1 files changed, 19 insertions, 9 deletions

diff --git a/src/access/screen.rs b/src/access/screen.rs
index 68e4db7..3a516e3 100644
--- a/src/access/screen.rs
+++ b/src/access/screen.rs
@@ -375,7 +375,7 @@ pub(crate) fn apply(
     }
 
     // The changes of the document each accepted one, not applied yet, builds on.
-    let mut frontier: HashMap<ChangeHash, Vec<ChangeHash>> = HashMap::new();
+    let mut frontier: HashMap<ChangeHash, std::rc::Rc<[ChangeHash]>> = HashMap::new();
     // Refused in this batch, each with the change that broke a rule.
     let mut refused: HashMap<ChangeHash, ChangeHash> = HashMap::new();
     let mut unsigned: HashSet<ChangeHash> = HashSet::new();
@@ -458,16 +458,26 @@ pub(crate) fn apply(
             unapplied.clear();
             signed = None;
         }
-        let mut seen_at: BTreeSet<ChangeHash> = BTreeSet::new();
-        for dep in change.deps() {
-            match frontier.get(dep) {
-                Some(heads) => seen_at.extend(heads.iter().copied()),
-                None => {
-                    seen_at.insert(*dep);
+        // Shared with what it builds on, if it builds on one alone (most do): the same
+        // heads, not a copy each.
+        let seen_at: std::rc::Rc<[ChangeHash]> = match change.deps() {
+            [dep] => frontier
+                .get(dep)
+                .cloned()
+                .unwrap_or_else(|| std::rc::Rc::from([*dep])),
+            deps => {
+                let mut seen_at: BTreeSet<ChangeHash> = BTreeSet::new();
+                for dep in deps {
+                    match frontier.get(dep) {
+                        Some(heads) => seen_at.extend(heads.iter().copied()),
+                        None => {
+                            seen_at.insert(*dep);
+                        }
+                    }
                 }
+                seen_at.into_iter().collect()
             }
-        }
-        let seen_at: Vec<ChangeHash> = seen_at.into_iter().collect();
+        };
         let root = change.deps().is_empty();
         let judged = if judging || root {
             rules::judge_reading(