From 9853cd91b81a89a763f56b9dcacc15e6f01577fa Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Sat, 11 Apr 2026 07:58:48 +0000
Subject: [PATCH] state: dedupe Reaction events by author

Each peer can react with a given emoji at most once. Switch
ChatMessage::reactions from BTreeMap<String, Vec<EndpointId>> to
BTreeMap<String, BTreeSet<EndpointId>> and use insert() in the
materializer so duplicate Reaction events from the same author
collapse to a single entry.

Without this change, an author replaying or retransmitting a
Reaction would inflate the reactor list, causing UI miscounts
and state-hash divergence between peers.

Closes #111
Progresses #108
---
 crates/state/src/materialize.rs |  2 +-
 crates/state/src/tests.rs       | 55 +++++++++++++++++++++++++++++++--
 crates/state/src/types.rs       |  6 ++--
 3 files changed, 58 insertions(+), 5 deletions(-)

diff --git a/crates/state/src/materialize.rs b/crates/state/src/materialize.rs
index 03473c66..cda4668a 100644
--- a/crates/state/src/materialize.rs
+++ b/crates/state/src/materialize.rs
@@ -386,7 +386,7 @@ fn apply_mutation(state: &mut ServerState, event: &Event) -> ApplyResult {
                 msg.reactions
                     .entry(emoji.clone())
                     .or_default()
-                    .push(event.author);
+                    .insert(event.author);
             }
         }
 
diff --git a/crates/state/src/tests.rs b/crates/state/src/tests.rs
index 93b82902..3374a130 100644
--- a/crates/state/src/tests.rs
+++ b/crates/state/src/tests.rs
@@ -628,8 +628,59 @@ fn duplicate_reaction_from_same_peer() {
     );
 
     let state = materialize(&dag);
-    // Implementation-dependent: may deduplicate or not.
-    assert!(!state.messages[0].reactions.is_empty());
+    // The same author reacting twice with the same emoji collapses to a
+    // single entry — reactions are stored as a set keyed by author.
+    let reactors = state.messages[0]
+        .reactions
+        .get("👍")
+        .expect("emoji should be present");
+    assert_eq!(reactors.len(), 1);
+    assert!(reactors.contains(&admin.endpoint_id()));
+}
+
+#[test]
+fn same_author_duplicate_reaction_is_idempotent() {
+    let admin = Identity::generate();
+    let mut dag = test_dag(&admin);
+
+    let msg = do_emit(
+        &mut dag,
+        &admin,
+        EventKind::Message {
+            channel_id: "ch-1".to_string(),
+            body: "react to me".to_string(),
+            reply_to: None,
+        },
+    );
+
+    // Apply two distinct Reaction events with the same emoji from the
+    // same author. The events themselves are unique (different hashes
+    // because of timestamps/parents), so dedup must happen at
+    // materialization time.
+    do_emit(
+        &mut dag,
+        &admin,
+        EventKind::Reaction {
+            message_id: msg.hash,
+            emoji: "🎉".to_string(),
+        },
+    );
+    do_emit(
+        &mut dag,
+        &admin,
+        EventKind::Reaction {
+            message_id: msg.hash,
+            emoji: "🎉".to_string(),
+        },
+    );
+
+    let state = materialize(&dag);
+    let reactors = state.messages[0]
+        .reactions
+        .get("🎉")
+        .expect("emoji should be present");
+    assert_eq!(reactors.len(), 1, "duplicate reactions must be deduped");
+    assert!(reactors.contains(&admin.endpoint_id()));
 }
 
 #[test]
diff --git a/crates/state/src/types.rs b/crates/state/src/types.rs
index b79496f1..13286801 100644
--- a/crates/state/src/types.rs
+++ b/crates/state/src/types.rs
@@ -70,8 +70,10 @@ pub struct ChatMessage {
     pub edited: bool,
     /// Whether this message has been soft-deleted.
     pub deleted: bool,
-    /// Reactions: emoji string -> list of reactor endpoint IDs.
-    pub reactions: BTreeMap<String, Vec<EndpointId>>,
+    /// Reactions: emoji string -> set of reactor endpoint IDs.
+    /// Stored as a `BTreeSet` so each peer can only react once with a
+    /// given emoji to a given message.
+    pub reactions: BTreeMap<String, BTreeSet<EndpointId>>,
     /// If this is a reply, the EventHash of the parent message.
     pub reply_to: Option<EventHash>,
 }