feat(sdk): Introduce event_cache::Deduplicator.

This patch introduces `Deduplicator`, an efficient type to detect
duplicated events in the event cache. It uses a bloom filter, and
decorates a collection of events with `Decoration`, which an enum that
marks whether an event is unique, duplicated or invalid.

crates/matrix-sdk/Cargo.toml

@@ -82,6 +82,7 @@ eyeball-im = { workspace = true }
 eyre = { version = "0.6.8", optional = true }
 futures-core = { workspace = true }
 futures-util = { workspace = true }
+growable-bloom-filter = { workspace = true }
 http = { workspace = true }
 imbl = { workspace = true, features = ["serde"] }
 indexmap = "2.0.2"

crates/matrix-sdk/src/event_cache/deduplicator.rs

@@ -0,0 +1,266 @@
+// Copyright 2024 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.
+
+//! Simple but efficient types to find duplicated events. See [`Deduplicator`]
+//! to learn more.
+
+use std::{collections::BTreeSet, sync::Mutex};
+
+use growable_bloom_filter::{GrowableBloom, GrowableBloomBuilder};
+
+use super::store::{Event, RoomEvents};
+
+/// `Deduplicator` is an efficient type to find duplicated events.
+///
+/// It uses a [bloom filter] to provide a fast probabilistic answer to: “has
+/// event E been seen already?”. False positives are possible, while false
+/// negatives are impossible. In the case of a positive reply, we fallback to a
+/// linear (backward) search on all events to check whether it's a false
+/// positive or not
+///
+/// [bloom filter]: https://en.wikipedia.org/wiki/Bloom_filter
+pub struct Deduplicator {
+    bloom_filter: Mutex<GrowableBloom>,
+}
+
+impl Deduplicator {
+    const APPROXIMATED_MAXIMUM_NUMBER_OF_EVENTS: usize = 800_000;
+    const DESIRED_FALSE_POSITIVE_RATE: f64 = 0.001;
+
+    /// Create a new `Deduplicator`.
+    pub fn new() -> Self {
+        Self {
+            bloom_filter: Mutex::new(
+                GrowableBloomBuilder::new()
+                    .estimated_insertions(Self::APPROXIMATED_MAXIMUM_NUMBER_OF_EVENTS)
+                    .desired_error_ratio(Self::DESIRED_FALSE_POSITIVE_RATE)
+                    .build(),
+            ),
+        }
+    }
+
+    /// Scan a collection of events and detect duplications.
+    ///
+    /// This method takes a collection of events `events_to_scan` and returns a
+    /// new collection of events, where each event is decorated by a
+    /// [`Decoration`], so that the caller can decide what to do with these
+    /// events.
+    ///
+    /// Each scanned event will update `Self`'s internal state.
+    ///
+    /// `existing_events` represents all events of a room that already exist.
+    pub fn scan_and_learn<'a, I>(
+        &'a self,
+        events_to_scan: I,
+        existing_events: &'a RoomEvents,
+    ) -> impl Iterator<Item = Decoration<I::Item>> + 'a
+    where
+        I: Iterator<Item = Event> + 'a,
+    {
+        let mut already_seen = BTreeSet::new();
+
+        events_to_scan.map(move |event| {
+            let Some(event_id) = event.event_id() else {
+                // The event has no `event_id`.
+                return Decoration::Invalid(event);
+            };
+
+            if self.bloom_filter.lock().unwrap().check_and_set(&event_id) {
+                // Bloom filter has false positives. We are NOT sure the event is NOT present.
+                // Even if the false positive rate is low, we need to iterate over all events to
+                // ensure it isn't present.
+
+                // But first, let's ensure `event` is not a duplicate from `events`, i.e. if the
+                // iterator itself contains duplicated events! We use a `BTreetSet`, otherwise
+                // using a bloom filter again may generate false positives.
+                if already_seen.contains(&event_id) {
+                    // The iterator contains a duplicated `event`.
+                    return Decoration::Duplicated(event);
+                }
+
+                // Now we can iterate over all events to ensure `event` is not present in
+                // `existing_events`.
+                let duplicated = existing_events.revents().any(|(_position, other_event)| {
+                    other_event.event_id().as_ref() == Some(&event_id)
+                });
+
+                already_seen.insert(event_id);
+
+                if duplicated {
+                    Decoration::Duplicated(event)
+                } else {
+                    Decoration::Unique(event)
+                }
+            } else {
+                already_seen.insert(event_id);
+
+                // Bloom filter has no false negatives. We are sure the event is NOT present: we
+                // can keep it in the iterator.
+                Decoration::Unique(event)
+            }
+        })
+    }
+}
+
+/// Information about the scanned collection of events.
+#[derive(Debug)]
+pub enum Decoration<I> {
+    /// This event is not duplicated.
+    Unique(I),
+
+    /// This event is duplicated.
+    Duplicated(I),
+
+    /// This event is invalid (i.e. not well formed).
+    Invalid(I),
+}
+
+#[cfg(test)]
+mod tests {
+    use assert_matches2::assert_let;
