Add a simple publish-subscribe mechanism.

src/data/pubsub.rs

@@ -0,0 +1,121 @@
+//! Publish-subscribe mechanism for internal events.
+
+use std::collections::VecDeque;
+
+/// The position of a subscriber on a pub-sub queue.
+#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
+pub struct PubSubCursor {
+    // Index of the next message to read.
+    id: u32,
+    next: u32,
+}
+
+impl PubSubCursor {
+    fn id(&self, num_deleted: u32) -> usize {
+        (self.id - num_deleted) as usize
+    }
+
+    fn next(&self, num_deleted: u32) -> usize {
+        (self.next - num_deleted) as usize
+    }
+}
+
+/// A pub-sub queue.
+#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
+pub struct PubSub<T> {
+    deleted_messages: u32,
+    deleted_offsets: u32,
+    messages: VecDeque<T>,
+    offsets: VecDeque<u32>,
+}
+
+impl<T> PubSub<T> {
+    /// Create a new empty pub-sub queue.
+    pub fn new() -> Self {
+        Self {
+            deleted_offsets: 0,
+            deleted_messages: 0,
+            messages: VecDeque::new(),
+            offsets: VecDeque::new(),
+        }
+    }
+
+    /// Publish a new message.
+    pub fn publish(&mut self, message: T) {
+        if self.offsets.is_empty() {
+            // No subscribers, drop the message.
+            return;
+        }
+
+        self.messages.push_back(message);
+    }
+
+    /// Subscribe to the queue.
+    ///
+    /// A subscription cannot be cancelled.
+    pub fn subscribe(&mut self) -> PubSubCursor {
+        let cursor = PubSubCursor {
+            next: self.messages.len() as u32 + self.deleted_messages,
+            id: self.offsets.len() as u32 + self.deleted_offsets,
+        };
+
+        self.offsets.push_back(cursor.next);
+        cursor
+    }
+
+    /// Get the messages not yet read by the given subscriber.
+    pub fn read(&self, cursor: &PubSubCursor) -> impl Iterator<Item = &T> {
+        let next = cursor.next(self.deleted_messages);
+
+        // TODO: use self.queue.range(next..) once it is stabilised.
+        MessageRange {
+            queue: &self.messages,
+            next,
+        }
+    }
+
+    /// Makes the given subscribe acknowledge all the messages in the queue.
+    ///
+    /// A subscriber cannot read acknowledged messages any more.
+    pub fn ack(&mut self, cursor: &mut PubSubCursor) {
+        // Update the cursor.
+        cursor.next = self.messages.len() as u32 + self.deleted_messages;
+        self.offsets[cursor.id(self.deleted_offsets)] = u32::MAX;
+        cursor.id = self.offsets.len() as u32 + self.deleted_offsets;
+        self.offsets.push_back(cursor.next);
+
+        // Now clear the messages we don't need to
+        // maintain in memory anymore.
+        while self.offsets.front() == Some(&u32::MAX) {
+            self.offsets.pop_front();
+            self.deleted_offsets += 1;
+        }
+
+        // There must be at least one offset otherwise
+        // that would mean we have no subscribers.
+        let next = self.offsets.front().unwrap();
+        let num_to_delete = *next - self.deleted_messages;
+
+        for _ in 0..num_to_delete {
+            self.messages.pop_front();
+        }
+
+        self.deleted_messages += num_to_delete;
+    }
+}
+
+struct MessageRange<'a, T> {
+    queue: &'a VecDeque<T>,
+    next: usize,
+}
+
+impl<'a, T> Iterator for MessageRange<'a, T> {
+    type Item = &'a T;
+
+    #[inline(always)]
+    fn next(&mut self) -> Option<&'a T> {
+        let result = self.queue.get(self.next);
+        self.next += 1;
+        result
+    }
+}