Add next_branch_head and prev_branch_head methods to history

src/history.rs

@@ -145,6 +145,28 @@ impl<E, S> History<E, S> {
         At::new(self.root, self.record.index)
     }
 
+    /// Returns the head of the next branch in the history.
+    ///
+    /// This will be the first edit that was stored in the branch.
+    /// This can be used in combination with [`History::go_to`] to go to the next branch.
+    pub fn next_branch_head(&self) -> Option<At> {
+        self.branches
+            .range(self.root + 1..)
+            .next()
+            .map(|(&id, branch)| At::new(id, branch.parent.index + 1))
+    }
+
+    /// Returns the head of the previous branch in the history.
+    ///
+    /// This will be the first edit that was stored in the branch.
+    /// This can be used in combination with [`History::go_to`] to go to the previous branch.
+    pub fn prev_branch_head(&self) -> Option<At> {
+        self.branches
+            .range(..self.root)
+            .next_back()
+            .map(|(&id, branch)| At::new(id, branch.parent.index + 1))
+    }
+
     /// Returns the entry at the index in the current root branch.
     ///
     /// Use [History::get_branch] if you want to get entry from other branches.

tests/history.rs

@@ -121,3 +121,26 @@ fn checkpoint() {
     checkpoint.cancel(&mut target);
     assert_eq!(target, "");
 }
+
+#[test]
+fn next_and_prev() {
+    let mut target = String::new();
+    let mut history = History::new();
+    history.edit(&mut target, A);
+    history.edit(&mut target, B);
+    history.edit(&mut target, C);
+    assert_eq!(history.next_branch_head(), None);
+    assert_eq!(history.prev_branch_head(), None);
+
+    history.undo(&mut target).unwrap();
+    history.undo(&mut target).unwrap();
+    history.edit(&mut target, D);
+    history.edit(&mut target, E);
+    assert_eq!(history.next_branch_head(), None);
+    let prev_head = history.prev_branch_head().unwrap();
+    assert_eq!(prev_head, At::new(0, 2));
+
+    history.go_to(&mut target, prev_head);
+    assert_eq!(history.next_branch_head(), Some(At::new(1, 2)));
+    assert_eq!(history.prev_branch_head(), None);
+}