add skeleton

Signed-off-by: Richard Chien <stdrc@outlook.com>

src/stream/src/executor/over_window/state/aggregate.rs

@@ -0,0 +1,51 @@
+// Copyright 2023 RisingWave Labs
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::collections::VecDeque;
+
+use risingwave_common::types::Datum;
+use smallvec::SmallVec;
+
+use super::{StateKey, StateOutput, StatePos, WindowState};
+
+struct BufferEntry {
+    key: StateKey,
+    args: SmallVec<[Datum; 2]>,
+}
+
+pub(super) struct AggregateState {
+    buffer: VecDeque<BufferEntry>,
+}
+
+impl AggregateState {
+    pub fn new() -> Self {
+        Self {
+            buffer: Default::default(),
+        }
+    }
+}
+
+impl WindowState for AggregateState {
+    fn append(&mut self, key: StateKey, args: SmallVec<[Datum; 2]>) {
+        todo!()
+    }
+
+    fn curr_window(&self) -> StatePos<'_> {
+        todo!()
+    }
+
+    fn slide(&mut self) -> StateOutput {
+        todo!()
+    }
+}

src/stream/src/executor/over_window/state/mod.rs

@@ -20,6 +20,7 @@ use smallvec::SmallVec;
 
 use super::MemcmpEncoded;
 
+mod aggregate;
 mod lag;
 mod lead;