fix: adjust aex9 balance history endpoint

lib/ae_mdw/aex9.ex

@@ -284,25 +284,54 @@ defmodule AeMdw.Aex9 do
           nil -> {0, DbUtil.last_gen!(state)}
         end
 
-      streamer = fn
-        :forward when first_gen <= cursor and cursor <= last_gen -> cursor..last_gen
-        :backward when cursor == nil -> last_gen..first_gen
-        :backward when first_gen <= cursor and cursor <= last_gen -> cursor..first_gen
-        _dir -> first_gen..last_gen
-      end
+      streamer =
+        balance_history_streamer(%{
+          account_pk: account_pk,
+          contract_pk: contract_pk,
+          range: {first_gen, last_gen},
+          cursor: cursor
+        })
 
       paginated_history =
         Collection.paginate(
           streamer,
           pagination,
-          &render_balance_history_item(contract_pk, account_pk, &1),
-          & &1
+          &render_balance_history_item/1,
+          &serialize_history_cursor/1
         )
 
       {:ok, paginated_history}
     end
   end
 
+  defp balance_history_streamer(%{
+         account_pk: account_pk,
+         contract_pk: contract_pk,
+         range: {first_gen, last_gen},
+         cursor: cursor
+       }) do
+    fn direction ->
+      direction
+      |> case do
+        :forward when first_gen <= cursor and cursor <= last_gen -> cursor..last_gen
+        :backward when cursor == nil -> last_gen..first_gen
+        :backward when first_gen <= cursor and cursor <= last_gen -> cursor..first_gen
+        _dir -> first_gen..last_gen
+      end
+      |> Stream.flat_map(fn gen ->
+        type_height_hash = {:key, gen, DbUtil.height_hash(gen)}
+
+        case Db.aex9_balance(contract_pk, account_pk, type_height_hash) do
+          {:ok, {amount_or_nil, _height_hash}} ->
+            [{contract_pk, account_pk, gen, amount_or_nil}]
+
+          _invalid ->
+            []
+        end
+      end)
+    end
+  end
+
   defp serialize_account_balance_cursor(cursor),
     do: cursor |> :erlang.term_to_binary() |> Base.encode64(padding: false)
 
@@ -335,15 +364,10 @@ defmodule AeMdw.Aex9 do
     }
   end
 
-  defp render_balance_history_item(contract_pk, account_pk, gen) do
-    type_height_hash = {:key, gen, DbUtil.height_hash(gen)}
-
-    {:ok, {amount_or_nil, _height_hash}} =
-      Db.aex9_balance(contract_pk, account_pk, type_height_hash)
-
-    balance = render_balance(contract_pk, {:address, account_pk}, amount_or_nil)
-
-    Map.put(balance, :height, gen)
+  defp render_balance_history_item({contract_pk, account_pk, gen, amount_or_nil}) do
+    contract_pk
+    |> render_balance({:address, account_pk}, amount_or_nil)
+    |> Map.put(:height, gen)
   end
 
   defp render_account_balance(state, type_height_hash, {account_pk, contract_pk}) do
@@ -385,6 +409,10 @@ defmodule AeMdw.Aex9 do
     end
   end
 
+  defp serialize_history_cursor({_contract_pk, _account_pk, gen, _amount_or_nil}) do
+    gen
+  end
+
   defp serialize_event_balances_cursor({_contract_pk, account_pk}), do: encode_account(account_pk)
 
   defp deserialize_event_balances_cursor(nil), do: {:ok, nil}