radicle/cob/identity: Use verdicts to count votes

Evaluation of this COB is implemented under the assumption that
delegates accept at most one revision.

This can lead to issues, if some delegates create new revisions eagerly,
without waiting for the others to vote on earlier revisions. As the
eager delegates start accepting newer and newer revisions, this shadows
their acceptance of earlier revisions, which leads to failure to
recognize that these earlier revisions were actually accepted by a
majority.

To avoid such situations, remove `heads`, and always count the number of
"accept" verdicts explicitly.

crates/radicle/src/cob/identity.rs

@@ -177,9 +177,6 @@ pub struct Identity {
     pub current: RevisionId,
     /// The initial revision of the document.
     pub root: RevisionId,
-    /// The latest revision that each delegate has accepted.
-    /// Delegates can only accept one revision at a time.
-    pub heads: BTreeMap<Did, RevisionId>,
 
     /// Revisions.
     revisions: BTreeMap<RevisionId, Option<Revision>>,
@@ -209,12 +206,6 @@ impl Identity {
             id: revision.blob.into(),
             root,
             current: root,
-            heads: revision
-                .delegates()
-                .iter()
-                .copied()
-                .map(|did| (did, root))
-                .collect(),
             revisions: BTreeMap::from_iter([(root, Some(revision))]),
             timeline: vec![root],
         }
@@ -493,7 +484,6 @@ impl Identity {
                 }
                 assert_eq!(revision.parent, Some(current.id));
 
-                self.heads.insert(author.into(), id);
                 revision.accept(author, signature, &current)?;
 
                 self.adopt(id);
@@ -606,7 +596,6 @@ impl Identity {
                 );
                 let id = revision.id;
 
-                self.heads.insert(author.into(), id);
                 self.revisions.insert(id, Some(revision));
 
                 if state == State::Active {
@@ -623,10 +612,9 @@ impl Identity {
             return;
         }
         let votes = self
-            .heads
-            .values()
-            .filter(|revision| **revision == id)
-            .count();
+            .revision(&id)
+            .map(|revision| revision.accepted().count())
+            .unwrap_or_default();
         if self.is_majority(votes) {
             self.current = id;
             self.current_mut().state = State::Accepted;