {'f': 'coffee'}}, self.F) \
+            == {'delete': {'c': {'e': 'coke'}}}
+
+    def test_update_and_delete(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                             {'a': 'hotdog', 'b': 'potatoes'}, self.F) \
+            == {'delete': {'c': {'e': 'coke', 'f': 'coffee'}}, 'update': {'a': 'hotdog', 'b': 'potatoes'}}
+
+    def test_nested_add_delete_update(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                             {'a': 'hotdog', 'b': 'potatoes', 'd': 'icecream'}, self.F) \
+            == {'add': {'d': 'icecream'}, 'delete': {'c': {'e': 'coke', 'f': 'coffee'}}, 'update': {'a': 'hotdog', 'b': 'potatoes'}}
+
+
+# ── jay_diff_full (combine_upd_add=True) ──────────────────────────────────────
+
+class TestJayDiffFullCombined:
+    T = True
+
+    def test_add_all(self):
+        assert jay_diff_full({}, {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}}, self.T) \
+            == {'update_add': {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}}}
+
+    def test_update_one(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'},
+                             {'a': 'hamburger', 'b': 'fries', 'c': 'coke'}, self.T) \
+            == {'update_add': {'c': 'coke'}}
+
+    def test_delete_one(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi', 'd': 'icecream'},
+                             {'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'}, self.T) \
+            == {'delete': {'d': 'icecream'}}
+
+    def test_delete_and_update_add(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'},
+                             {'a': 'hamburger', 'd': 'icecream', 'c': 'coke'}, self.T) \
+            == {'delete': {'b': 'fries'}, 'update_add': {'c': 'coke', 'd': 'icecream'}}
+
+    def test_identical(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': 'coke'},
+                             {'a': 'hamburger', 'b': 'fries', 'c': 'coke'}, self.T) == {}
+
+    def test_delete_nested(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                             {'a': 'hamburger', 'b': 'fries'}, self.T) \
+            == {'delete': {'c': {'e': 'coke', 'f': 'coffee'}}}
+
+    def test_update_and_delete_nested(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                             {'a': 'hotdog', 'b': 'potatoes'}, self.T) \
+            == {'delete': {'c': {'e': 'coke', 'f': 'coffee'}}, 'update_add': {'a': 'hotdog', 'b': 'potatoes'}}
+
+    def test_nested_update_add_and_delete(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                             {'c': {'e': 'pepsi', 'g': 'tea', 'h': 'cake'}, 'b': 'potatoes'}, self.T) \
+            == {'delete': {'a': 'hamburger', 'c': {'f': 'coffee'}},
+                'update_add': {'b': 'potatoes', 'c': {'e': 'pepsi', 'g': 'tea', 'h': 'cake'}}}
+
+    def test_nested_add_subkey(self):
+        assert jay_diff_full({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                             {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'pepsi', 'f': 'coffee', 'h': 'cake'}}, self.T) \
+            == {'update_add': {'c': {'e': 'pepsi', 'h': 'cake'}}}
+
+
+# ── jay_merge_* ───────────────────────────────────────────────────────────────
+
+class TestJayMerge:
+    def test_merge_add_to_empty(self):
+        assert jay_merge_add({}, {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}}
+
+    def test_merge_add_new_key(self):
+        assert jay_merge_add({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                             {'d': 'icecream'}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}, 'd': 'icecream'}
+
+    def test_merge_add_existing_unchanged(self):
+        assert jay_merge_add({'a': 'hamburger', 'b': 'fries', 'c': {'f': 'coffee'}, 'd': 'icecream'},
+                             {'d': 'icecream'}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': {'f': 'coffee'}, 'd': 'icecream'}
+
+    def test_merge_delete_top_key(self):
+        assert jay_merge_delete({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}, 'd': 'icecream'},
