Unified search query syntax using the full-text search capabilities of the underlying DB. (#11635)

Support a unified search query syntax which leverages more of the full-text
search of each database supported by Synapse.

Supports, with the same syntax across Postgresql 11+ and Sqlite:

- quoted "search terms"
- `AND`, `OR`, `-` (negation) operators
- Matching words based on their stem, e.g. searches for "dog" matches
  documents containing "dogs". 

This is achieved by 

- If on postgresql 11+, pass the user input to `websearch_to_tsquery`
- If on sqlite, manually parse the query and transform it into the sqlite-specific
  query syntax.

Note that postgresql 10, which is close to end-of-life, falls back to using
`phraseto_tsquery`, which only supports a subset of the features.

Multiple terms separated by a space are implicitly ANDed.

Note that:

1. There is no escaping of full-text syntax that might be supported by the database;
  e.g. `NOT`, `NEAR`, `*` in sqlite. This runs the risk that people might discover this
  as accidental functionality and depend on something we don't guarantee.
2. English text is assumed for stemming. To support other languages, either the target
  language needs to be known at the time of indexing the message (via room metadata,
  or otherwise), or a separate index for each language supported could be created.

Sqlite docs: https://www.sqlite.org/fts3.html#full_text_index_queries
Postgres docs: https://www.postgresql.org/docs/11/textsearch-controls.html

changelog.d/11635.feature

@@ -0,0 +1 @@
+Allow use of postgres and sqllite full-text search operators in search queries.

synapse/storage/databases/main/search.py

@@ -11,10 +11,22 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
+import enum
 import logging
 import re
-from typing import TYPE_CHECKING, Any, Collection, Iterable, List, Optional, Set, Tuple
+from collections import deque
+from dataclasses import dataclass
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Collection,
+    Iterable,
+    List,
+    Optional,
+    Set,
+    Tuple,
+    Union,
+)
 
 import attr
 
@@ -27,7 +39,7 @@ from synapse.storage.database import (
     LoggingTransaction,
 )
 from synapse.storage.databases.main.events_worker import EventRedactBehaviour
-from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine, Sqlite3Engine
+from synapse.storage.engines import PostgresEngine, Sqlite3Engine
 from synapse.types import JsonDict
 
 if TYPE_CHECKING:
@@ -421,8 +433,6 @@ class SearchStore(SearchBackgroundUpdateStore):
         """
         clauses = []
 
-        search_query = _parse_query(self.database_engine, search_term)
-
         args: List[Any] = []
 
         # Make sure we don't explode because the person is in too many rooms.
@@ -444,20 +454,24 @@ class SearchStore(SearchBackgroundUpdateStore):
         count_clauses = clauses
 
         if isinstance(self.database_engine, PostgresEngine):
+            search_query = search_term
+            tsquery_func = self.database_engine.tsquery_func
             sql = (
-                "SELECT ts_rank_cd(vector, to_tsquery('english', ?)) AS rank,"
+                f"SELECT ts_rank_cd(vector, {tsquery_func}('english', ?)) AS rank,"
                 " room_id, event_id"
                 " FROM event_search"
-                " WHERE vector @@ to_tsquery('english', ?)"
+                f" WHERE vector @@  {tsquery_func}('english', ?)"
             )
             args = [search_query, search_query] + args
 
             count_sql = (
                 "SELECT room_id, count(*) as count FROM event_search"
-                " WHERE vector @@ to_tsquery('english', ?)"
+                f" WHERE vector @@ {tsquery_func}('english', ?)"
             )
             count_args = [search_query] + count_args
         elif isinstance(self.database_engine, Sqlite3Engine):
+            search_query = _parse_query_for_sqlite(search_term)
+
             sql = (
                 "SELECT rank(matchinfo(event_search)) as rank, room_id, event_id"
                 " FROM event_search"
@@ -469,7 +483,7 @@ class SearchStore(SearchBackgroundUpdateStore):
                 "SELECT room_id, count(*) as count FROM event_search"
                 " WHERE value MATCH ?"
             )
-            count_args = [search_term] + count_args
+            count_args = [search_query] + count_args
         else:
             # This should be unreachable.
             raise Exception("Unrecognized database engine")
@@ -501,7 +515,9 @@ class SearchStore(SearchBackgroundUpdateStore):
 
         highlights = None
         if isinstance(self.database_engine, PostgresEngine):
-            highlights = await self._find_highlights_in_postgres(search_query, events)
+            highlights = await self._find_highlights_in_postgres(
+                search_query, events, tsquery_func
+            )
 
         count_sql += " GROUP BY room_id"
 
