factor out InMemAccountsIndex to prepare for disk index (#19773)

runtime/src/in_mem_accounts_index.rs

@@ -0,0 +1,51 @@
+use solana_sdk::pubkey::Pubkey;
+use std::collections::{
+    hash_map::{Entry, Iter, Keys},
+    HashMap,
+};
+use std::fmt::Debug;
+
+type K = Pubkey;
+
+// one instance of this represents one bin of the accounts index.
+#[derive(Debug, Default)]
+pub struct InMemAccountsIndex<V> {
+    // backing store
+    map: HashMap<Pubkey, V>,
+}
+
+impl<V> InMemAccountsIndex<V> {
+    pub fn new() -> Self {
+        Self {
+            map: HashMap::new(),
+        }
+    }
+
+    pub fn entry(&mut self, pubkey: Pubkey) -> Entry<K, V> {
+        self.map.entry(pubkey)
+    }
+
+    pub fn iter(&self) -> Iter<K, V> {
+        self.map.iter()
+    }
+
+    pub fn keys(&self) -> Keys<K, V> {
+        self.map.keys()
+    }
+
+    pub fn get(&self, key: &K) -> Option<&V> {
+        self.map.get(key)
+    }
+
+    pub fn remove(&mut self, key: &K) {
+        self.map.remove(key);
+    }
+
+    pub fn len(&self) -> usize {
+        self.map.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+}