Add TreeDiff trait to reuse tree functions (#11046) (#11048)

Co-authored-by: Carl <carl@solana.com>
(cherry picked from commit e9cbdf711b)

Co-authored-by: carllin <wumu727@gmail.com>

core/src/tree_diff.rs

@@ -0,0 +1,32 @@
+use solana_sdk::clock::Slot;
+use std::collections::HashSet;
+
+pub trait TreeDiff {
+    fn children(&self, slot: Slot) -> Option<&[Slot]>;
+
+    fn contains_slot(&self, slot: Slot) -> bool;
+
+    // Find all nodes reachable from `root1`, excluding subtree at `root2`
+    fn subtree_diff(&self, root1: Slot, root2: Slot) -> HashSet<Slot> {
+        if !self.contains_slot(root1) {
+            return HashSet::new();
+        }
+        let mut pending_slots = vec![root1];
+        let mut reachable_set = HashSet::new();
+        while !pending_slots.is_empty() {
+            let current_slot = pending_slots.pop().unwrap();
+            if current_slot == root2 {
+                continue;
+            }
+            reachable_set.insert(current_slot);
+            for child in self
+                .children(current_slot)
+                .expect("slot was discovered earlier, must exist")
+            {
+                pending_slots.push(*child);
+            }
+        }
+
+        reachable_set
+    }
+}