@@ -510,7 +526,6 @@ class SearchStore(SearchBackgroundUpdateStore):
         )
 
         count = sum(row["count"] for row in count_results if row["room_id"] in room_ids)
-
         return {
             "results": [
                 {"event": event_map[r["event_id"]], "rank": r["rank"]}
@@ -542,9 +557,6 @@ class SearchStore(SearchBackgroundUpdateStore):
             Each match as a dictionary.
         """
         clauses = []
-
-        search_query = _parse_query(self.database_engine, search_term)
-
         args: List[Any] = []
 
         # Make sure we don't explode because the person is in too many rooms.
@@ -582,20 +594,23 @@ class SearchStore(SearchBackgroundUpdateStore):
             args.extend([origin_server_ts, origin_server_ts, stream])
 
         if isinstance(self.database_engine, PostgresEngine):
+            search_query = search_term
+            tsquery_func = self.database_engine.tsquery_func
             sql = (
-                "SELECT ts_rank_cd(vector, to_tsquery('english', ?)) as rank,"
+                f"SELECT ts_rank_cd(vector, {tsquery_func}('english', ?)) as rank,"
                 " origin_server_ts, stream_ordering, room_id, event_id"
                 " FROM event_search"
-                " WHERE vector @@ to_tsquery('english', ?) AND "
+                f" WHERE vector @@ {tsquery_func}('english', ?) AND "
             )
             args = [search_query, search_query] + args
 
             count_sql = (
                 "SELECT room_id, count(*) as count FROM event_search"
-                " WHERE vector @@ to_tsquery('english', ?) AND "
+                f" WHERE vector @@ {tsquery_func}('english', ?) AND "
             )
             count_args = [search_query] + count_args
         elif isinstance(self.database_engine, Sqlite3Engine):
+
             # We use CROSS JOIN here to ensure we use the right indexes.
             # https://sqlite.org/optoverview.html#crossjoin
             #
@@ -614,13 +629,14 @@ class SearchStore(SearchBackgroundUpdateStore):
                 " CROSS JOIN events USING (event_id)"
                 " WHERE "
             )
+            search_query = _parse_query_for_sqlite(search_term)
             args = [search_query] + args
 
             count_sql = (
                 "SELECT room_id, count(*) as count FROM event_search"
                 " WHERE value MATCH ? AND "
             )
-            count_args = [search_term] + count_args
+            count_args = [search_query] + count_args
         else:
             # This should be unreachable.
             raise Exception("Unrecognized database engine")
@@ -660,7 +676,9 @@ class SearchStore(SearchBackgroundUpdateStore):
 
         highlights = None
         if isinstance(self.database_engine, PostgresEngine):
-            highlights = await self._find_highlights_in_postgres(search_query, events)
+            highlights = await self._find_highlights_in_postgres(
+                search_query, events, tsquery_func
+            )
 
         count_sql += " GROUP BY room_id"
 
@@ -686,7 +704,7 @@ class SearchStore(SearchBackgroundUpdateStore):
         }
 
     async def _find_highlights_in_postgres(
-        self, search_query: str, events: List[EventBase]
+        self, search_query: str, events: List[EventBase], tsquery_func: str
     ) -> Set[str]:
         """Given a list of events and a search term, return a list of words
         that match from the content of the event.
@@ -697,6 +715,7 @@ class SearchStore(SearchBackgroundUpdateStore):
         Args:
             search_query
             events: A list of events
+            tsquery_func: The tsquery_* function to use when making queries
 
         Returns:
             A set of strings.
@@ -729,7 +748,7 @@ class SearchStore(SearchBackgroundUpdateStore):
                 while stop_sel in value:
                     stop_sel += ">"
 
