/*
 * 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.streaming.api.datastream;

import org.apache.flink.annotation.Public;
import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.api.common.functions.AggregateFunction;
import org.apache.flink.api.common.functions.ReduceFunction;
import org.apache.flink.api.common.functions.RichFunction;
import org.apache.flink.api.common.state.AggregatingStateDescriptor;
import org.apache.flink.api.common.state.ListStateDescriptor;
import org.apache.flink.api.common.state.ReducingStateDescriptor;
import org.apache.flink.api.common.typeinfo.TypeInformation;
import org.apache.flink.api.common.typeutils.TypeSerializer;
import org.apache.flink.api.java.Utils;
import org.apache.flink.api.java.functions.KeySelector;
import org.apache.flink.api.java.functions.NullByteKeySelector;
import org.apache.flink.api.java.typeutils.TypeExtractor;
import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
import org.apache.flink.streaming.api.functions.aggregation.AggregationFunction;
import org.apache.flink.streaming.api.functions.aggregation.ComparableAggregator;
import org.apache.flink.streaming.api.functions.aggregation.SumAggregator;
import org.apache.flink.streaming.api.functions.windowing.AggregateApplyAllWindowFunction;
import org.apache.flink.streaming.api.functions.windowing.AllWindowFunction;
import org.apache.flink.streaming.api.functions.windowing.PassThroughAllWindowFunction;
import org.apache.flink.streaming.api.functions.windowing.ProcessAllWindowFunction;
import org.apache.flink.streaming.api.functions.windowing.ReduceApplyAllWindowFunction;
import org.apache.flink.streaming.api.functions.windowing.ReduceApplyProcessAllWindowFunction;
import org.apache.flink.streaming.api.operators.OneInputStreamOperator;
import org.apache.flink.streaming.api.windowing.assigners.MergingWindowAssigner;
import org.apache.flink.streaming.api.windowing.assigners.WindowAssigner;
import org.apache.flink.streaming.api.windowing.evictors.Evictor;
import org.apache.flink.streaming.api.windowing.time.Time;
import org.apache.flink.streaming.api.windowing.triggers.Trigger;
import org.apache.flink.streaming.api.windowing.windows.Window;
import org.apache.flink.streaming.runtime.operators.windowing.EvictingWindowOperator;
import org.apache.flink.streaming.runtime.operators.windowing.WindowOperator;
import org.apache.flink.streaming.runtime.operators.windowing.functions.InternalAggregateProcessAllWindowFunction;
import org.apache.flink.streaming.runtime.operators.windowing.functions.InternalIterableAllWindowFunction;
import org.apache.flink.streaming.runtime.operators.windowing.functions.InternalIterableProcessAllWindowFunction;
import org.apache.flink.streaming.runtime.operators.windowing.functions.InternalSingleValueAllWindowFunction;
import org.apache.flink.streaming.runtime.operators.windowing.functions.InternalSingleValueProcessAllWindowFunction;
import org.apache.flink.streaming.runtime.operators.windowing.functions.InternalWindowFunction;
import org.apache.flink.streaming.runtime.streamrecord.StreamElementSerializer;
import org.apache.flink.streaming.runtime.streamrecord.StreamRecord;
import org.apache.flink.util.OutputTag;
import org.apache.flink.util.Preconditions;

import static org.apache.flink.util.Preconditions.checkArgument;
import static org.apache.flink.util.Preconditions.checkNotNull;

/**
 * A {@code AllWindowedStream} represents a data stream where the stream of elements is split into
 * windows based on a {@link org.apache.flink.streaming.api.windowing.assigners.WindowAssigner}.
 * Window emission is triggered based on a {@link
 * org.apache.flink.streaming.api.windowing.triggers.Trigger}.
 *
 * <p>If an {@link org.apache.flink.streaming.api.windowing.evictors.Evictor} is specified it will
 * be used to evict elements from the window after evaluation was triggered by the {@code Trigger}
 * but before the actual evaluation of the window. When using an evictor, window performance will
 * degrade significantly, since pre-aggregation of window results cannot be used.
 *
 * <p>Note that the {@code AllWindowedStream} is purely an API construct, during runtime the {@code
 * AllWindowedStream} will be collapsed together with the operation over the window into one single
 * operation.
 *
 * @param <T> The type of elements in the stream.
 * @param <W> The type of {@code Window} that the {@code WindowAssigner} assigns the elements to.
 */
@Public
public class AllWindowedStream<T, W extends Window> {

    /** The keyed data stream that is windowed by this stream. */
    private final KeyedStream<T, Byte> input;

