1 files changed, 53 insertions, 0 deletions
diff --git a/src/remake/snapshot.rs b/src/remake/snapshot.rs
index ca50c86..3c180ba 100644
--- a/src/remake/snapshot.rs
+++ b/src/remake/snapshot.rs
@@ -226,6 +226,38 @@ pub(crate) fn verify_document(doc: &mut AutoCommit, epoch: Epoch) -> Result<Epoc
     parse_metadata(&at_snapshot, epoch.generation)
 }
 
+/// The epoch `doc` is on: genesis if it holds no epoch metadata; otherwise the newest
+/// generation it has metadata for, whose snapshot change is the document's only change
+/// without dependencies (verified). For a document loaded from a backup.
+pub(crate) fn epoch_of(doc: &mut AutoCommit) -> Result<Epoch, String> {
+    let generation = store::keys_at(doc, &[META_ROOT.to_owned(), EPOCHS.to_owned()])
+        .map_err(|error| error.to_string())?
+        .iter()
+        .filter_map(|key| key.parse::<u64>().ok())
+        .max();
+    let Some(generation) = generation.filter(|generation| *generation > 0) else {
+        return Ok(Epoch::GENESIS);
+    };
+    let roots: Vec<ChangeHash> = doc
+        .get_changes(&[])
+        .iter()
+        .filter(|change| change.deps().is_empty())
+        .map(|change| change.hash())
+        .collect();
+    let [root] = roots.as_slice() else {
+        return Err(format!(
+            "generation {generation} needs exactly one change without dependencies (its snapshot), found {}",
+            roots.len()
+        ));
+    };
+    let epoch = Epoch {
+        generation,
+        id: root.0,
+    };
+    verify_document(doc, epoch)?;
+    Ok(epoch)
+}
+
 /// Metadata of an earlier generation, as the current document holds it. Snapshots
 /// carry the metadata of all earlier epochs forward (only remake records stay
 /// behind), so a node many epochs behind can still find the epoch that followed its own.
@@ -523,6 +555,27 @@ mod tests {
     }
 
     #[test]
+    fn epoch_of_a_backup() {
+        let mut a = doc(b"actor-a");
+        store::write_path(&mut a, &["k".into()], &json!(1)).unwrap();
+        a.commit();
+        assert_eq!(epoch_of(&mut a), Ok(Epoch::GENESIS));
+
+        let heads = a.get_heads();
+        let policy = json!({});
+        let mut snap = build(&mut a, &params(&heads, &policy)).unwrap();
+        // Saved and loaded, with a write since, as a backup would be.
+        let mut later = snap
+            .doc
+            .fork()
+            .with_actor(ActorId::from(b"actor-b".as_slice()));
+        store::write_path(&mut later, &["k".into()], &json!(2)).unwrap();
+        later.commit();
+        let mut backup = AutoCommit::load(&later.save()).unwrap();
+        assert_eq!(epoch_of(&mut backup), Ok(snap.epoch));
+    }
+
+    #[test]
     fn metadata_written_and_remake_records_dropped() {
         let mut a = doc(b"actor-a");
         store::write_path(