-                query = "SELECT ts_headline(?, to_tsquery('english', ?), %s)" % (
+                query = f"SELECT ts_headline(?, {tsquery_func}('english', ?), %s)" % (
                     _to_postgres_options(
                         {
                             "StartSel": start_sel,
@@ -760,20 +779,128 @@ def _to_postgres_options(options_dict: JsonDict) -> str:
     return "'%s'" % (",".join("%s=%s" % (k, v) for k, v in options_dict.items()),)
 
 
-def _parse_query(database_engine: BaseDatabaseEngine, search_term: str) -> str:
+@dataclass
-    """Takes a plain unicode string from the user and converts it into a form
+class Phrase:
-    that can be passed to database.
+    phrase: List[str]
-    We use this so that we can add prefix matching, which isn't something
+
-    that is supported by default.
+
+class SearchToken(enum.Enum):
+    Not = enum.auto()
+    Or = enum.auto()
+    And = enum.auto()
+
+
+Token = Union[str, Phrase, SearchToken]
+TokenList = List[Token]
+
+
+def _is_stop_word(word: str) -> bool:
+    # TODO Pull these out of the dictionary:
+    #  https://github.com/postgres/postgres/blob/master/src/backend/snowball/stopwords/english.stop
+    return word in {"the", "a", "you", "me", "and", "but"}
+
+
+def _tokenize_query(query: str) -> TokenList:
     """
+    Convert the user-supplied `query` into a TokenList, which can be translated into
+    some DB-specific syntax.
+
+    The following constructs are supported:
+
+    - phrase queries using "double quotes"
+    - case-insensitive `or` and `and` operators
+    - negation of a keyword via unary `-`
+    - unary hyphen to denote NOT e.g. 'include -exclude'
+
+    The following differs from websearch_to_tsquery:
+
+    - Stop words are not removed.
+    - Unclosed phrases are treated differently.
+
+    """
+    tokens: TokenList = []
+
+    # Find phrases.
+    in_phrase = False
+    parts = deque(query.split('"'))
+    for i, part in enumerate(parts):
+        # The contents inside double quotes is treated as a phrase, a trailing
+        # double quote is not implied.
+        in_phrase = bool(i % 2) and i != (len(parts) - 1)
 
         # Pull out the individual words, discarding any non-word characters.
-    results = re.findall(r"([\w\-]+)", search_term, re.UNICODE)
+        words = deque(re.findall(r"([\w\-]+)", part, re.UNICODE))
 
-    if isinstance(database_engine, PostgresEngine):
+        # Phrases have simplified handling of words.
-        return " & ".join(result + ":*" for result in results)
+        if in_phrase:
-    elif isinstance(database_engine, Sqlite3Engine):
+            # Skip stop words.
-        return " & ".join(result + "*" for result in results)
+            phrase = [word for word in words if not _is_stop_word(word)]
+
+            # Consecutive words are implicitly ANDed together.
+            if tokens and tokens[-1] not in (SearchToken.Not, SearchToken.Or):
+                tokens.append(SearchToken.And)
+
+            # Add the phrase.
+            tokens.append(Phrase(phrase))
+            continue
+
+        # Otherwise, not in a phrase.
+        while words:
+            word = words.popleft()
+
+            if word.startswith("-"):
+                tokens.append(SearchToken.Not)
+
+                # If there's more word, put it back to be processed again.
+                word = word[1:]
+                if word:
+                    words.appendleft(word)
+            elif word.lower() == "or":
+                tokens.append(SearchToken.Or)
             else:
-        # This should be unreachable.
+                # Skip stop words.
-        raise Exception("Unrecognized database engine")
+                if _is_stop_word(word):
+                    continue
+
+                # Consecutive words are implicitly ANDed together.
+                if tokens and tokens[-1] not in (SearchToken.Not, SearchToken.Or):
+                    tokens.append(SearchToken.And)
+
+                # Add the search term.
+                tokens.append(word)
+
+    return tokens
+
+
+def _tokens_to_sqlite_match_query(tokens: TokenList) -> str:
+    """
+    Convert the list of tokens to a string suitable for passing to sqlite's MATCH.
+    Assume sqlite was compiled with enhanced query syntax.
+
+    Ref: https://www.sqlite.org/fts3.html#full_text_index_queries
+    """
+    match_query = []
+    for token in tokens:
+        if isinstance(token, str):
+            match_query.append(token)
+        elif isinstance(token, Phrase):
+            match_query.append('"' + " ".join(token.phrase) + '"')
+        elif token == SearchToken.Not:
+            # TODO: SQLite treats NOT as a *binary* operator. Hopefully a search
+            # term has already been added before this.
+            match_query.append(" NOT ")
+        elif token == SearchToken.Or:
+            match_query.append(" OR ")
+        elif token == SearchToken.And:
+            match_query.append(" AND ")
+        else:
+            raise ValueError(f"unknown token {token}")
+
+    return "".join(match_query)
+
+
+def _parse_query_for_sqlite(search_term: str) -> str:
+    """Takes a plain unicode string from the user and converts it into a form
+    that can be passed to sqllite's matchinfo().
+    """
+    return _tokens_to_sqlite_match_query(_tokenize_query(search_term))

