answering behind counted

1 files changed, 17 insertions, 0 deletions

diff --git a/src/document/sync.rs b/src/document/sync.rs
index f9f0295..179df78 100644
--- a/src/document/sync.rs
+++ b/src/document/sync.rs
@@ -700,6 +700,23 @@ impl SyncSession {
         }
     }
 
+    /// Whether the peer, as the message just applied says, is behind us: all its heads
+    /// in our history, and not all of ours. Asked before `answered_as_delivered`: a
+    /// message taken as delivered from a peer behind us is answered with what it lacks
+    /// (a node that got its writer's changes before its parent did, by another way).
+    pub(crate) fn peer_behind(
+        &self,
+        doc: &Document,
+        heads: &[crate::document::ChangeHash],
+    ) -> bool {
+        self.state.their_heads.as_ref().is_some_and(|theirs| {
+            theirs
+                .iter()
+                .all(|hash| doc.get_change_by_hash(hash).is_some())
+                && heads.iter().any(|hash| !theirs.contains(hash))
+        })
+    }
+
     /// We applied a message its sender took as delivered (`assume_delivered`,
     /// `SyncEnvelope::delivered`) to `doc`: if we hold exactly its sender's heads now,
     /// Automerge's answer, our heads, would tell it nothing. So none goes: we count as