Compute Switch Threshold (#9218)

* Add switching threshold check

Co-authored-by: Carl <carl@solana.com>

core/src/pubkey_references.rs

@@ -0,0 +1,21 @@
+use solana_sdk::pubkey::Pubkey;
+use std::{collections::HashSet, rc::Rc};
+
+#[derive(Default)]
+pub struct PubkeyReferences(HashSet<Rc<Pubkey>>);
+
+impl PubkeyReferences {
+    pub fn get_or_insert(&mut self, pubkey: &Pubkey) -> Rc<Pubkey> {
+        let mut cached_pubkey: Option<Rc<Pubkey>> = self.0.get(pubkey).cloned();
+        if cached_pubkey.is_none() {
+            let new_pubkey = Rc::new(*pubkey);
+            self.0.insert(new_pubkey.clone());
+            cached_pubkey = Some(new_pubkey);
+        }
+        cached_pubkey.unwrap()
+    }
+
+    pub fn purge(&mut self) {
+        self.0.retain(|x| Rc::strong_count(x) > 1);
+    }
+}