+                                {'d': 'icecream'}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}}
+
+    def test_merge_delete_nested_key(self):
+        assert jay_merge_delete({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                                {'c': {'f': 'coffee'}}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke'}}
+
+    def test_merge_delete_nested_leaves_sibling(self):
+        assert jay_merge_delete({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                                {'c': {'e': 'coke'}}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': {'f': 'coffee'}}
+
+
+class TestJayMergeFull:
+    def test_passthrough_when_old_empty(self):
+        assert jay_merge_full({}, {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}}
+
+    def test_empty_diff_unchanged(self):
+        assert jay_merge_full({'a': 'hamburger', 'b': 'fries', 'c': 'coke'}, {}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': 'coke'}
+
+    def test_update(self):
+        assert jay_merge_full({'a': 'hamburger', 'b': 'fries', 'c': 'coke'}, {'update': {'c': 'pepsi'}}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'}
+
+    def test_update_add(self):
+        assert jay_merge_full({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'}, {'update_add': {'c': 'coke'}}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': 'coke'}
+
+    def test_delete(self):
+        assert jay_merge_full({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi', 'd': 'icecream'},
+                              {'delete': {'d': 'icecream'}}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'}
+
+    def test_add_delete_update_combined(self):
+        assert jay_merge_full({'a': 'hamburger', 'b': 'fries', 'c': 'pepsi'},
+                              {'add': {'d': 'icecream'}, 'delete': {'b': 'fries'}, 'update': {'c': 'coke'}}) \
+            == {'a': 'hamburger', 'd': 'icecream', 'c': 'coke'}
+
+    def test_nested_update_and_delete(self):
+        assert jay_merge_full({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                              {'delete': {'a': 'hamburger', 'b': 'fries', 'c': {'f': 'coffee'}},
+                               'update': {'c': {'e': 'pepsi'}}}) \
+            == {'c': {'e': 'pepsi'}}
+
+    def test_update_add_new_subkey(self):
+        assert jay_merge_full({'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}},
+                              {'update_add': {'c': {'e': 'pepsi', 'h': 'cake'}}}) \
+            == {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'pepsi', 'f': 'coffee', 'h': 'cake'}}
+
+    def test_real_world_snapshot_update(self):
+        old = {'charging': {
+            'batteryStatus': {'value': {'carCapturedTimestamp': '2024-10-19T05:34:34Z', 'cruisingRangeElectric_km': 192}},
+            'chargingStatus': {'value': {'carCapturedTimestamp': '2024-10-19T05:34:34Z'}},
+            'chargingSettings': {'value': {'carCapturedTimestamp': '2024-10-19T05:34:30Z'}},
+            'plugStatus': {'value': {'carCapturedTimestamp': '2024-10-19T05:34:29Z'}}}}
+        update = {'charging': {
+            'batteryStatus': {'value': {'carCapturedTimestamp': '2024-10-19T05:35:46Z', 'cruisingRangeElectric_km': 191}},
+            'chargingStatus': {'value': {'carCapturedTimestamp': '2024-10-19T05:35:46Z'}}}}
+        add = {'charging': {'batteryStatus': {'value': {'currentSOC_pct': 55}}},
+               'fuelStatus': {'rangeStatus': {'value': {'primaryEngine': {'currentSOC_pct': 55}}}},
+               'measurements': {'fuelLevelStatus': {'value': {'currentSOC_pct': 55}}}}
+        delete = {'charging': {
+            'chargingSettings': {'value': {'carCapturedTimestamp': '2024-10-19T05:34:30Z'}},
+            'plugStatus': {'value': {'carCapturedTimestamp': '2024-10-19T05:34:29Z'}}}}
+        result = jay_merge_full(old, {'update': update, 'add': add, 'delete': delete})
+        assert result == old
+
+
+# ── Corner cases: data types ───────────────────────────────────────────────────
+
+class TestNumericAndScalarTypes:
