refactor: simplify merge_db_account_data (#9223)

* refactor: simplify `merge_db_account_data`

* chore: fmt

Author: Karrq

crates/evm/core/src/backend/mod.rs

@@ -1877,25 +1877,31 @@ fn merge_db_account_data<ExtDB: DatabaseRef>(
 ) {
     trace!(?addr, "merging database data");
 
-    let mut acc = if let Some(acc) = active.accounts.get(&addr).cloned() {
-        acc
-    } else {
-        // Account does not exist
-        return;
-    };
-
-    if let Some(code) = active.contracts.get(&acc.info.code_hash).cloned() {
-        fork_db.contracts.insert(acc.info.code_hash, code);
-    }
-
-    if let Some(fork_account) = fork_db.accounts.get_mut(&addr) {
-        // This will merge the fork's tracked storage with active storage and update values
-        fork_account.storage.extend(std::mem::take(&mut acc.storage));
-        // swap them so we can insert the account as whole in the next step
-        std::mem::swap(&mut fork_account.storage, &mut acc.storage);
+    let Some(acc) = active.accounts.get(&addr) else { return };
+
+    // port contract cache over
+    if let Some(code) = active.contracts.get(&acc.info.code_hash) {
+        trace!("merging contract cache");
+        fork_db.contracts.insert(acc.info.code_hash, code.clone());
+    }
+
+    // port account storage over
+    use std::collections::hash_map::Entry;
+    match fork_db.accounts.entry(addr) {
+        Entry::Vacant(vacant) => {
+            trace!("target account not present - inserting from active");
+            // if the fork_db doesn't have the target account
+            // insert the entire thing
+            vacant.insert(acc.clone());
+        }
+        Entry::Occupied(mut occupied) => {
+            trace!("target account present - merging storage slots");
+            // if the fork_db does have the system,
+            // extend the existing storage (overriding)
+            let fork_account = occupied.get_mut();
+            fork_account.storage.extend(&acc.storage);
+        }
     }
-
-    fork_db.accounts.insert(addr, acc);
 }
 
 /// Returns true of the address is a contract