synapse/storage/engines/postgres.py

@@ -170,6 +170,22 @@ class PostgresEngine(
         """Do we support the `RETURNING` clause in insert/update/delete?"""
         return True
 
+    @property
+    def tsquery_func(self) -> str:
+        """
+        Selects a tsquery_* func to use.
+
+        Ref: https://www.postgresql.org/docs/current/textsearch-controls.html
+
+        Returns:
+            The function name.
+        """
+        # Postgres 11 added support for websearch_to_tsquery.
+        assert self._version is not None
+        if self._version >= 110000:
+            return "websearch_to_tsquery"
+        return "plainto_tsquery"
+
     def is_deadlock(self, error: Exception) -> bool:
         if isinstance(error, psycopg2.DatabaseError):
             # https://www.postgresql.org/docs/current/static/errcodes-appendix.html

synapse/storage/schema/main/delta/73/10_update_sqlite_fts4_tokenizer.py

@@ -0,0 +1,62 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import json
+
+from synapse.storage.engines import BaseDatabaseEngine, Sqlite3Engine
+from synapse.storage.types import Cursor
+
+
+def run_create(cur: Cursor, database_engine: BaseDatabaseEngine) -> None:
+    """
+    Upgrade the event_search table to use the porter tokenizer if it isn't already
+
+    Applies only for sqlite.
+    """
+    if not isinstance(database_engine, Sqlite3Engine):
+        return
+
+    # Rebuild the table event_search table with tokenize=porter configured.
+    cur.execute("DROP TABLE event_search")
+    cur.execute(
+        """
+        CREATE VIRTUAL TABLE event_search
+        USING fts4 (tokenize=porter, event_id, room_id, sender, key, value )
+        """
+    )
+
+    # Re-run the background job to re-populate the event_search table.
+    cur.execute("SELECT MIN(stream_ordering) FROM events")
+    row = cur.fetchone()
+    min_stream_id = row[0]
+
+    # If there are not any events, nothing to do.
+    if min_stream_id is None:
+        return
+
+    cur.execute("SELECT MAX(stream_ordering) FROM events")
+    row = cur.fetchone()
+    max_stream_id = row[0]
+
+    progress = {
+        "target_min_stream_id_inclusive": min_stream_id,
+        "max_stream_id_exclusive": max_stream_id + 1,
+    }
+    progress_json = json.dumps(progress)
+
+    sql = """
+    INSERT into background_updates (ordering, update_name, progress_json)
+    VALUES (?, ?, ?)
+    """
+
+    cur.execute(sql, (7310, "event_search", progress_json))

tests/storage/test_room_search.py

@@ -12,11 +12,22 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import List, Tuple, Union
+from unittest.case import SkipTest
+from unittest.mock import PropertyMock, patch
+
+from twisted.test.proto_helpers import MemoryReactor
+
 import synapse.rest.admin
 from synapse.api.constants import EventTypes
 from synapse.api.errors import StoreError
 from synapse.rest.client import login, room
+from synapse.server import HomeServer
+from synapse.storage.databases.main import DataStore
+from synapse.storage.databases.main.search import Phrase, SearchToken, _tokenize_query
 from synapse.storage.engines import PostgresEngine
+from synapse.storage.engines.sqlite import Sqlite3Engine
+from synapse.util import Clock
 
