rpc: Speedup getaddressesbylabel

Author: promag

src/wallet/rpcwallet.cpp

@@ -3790,9 +3790,20 @@ static UniValue getaddressesbylabel(const JSONRPCRequest& request)
 
     // Find all addresses that have the given label
     UniValue ret(UniValue::VOBJ);
+    std::set<std::string> addresses;
     for (const std::pair<const CTxDestination, CAddressBookData>& item : pwallet->mapAddressBook) {
         if (item.second.name == label) {
-            ret.pushKV(EncodeDestination(item.first), AddressBookDataToJSON(item.second, false));
+            std::string address = EncodeDestination(item.first);
+            // CWallet::mapAddressBook is not expected to contain duplicate
+            // address strings, but build a separate set as a precaution just in
+            // case it does.
+            bool unique = addresses.emplace(address).second;
+            assert(unique);
+            // UniValue::pushKV checks if the key exists in O(N)
+            // and since duplicate addresses are unexpected (checked with
+            // std::set in O(log(N))), UniValue::__pushKV is used instead,
+            // which currently is O(1).
+            ret.__pushKV(address, AddressBookDataToJSON(item.second, false));
         }
     }