actions.py: Refactor generate_topic_history_from_db_rows for clarity.

This refactors the generate_topic_history_from_db_rows function to not
depend upon the assumption of rows passed as parameter to be sorted in
reverse order of max_message_id field.

Additionally, we add sorting and some tests that verify correct
handling of these cases.

zerver/lib/actions.py

@@ -214,26 +214,25 @@ def activity_change_requires_seat_update(user: UserProfile) -> bool:
     return user.realm.has_seat_based_plan and not user.is_bot
 
 def generate_topic_history_from_db_rows(rows: List[Tuple[str, int]]) -> List[Dict[str, Any]]:
-    canonical_topic_names = set()  # type: Set[str]
-    history = []
-    # This algorithm relies on the fact that `rows` is reverse-sorted
-    # by `max_message_id`, so that we will correctly use the highest
-    # max_message_id where multiple topics have the same canonical
-    # name.
-    #
-    # Arguably, we should just reimplment this using a dict rather
-    # than a set to remove that assumption for better readability.
+    canonical_topic_names = {}  # type: Dict[str, Tuple[int, str]]
+
     for (topic_name, max_message_id) in rows:
         canonical_name = topic_name.lower()
-        if canonical_name in canonical_topic_names:
+
+        if canonical_name not in canonical_topic_names:
+            canonical_topic_names[canonical_name] = (max_message_id, topic_name)
             continue
 
-        canonical_topic_names.add(canonical_name)
+        if max_message_id > canonical_topic_names[canonical_name][0]:
+            canonical_topic_names[canonical_name] = (max_message_id, topic_name)
+
+    history = []
+    for canonical_topic, (max_message_id, topic_name) in canonical_topic_names.items():
         history.append(dict(
             name=topic_name,
-            max_id=max_message_id))
-
-    return history
+            max_id=max_message_id)
+        )
+    return sorted(history, key=lambda x: -x['max_id'])
 
 def get_topic_history_for_stream(user_profile: UserProfile,
                                  recipient: Recipient,