 from tests.unittest import HomeserverTestCase, skip_unless
 from tests.utils import USE_POSTGRES_FOR_TESTS
@@ -187,3 +198,205 @@ class EventSearchInsertionTest(HomeserverTestCase):
             ),
         )
         self.assertCountEqual(values, ["hi", "2"])
+
+
+class MessageSearchTest(HomeserverTestCase):
+    """
+    Check message search.
+
+    A powerful way to check the behaviour is to run the following in Postgres >= 11:
+
+        # SELECT websearch_to_tsquery('english', <your string>);
+
+    The result can be compared to the tokenized version for SQLite and Postgres < 11.
+
+    """
+
+    servlets = [
+        synapse.rest.admin.register_servlets_for_client_rest_resource,
+        login.register_servlets,
+        room.register_servlets,
+    ]
+
+    PHRASE = "the quick brown fox jumps over the lazy dog"
+
+    # Each entry is a search query, followed by either a boolean of whether it is
+    # in the phrase OR a tuple of booleans: whether it matches using websearch
+    # and using plain search.
+    COMMON_CASES: List[Tuple[str, Union[bool, Tuple[bool, bool]]]] = [
+        ("nope", False),
+        ("brown", True),
+        ("quick brown", True),
+        ("brown quick", True),
+        ("quick \t brown", True),
+        ("jump", True),
+        ("brown nope", False),
+        ('"brown quick"', (False, True)),
+        ('"jumps over"', True),
+        ('"quick fox"', (False, True)),
+        ("nope OR doublenope", False),
+        ("furphy OR fox", (True, False)),
+        ("fox -nope", (True, False)),
+        ("fox -brown", (False, True)),
+        ('"fox" quick', True),
+        ('"fox quick', True),
+        ('"quick brown', True),
+        ('" quick "', True),
+        ('" nope"', False),
+    ]
+    # TODO Test non-ASCII cases.
+
+    # Case that fail on SQLite.
+    POSTGRES_CASES: List[Tuple[str, Union[bool, Tuple[bool, bool]]]] = [
+        # SQLite treats NOT as a binary operator.
+        ("- fox", (False, True)),
+        ("- nope", (True, False)),
+        ('"-fox quick', (False, True)),
+        # PostgreSQL skips stop words.
+        ('"the quick brown"', True),
+        ('"over lazy"', True),
+    ]
+
+    def prepare(
+        self, reactor: MemoryReactor, clock: Clock, homeserver: HomeServer
+    ) -> None:
+        # Register a user and create a room, create some messages
+        self.register_user("alice", "password")
+        self.access_token = self.login("alice", "password")
+        self.room_id = self.helper.create_room_as("alice", tok=self.access_token)
+
+        # Send the phrase as a message and check it was created
+        response = self.helper.send(self.room_id, self.PHRASE, tok=self.access_token)
+        self.assertIn("event_id", response)
+
+    def test_tokenize_query(self) -> None:
+        """Test the custom logic to tokenize a user's query."""
+        cases = (
+            ("brown", ["brown"]),
+            ("quick brown", ["quick", SearchToken.And, "brown"]),
+            ("quick \t brown", ["quick", SearchToken.And, "brown"]),
+            ('"brown quick"', [Phrase(["brown", "quick"])]),
+            ("furphy OR fox", ["furphy", SearchToken.Or, "fox"]),
+            ("fox -brown", ["fox", SearchToken.Not, "brown"]),
+            ("- fox", [SearchToken.Not, "fox"]),
+            ('"fox" quick', [Phrase(["fox"]), SearchToken.And, "quick"]),
+            # No trailing double quoe.
+            ('"fox quick', ["fox", SearchToken.And, "quick"]),
+            ('"-fox quick', [SearchToken.Not, "fox", SearchToken.And, "quick"]),
+            ('" quick "', [Phrase(["quick"])]),
+            (
+                'q"uick brow"n',
+                [
+                    "q",
+                    SearchToken.And,
+                    Phrase(["uick", "brow"]),
+                    SearchToken.And,
+                    "n",
+                ],
+            ),
+            (
+                '-"quick brown"',
+                [SearchToken.Not, Phrase(["quick", "brown"])],
+            ),
+        )
+
+        for query, expected in cases:
+            tokenized = _tokenize_query(query)
