1 files changed, 27 insertions, 2 deletions
diff --git a/src/document/replica.rs b/src/document/replica.rs
index af807ea..e24f93c 100644
--- a/src/document/replica.rs
+++ b/src/document/replica.rs
@@ -9,7 +9,7 @@
 use std::fmt;
 use std::ops::{Deref, DerefMut};
 
-use automerge::AutoCommit;
+use automerge::{AutoCommit, ChangeHash};
 
 /// One generation of the swarm document.
 ///
@@ -77,14 +77,39 @@ impl fmt::Debug for Epoch {
 ///
 /// Derefs to the document, so code that only needs the `AutoCommit` keeps working
 /// with a guard of `Replica`.
+///
+/// The heads are cached: `AutoCommit::get_heads` takes `&mut self` (it closes a pending
+/// transaction), so without the cache every keepalive and status read would need the
+/// write lock. Every path that changes the document under the write lock calls
+/// [`Replica::refresh_heads`] before releasing it (`AppState::write_doc`, `mutate_doc`,
+/// `switch_epoch`, the sync apply); readers take the read lock and use
+/// [`Replica::heads`].
 pub struct Replica {
     pub epoch: Epoch,
     pub doc: AutoCommit,
+    heads: Vec<ChangeHash>,
 }
 
 impl Replica {
     pub fn new(epoch: Epoch, doc: AutoCommit) -> Self {
-        Self { epoch, doc }
+        let mut replica = Self {
+            epoch,
+            doc,
+            heads: Vec::new(),
+        };
+        replica.refresh_heads();
+        replica
+    }
+
+    /// The document's heads as of the last refresh, sorted.
+    pub fn heads(&self) -> &[ChangeHash] {
+        &self.heads
+    }
+
+    /// Call after changing the document (under the write lock).
+    pub fn refresh_heads(&mut self) {
+        self.heads = self.doc.get_heads();
+        self.heads.sort_unstable();
     }
 }