Github user haohui commented on a diff in the pull request: https://github.com/apache/flink/pull/4556#discussion_r140824819 --- Diff: flink-libraries/flink-table/src/main/scala/org/apache/flink/table/functions/aggfunctions/FirstValueAggFunctionWithRetract.scala --- @@ -0,0 +1,129 @@ +/* + * Licensed to the Apache Software Foundation (ASF) under one + * or more contributor license agreements. See the NOTICE file + * distributed with this work for additional information + * regarding copyright ownership. The ASF licenses this file + * to you 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. + */ +package org.apache.flink.table.functions.aggfunctions + +import java.math.BigDecimal +import org.apache.flink.api.common.typeinfo.TypeInformation +import org.apache.flink.table.api.dataview.{ListView, MapView} +import org.apache.flink.table.functions.AggregateFunction + +/** The initial accumulator for first value with retraction aggregate function */ +class FirstValueWithRetractAccumulator[T] { + var data: ListView[T] = _ + var retractedData: MapView[T, Byte] = _ + + override def equals(obj: scala.Any): Boolean = { + if (obj.isInstanceOf[FirstValueWithRetractAccumulator[T]]) { + val target = obj.asInstanceOf[FirstValueWithRetractAccumulator[T]] + if (target.data.equals(this.data) && target.retractedData.equals(this.retractedData)) { + return true + } + } + false + } +} + +/** + * Base class for built-in first value with retraction aggregate function + * + * @tparam T the type for the aggregation result + */ +abstract class FirstValueWithRetractAggFunction[T] + extends AggregateFunction[T, FirstValueWithRetractAccumulator[T]] { + + override def createAccumulator(): FirstValueWithRetractAccumulator[T] = { + val acc = new FirstValueWithRetractAccumulator[T] + acc.data = new ListView[T] + acc.retractedData = new MapView[T, Byte] + acc + } + + def accumulate(acc: FirstValueWithRetractAccumulator[T], value: Any): Unit = { + if (null != value) { + val v = value.asInstanceOf[T] + acc.data.add(v) + } + } + + def retract(acc: FirstValueWithRetractAccumulator[T], value: Any): Unit = { --- End diff -- It seems that we can do better than having two copies of state here. It might make sense to extend the ListView interface or to introduce a TreeMap like view.
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