+            self.assertEqual(
+                tokenized, expected, f"{tokenized} != {expected} for {query}"
+            )
+
+    def _check_test_cases(
+        self,
+        store: DataStore,
+        cases: List[Tuple[str, Union[bool, Tuple[bool, bool]]]],
+        index=0,
+    ) -> None:
+        # Run all the test cases versus search_msgs
+        for query, expect_to_contain in cases:
+            if isinstance(expect_to_contain, tuple):
+                expect_to_contain = expect_to_contain[index]
+
+            result = self.get_success(
+                store.search_msgs([self.room_id], query, ["content.body"])
+            )
+            self.assertEquals(
+                result["count"],
+                1 if expect_to_contain else 0,
+                f"expected '{query}' to match '{self.PHRASE}'"
+                if expect_to_contain
+                else f"'{query}' unexpectedly matched '{self.PHRASE}'",
+            )
+            self.assertEquals(
+                len(result["results"]),
+                1 if expect_to_contain else 0,
+                "results array length should match count",
+            )
+
+        # Run them again versus search_rooms
+        for query, expect_to_contain in cases:
+            if isinstance(expect_to_contain, tuple):
+                expect_to_contain = expect_to_contain[index]
+
+            result = self.get_success(
+                store.search_rooms([self.room_id], query, ["content.body"], 10)
+            )
+            self.assertEquals(
+                result["count"],
+                1 if expect_to_contain else 0,
+                f"expected '{query}' to match '{self.PHRASE}'"
+                if expect_to_contain
+                else f"'{query}' unexpectedly matched '{self.PHRASE}'",
+            )
+            self.assertEquals(
+                len(result["results"]),
+                1 if expect_to_contain else 0,
+                "results array length should match count",
+            )
+
+    def test_postgres_web_search_for_phrase(self):
+        """
+        Test searching for phrases using typical web search syntax, as per postgres' websearch_to_tsquery.
+        This test is skipped unless the postgres instance supports websearch_to_tsquery.
+        """
+
+        store = self.hs.get_datastores().main
+        if not isinstance(store.database_engine, PostgresEngine):
+            raise SkipTest("Test only applies when postgres is used as the database")
+
+        if store.database_engine.tsquery_func != "websearch_to_tsquery":
+            raise SkipTest(
+                "Test only applies when postgres supporting websearch_to_tsquery is used as the database"
+            )
+
+        self._check_test_cases(store, self.COMMON_CASES + self.POSTGRES_CASES, index=0)
+
+    def test_postgres_non_web_search_for_phrase(self):
+        """
+        Test postgres searching for phrases without using web search, which is used when websearch_to_tsquery isn't
+        supported by the current postgres version.
+        """
+
+        store = self.hs.get_datastores().main
+        if not isinstance(store.database_engine, PostgresEngine):
+            raise SkipTest("Test only applies when postgres is used as the database")
+
+        # Patch supports_websearch_to_tsquery to always return False to ensure we're testing the plainto_tsquery path.
+        with patch(
+            "synapse.storage.engines.postgres.PostgresEngine.tsquery_func",
+            new_callable=PropertyMock,
+        ) as supports_websearch_to_tsquery:
+            supports_websearch_to_tsquery.return_value = "plainto_tsquery"
+            self._check_test_cases(
+                store, self.COMMON_CASES + self.POSTGRES_CASES, index=1
+            )
+
+    def test_sqlite_search(self):
+        """
+        Test sqlite searching for phrases.
+        """
+        store = self.hs.get_datastores().main
+        if not isinstance(store.database_engine, Sqlite3Engine):
+            raise SkipTest("Test only applies when sqlite is used as the database")
+
+        self._check_test_cases(store, self.COMMON_CASES, index=0)