+    use matrix_sdk_base::deserialized_responses::SyncTimelineEvent;
+    use matrix_sdk_test::{EventBuilder, ALICE};
+    use ruma::{events::room::message::RoomMessageEventContent, owned_event_id, EventId};
+
+    use super::*;
+
+    fn sync_timeline_event(event_builder: &EventBuilder, event_id: &EventId) -> SyncTimelineEvent {
+        SyncTimelineEvent::new(event_builder.make_sync_message_event_with_id(
+            *ALICE,
+            event_id,
+            RoomMessageEventContent::text_plain("foo"),
+        ))
+    }
+
+    #[test]
+    fn test_filter_no_duplicate() {
+        let event_builder = EventBuilder::new();
+
+        let event_id_0 = owned_event_id!("$ev0");
+        let event_id_1 = owned_event_id!("$ev1");
+        let event_id_2 = owned_event_id!("$ev2");
+
+        let event_0 = sync_timeline_event(&event_builder, &event_id_0);
+        let event_1 = sync_timeline_event(&event_builder, &event_id_1);
+        let event_2 = sync_timeline_event(&event_builder, &event_id_2);
+
+        let deduplicator = Deduplicator::new();
+        let existing_events = RoomEvents::new();
+
+        let mut events =
+            deduplicator.scan_and_learn([event_0, event_1, event_2].into_iter(), &existing_events);
+
+        assert_let!(Some(Decoration::Unique(event)) = events.next());
+        assert_eq!(event.event_id(), Some(event_id_0));
+
+        assert_let!(Some(Decoration::Unique(event)) = events.next());
+        assert_eq!(event.event_id(), Some(event_id_1));
+
+        assert_let!(Some(Decoration::Unique(event)) = events.next());
+        assert_eq!(event.event_id(), Some(event_id_2));
+
+        assert!(events.next().is_none());
+    }
+
+    #[test]
+    fn test_filter_duplicates_in_new_events() {
+        let event_builder = EventBuilder::new();
+
+        let event_id_0 = owned_event_id!("$ev0");
+        let event_id_1 = owned_event_id!("$ev1");
+
+        let event_0 = sync_timeline_event(&event_builder, &event_id_0);
+        let event_1 = sync_timeline_event(&event_builder, &event_id_1);
+
+        let deduplicator = Deduplicator::new();
+        let existing_events = RoomEvents::new();
+
+        let mut events = deduplicator.scan_and_learn(
+            [
+                event_0.clone(), // OK
+                event_0,         // Not OK
+                event_1,         // OK
+            ]
+            .into_iter(),
+            &existing_events,
+        );
+
+        assert_let!(Some(Decoration::Unique(event)) = events.next());
+        assert_eq!(event.event_id(), Some(event_id_0.clone()));
+
+        assert_let!(Some(Decoration::Duplicated(event)) = events.next());
+        assert_eq!(event.event_id(), Some(event_id_0));
+
+        assert_let!(Some(Decoration::Unique(event)) = events.next());
+        assert_eq!(event.event_id(), Some(event_id_1));
+
+        assert!(events.next().is_none());
+    }
+
+    #[test]
+    fn test_filter_duplicates_with_existing_events() {
+        let event_builder = EventBuilder::new();
+
+        let event_id_0 = owned_event_id!("$ev0");
+        let event_id_1 = owned_event_id!("$ev1");
+        let event_id_2 = owned_event_id!("$ev2");
+
+        let event_0 = sync_timeline_event(&event_builder, &event_id_0);
+        let event_1 = sync_timeline_event(&event_builder, &event_id_1);
+        let event_2 = sync_timeline_event(&event_builder, &event_id_2);
+
+        let deduplicator = Deduplicator::new();
+        let mut existing_events = RoomEvents::new();
+
+        // Simulate `event_1` is inserted inside `existing_events`.
+        {
+            let mut events =
+                deduplicator.scan_and_learn([event_1.clone()].into_iter(), &existing_events);
+
+            assert_let!(Some(Decoration::Unique(event_1)) = events.next());
+            assert_eq!(event_1.event_id(), Some(event_id_1.clone()));
+
+            assert!(events.next().is_none());
+
+            drop(events); // make the borrow checker happy.
+
+            // Now we can push `event_1` inside `existing_events`.
+            existing_events.push_events([event_1]);
+        }
+
+        // `event_1` will be duplicated.
+        {
+            let mut events = deduplicator.scan_and_learn(
+                [
+                    event_0, // OK
+                    event_1, // Not OK
+                    event_2, // Ok
+                ]
+                .into_iter(),
+                &existing_events,
+            );
+
+            assert_let!(Some(Decoration::Unique(event)) = events.next());
+            assert_eq!(event.event_id(), Some(event_id_0));
+
+            assert_let!(Some(Decoration::Duplicated(event)) = events.next());
+            assert_eq!(event.event_id(), Some(event_id_1));
+
+            assert_let!(Some(Decoration::Unique(event)) = events.next());
+            assert_eq!(event.event_id(), Some(event_id_2));
+
+            assert!(events.next().is_none());
+        }
+    }
+}

crates/matrix-sdk/src/event_cache/mod.rs

@@ -62,6 +62,7 @@ use self::{
 };
 use crate::{client::WeakClient, room::WeakRoom, Client};
 
+mod deduplicator;
 mod linked_chunk;
 mod pagination;
 mod store;