+    def test_diff_int_value_changed(self):
+        assert jay_diff_upd({'a': 1, 'b': 2}, {'a': 1, 'b': 3}) == {'b': 3}
+
+    def test_diff_float_value_changed(self):
+        assert jay_diff_upd({'a': 1.0}, {'a': 1.1}) == {'a': 1.1}
+
+    def test_diff_int_unchanged(self):
+        assert jay_diff_upd({'a': 42}, {'a': 42}) == {}
+
+    def test_diff_bool_changed(self):
+        assert jay_diff_upd({'a': True}, {'a': False}) == {'a': False}
+
+    def test_diff_bool_false_to_true(self):
+        assert jay_diff_upd({'online': False}, {'online': True}) == {'online': True}
+
+    def test_diff_none_value_changed(self):
+        assert jay_diff_upd({'a': None}, {'a': 'something'}) == {'a': 'something'}
+
+    def test_diff_value_to_none(self):
+        assert jay_diff_upd({'a': 'something'}, {'a': None}) == {'a': None}
+
+    def test_diff_list_unchanged(self):
+        assert jay_diff_upd({'a': [1, 2, 3]}, {'a': [1, 2, 3]}) == {}
+
+    def test_diff_list_changed(self):
+        assert jay_diff_upd({'a': [1, 2]}, {'a': [1, 2, 3]}) == {'a': [1, 2, 3]}
+
+    def test_add_numeric_key(self):
+        assert jay_diff_add({'a': 1}, {'a': 1, 'b': 2}) == {'b': 2}
+
+    def test_full_combined_numeric(self):
+        old = {'soc': 80, 'range': 300, 'temp': 22.5}
+        new = {'soc': 75, 'range': 280, 'temp': 22.5}
+        diff = jay_diff_full(old, new)
+        assert diff == {'update_add': {'soc': 75, 'range': 280}}
+
+    def test_merge_numeric_update(self):
+        assert jay_merge_full({'soc': 80, 'range': 300}, {'update_add': {'soc': 75}}) \
+            == {'soc': 75, 'range': 300}
+
+
+# ── Corner cases: type changes (dict ↔ scalar) ────────────────────────────────
+
+class TestTypeChanges:
+    def test_dict_replaced_by_scalar_in_diff(self):
+        # A key that was a dict becomes a scalar — upd treats the whole value as changed
+        old = {'a': {'b': 'c'}}
+        new = {'a': 'flat'}
+        diff = jay_diff_upd(old, new)
+        assert diff == {'a': 'flat'}
+
+    def test_merge_dict_to_scalar(self):
+        old = {'a': {'b': 'c'}, 'x': 1}
+        diff = {'update_add': {'a': 'flat'}}
+        result = jay_merge_full(old, diff)
+        assert result == {'a': 'flat', 'x': 1}
+
+    def test_diff_full_dict_to_scalar_combined(self):
+        old = {'status': {'value': 42}}
+        new = {'status': 'unknown'}
+        diff = jay_diff_full(old, new)
+        result = jay_merge_full(deepcopy(old), diff)
+        assert result == new
+
+    def test_diff_add_new_nested_structure(self):
+        # Key absent in old, present as dict in new — treated as addition
+        old = {'a': 1}
+        new = {'a': 1, 'b': {'c': 2}}
+        diff = jay_diff_add(old, new)
+        assert diff == {'b': {'c': 2}}
+
+
+# ── Corner cases: completely different structures ─────────────────────────────
+
+class TestCompletelyDifferentStructure:
+    def test_all_keys_replaced(self):
+        old = {'a': 1, 'b': 2}
+        new = {'x': 10, 'y': 20}
+        diff = jay_diff_full(old, new)
+        assert diff == {'update_add': {'x': 10, 'y': 20}, 'delete': {'a': 1, 'b': 2}}
+
+    def test_all_keys_replaced_deep(self):
+        old = {'charging': {'state': 'charging', 'power': 11}}
+        new = {'electric_drive': {'range': 300, 'soc': 80}}
+        diff = jay_diff_full(old, new)
+        assert diff == {
+            'update_add': {'electric_drive': {'range': 300, 'soc': 80}},
+            'delete': {'charging': {'state': 'charging', 'power': 11}},
+        }
+
+    def test_merge_round_trip_all_keys_replaced(self):
+        old = {'a': 1, 'b': 2}
+        new = {'x': 10, 'y': 20}
+        diff = jay_diff_full(old, new)
+        result = jay_merge_full(deepcopy(old), diff)
+        assert result == new
+
+    def test_diff_disjoint_no_update(self):
+        # No shared keys → no updates, only add/delete
+        old = {'a': 1}
+        new = {'b': 2}
+        diff = jay_diff_upd(old, new)
+        assert diff == {}
+
+    def test_diff_disjoint_add(self):
+        assert jay_diff_add({'a': 1}, {'b': 2}) == {'b': 2}
