2 files changed, 15 insertions, 5 deletions

diff --git a/src/data/mod.rs b/src/data/mod.rs
index 7c00442..5d3efa6 100644
--- a/src/data/mod.rs
+++ b/src/data/mod.rs
@@ -2,3 +2,4 @@
 
 pub mod arena;
 pub(crate) mod graph;
+pub mod pubsub;
diff --git a/src/data/pubsub.rs b/src/data/pubsub.rs
index b3b263c..1ac7498 100644
--- a/src/data/pubsub.rs
+++ b/src/data/pubsub.rs
@@ -1,16 +1,18 @@
 //! Publish-subscribe mechanism for internal events.
 
+use serde::export::PhantomData;
 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 {
+pub struct PubSubCursor<T> {
     // Index of the next message to read.
     id: u32,
     next: u32,
+    _phantom: PhantomData<T>,
 }
 
-impl PubSubCursor {
+impl<T> PubSubCursor<T> {
     fn id(&self, num_deleted: u32) -> usize {
         (self.id - num_deleted) as usize
     }
@@ -53,18 +55,25 @@ impl<T> PubSub<T> {
     /// Subscribe to the queue.
     ///
     /// A subscription cannot be cancelled.
-    pub fn subscribe(&mut self) -> PubSubCursor {
+    pub fn subscribe(&mut self) -> PubSubCursor<T> {
         let cursor = PubSubCursor {
             next: self.messages.len() as u32 + self.deleted_messages,
             id: self.offsets.len() as u32 + self.deleted_offsets,
+            _phantom: PhantomData,
         };
 
         self.offsets.push_back(cursor.next);
         cursor
     }
 
+    /// Read the i-th message not yet read by the given subsciber.
+    pub fn read_ith(&self, cursor: &PubSubCursor<T>, i: usize) -> Option<&T> {
+        self.messages
+            .get(cursor.next(self.deleted_messages) as usize + i)
+    }
+
     /// Get the messages not yet read by the given subscriber.
-    pub fn read(&self, cursor: &PubSubCursor) -> impl Iterator<Item = &T> {
+    pub fn read(&self, cursor: &PubSubCursor<T>) -> impl Iterator<Item = &T> {
         let next = cursor.next(self.deleted_messages);
 
         // TODO: use self.queue.range(next..) once it is stabilised.
@@ -77,7 +86,7 @@ impl<T> PubSub<T> {
     /// 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) {
+    pub fn ack(&mut self, cursor: &mut PubSubCursor<T>) {
         // Update the cursor.
         cursor.next = self.messages.len() as u32 + self.deleted_messages;
         self.offsets[cursor.id(self.deleted_offsets)] = u32::MAX;