+
+    def test_diff_disjoint_del(self):
+        assert jay_diff_del({'a': 1}, {'b': 2}) == {'a': 1}
+
+
+# ── Corner cases: empty containers ────────────────────────────────────────────
+
+class TestEmptyContainers:
+    def test_diff_both_empty(self):
+        assert jay_diff_full({}, {}) == {}
+
+    def test_diff_old_empty_new_has_data(self):
+        diff = jay_diff_full({}, {'a': 1})
+        assert diff == {'update_add': {'a': 1}}
+
+    def test_diff_new_empty_old_has_data(self):
+        diff = jay_diff_full({'a': 1}, {})
+        assert diff == {'delete': {'a': 1}}
+
+    def test_merge_delete_all_keys(self):
+        result = jay_merge_full({'a': 1, 'b': 2}, {'delete': {'a': 1, 'b': 2}})
+        assert result == {}
+
+    def test_delete_empty_dict_value(self):
+        # Deleting a key whose value is {} — should not recurse (fixed bug)
+        result = jay_merge_delete({'a': 'keep', 'b': {}}, {'b': {}})
+        assert result == {'a': 'keep'}
+
+    def test_diff_nested_empty_dict(self):
+        old = {'a': {'b': 'c'}}
+        new = {'a': {}}
+        diff = jay_diff_del(old, new)
+        assert diff == {'a': {'b': 'c'}}
+
+
+# ── Corner cases: deep nesting ────────────────────────────────────────────────
+
+class TestDeepNesting:
+    def test_diff_three_levels(self):
+        old = {'l1': {'l2': {'l3': 'old'}}}
+        new = {'l1': {'l2': {'l3': 'new'}}}
+        assert jay_diff_upd(old, new) == {'l1': {'l2': {'l3': 'new'}}}
+
+    def test_merge_three_levels(self):
+        old = {'l1': {'l2': {'l3': 'old', 'keep': 'yes'}}}
+        diff = {'update_add': {'l1': {'l2': {'l3': 'new'}}}}
+        result = jay_merge_full(deepcopy(old), diff)
+        assert result == {'l1': {'l2': {'l3': 'new', 'keep': 'yes'}}}
+
+    def test_diff_full_three_levels(self):
+        old = {'a': {'b': {'c': {'d': 'deep'}}}}
+        new = {'a': {'b': {'c': {'d': 'deeper'}}}}
+        diff = jay_diff_full(old, new)
+        result = jay_merge_full(deepcopy(old), diff)
+        assert result == new
+
+
+# ── Round-trip: merge(old, diff(old, new)) == new ────────────────────────────
+
+class TestRoundTrip:
+    """Verify that applying a diff to old always produces new."""
+
+    @pytest.mark.parametrize("old,new", [
+        ({'a': 1}, {'a': 2}),
+        ({'a': 1, 'b': 2}, {'b': 3, 'c': 4}),
+        ({'x': {'y': 'old'}}, {'x': {'y': 'new', 'z': 'added'}}),
+        ({'a': {'b': 1}, 'c': 2}, {'a': {'b': 1}, 'd': 3}),
+        ({'soc': 80, 'range': 300}, {'soc': 60, 'range': 240, 'temp': 25}),
+        ({'a': True, 'b': None}, {'a': False, 'c': 42}),
+        ({'l1': {'l2': {'v': 1}}}, {'l1': {'l2': {'v': 2}}}),
+        ({}, {'a': 1}),
+        ({'a': 1}, {}),
+    ])
+    def test_round_trip_combined(self, old, new):
+        diff = jay_diff_full(deepcopy(old), deepcopy(new), combine_upd_add=True)
+        result = jay_merge_full(deepcopy(old), diff)
+        assert result == new
+
+    @pytest.mark.parametrize("old,new", [
+        ({'a': 1}, {'a': 2}),
+        ({'a': 1, 'b': 2}, {'b': 3, 'c': 4}),
+        ({'x': {'y': 'old'}}, {'x': {'y': 'new', 'z': 'added'}}),
+        ({}, {'a': 1}),
+        ({'a': 1}, {}),
+    ])
+    def test_round_trip_separate(self, old, new):
+        diff = jay_diff_full(deepcopy(old), deepcopy(new), combine_upd_add=False)
+        result = jay_merge_full(deepcopy(old), diff)
+        assert result == new

utils/jay_diff.py

@@ -3,6 +3,8 @@ from copy import deepcopy
 
 def jay_diff_rev(_old, _new):
 	delta = {}
+	if not isinstance(_old, dict):
+		return delta
 	for k in _new.keys():
 		if k in _old:
 			if isinstance(_new[k], dict):
@@ -131,7 +133,7 @@ def jay_merge_full(_old, _diff):
 
 	return res
 
-def	main() -> None:
+def _legacy_assertions() -> None:
 	result = jay_diff_add({}, {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}})
 	assert result == {'a':'hamburger', 'b':'fries', 'c': {'e': 'coke', 'f': 'coffee'}}
 
@@ -579,4 +581,4 @@ def	main() -> None:
 	assert result == old
 
 if __name__ == '__main__':
-	main()
+	_legacy_assertions()