    /** The window assigner. */
    private final WindowAssigner<? super T, W> windowAssigner;

    /** The trigger that is used for window evaluation/emission. */
    private Trigger<? super T, ? super W> trigger;

    /** The evictor that is used for evicting elements before window evaluation. */
    private Evictor<? super T, ? super W> evictor;

    /** The user-specified allowed lateness. */
    private long allowedLateness = 0L;

    /**
     * Side output {@code OutputTag} for late data. If no tag is set late data will simply be
     * dropped.
     */
    private OutputTag<T> lateDataOutputTag;

    @PublicEvolving
    public AllWindowedStream(DataStream<T> input, WindowAssigner<? super T, W> windowAssigner) {
        this.input = input.keyBy(new NullByteKeySelector<T>());
        this.windowAssigner = windowAssigner;
        this.trigger = windowAssigner.getDefaultTrigger(input.getExecutionEnvironment());
    }

    /** Sets the {@code Trigger} that should be used to trigger window emission. */
    @PublicEvolving
    public AllWindowedStream<T, W> trigger(Trigger<? super T, ? super W> trigger) {
        if (windowAssigner instanceof MergingWindowAssigner && !trigger.canMerge()) {
            throw new UnsupportedOperationException(
                    "A merging window assigner cannot be used with a trigger that does not support merging.");
        }

        this.trigger = trigger;
        return this;
    }

    /**
     * Sets the time by which elements are allowed to be late. Elements that arrive behind the
     * watermark by more than the specified time will be dropped. By default, the allowed lateness
     * is {@code 0L}.
     *
     * <p>Setting an allowed lateness is only valid for event-time windows.
     */
    @PublicEvolving
    public AllWindowedStream<T, W> allowedLateness(Time lateness) {
        final long millis = lateness.toMilliseconds();
        checkArgument(millis >= 0, "The allowed lateness cannot be negative.");

        this.allowedLateness = millis;
        return this;
    }

    /**
     * Send late arriving data to the side output identified by the given {@link OutputTag}. Data is
     * considered late after the watermark has passed the end of the window plus the allowed
     * lateness set using {@link #allowedLateness(Time)}.
     *
     * <p>You can get the stream of late data using {@link
     * SingleOutputStreamOperator#getSideOutput(OutputTag)} on the {@link
     * SingleOutputStreamOperator} resulting from the windowed operation with the same {@link
     * OutputTag}.
     */
    @PublicEvolving
    public AllWindowedStream<T, W> sideOutputLateData(OutputTag<T> outputTag) {
        Preconditions.checkNotNull(outputTag, "Side output tag must not be null.");
        this.lateDataOutputTag = input.getExecutionEnvironment().clean(outputTag);
        return this;
    }

    /**
     * Sets the {@code Evictor} that should be used to evict elements from a window before emission.
     *
     * <p>Note: When using an evictor window performance will degrade significantly, since
     * incremental aggregation of window results cannot be used.
     */
    @PublicEvolving
    public AllWindowedStream<T, W> evictor(Evictor<? super T, ? super W> evictor) {

        this.evictor = evictor;
        return this;
    }

    // ------------------------------------------------------------------------
    //  Operations on the keyed windows
    // ------------------------------------------------------------------------

    /**
     * Applies a reduce function to the window. The window function is called for each evaluation of
     * the window for each key individually. The output of the reduce function is interpreted as a
     * regular non-windowed stream.
     *
     * <p>This window will try and incrementally aggregate data as much as the window policies
     * permit. For example, tumbling time windows can aggregate the data, meaning that only one
     * element per key is stored. Sliding time windows will aggregate on the granularity of the
     * slide interval, so a few elements are stored per key (one per slide interval). Custom windows
     * may not be able to incrementally aggregate, or may need to store extra values in an
     * aggregation tree.
     *
     * @param function The reduce function.
     * @return The data stream that is the result of applying the reduce function to the window.
     */
    @SuppressWarnings("unchecked")
    public SingleOutputStreamOperator<T> reduce(ReduceFunction<T> function) {
        if (function instanceof RichFunction) {
            throw new UnsupportedOperationException(
                    "ReduceFunction of reduce can not be a RichFunction. "
                            + "Please use reduce(ReduceFunction, WindowFunction) instead.");
        }

        // clean the closure
        function = input.getExecutionEnvironment().clean(function);

        String callLocation = Utils.getCallLocationName();
        String udfName = "AllWindowedStream." + callLocation;

        return reduce(function, new PassThroughAllWindowFunction<W, T>());
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window for each key individually. The output of the window function is
     * interpreted as a regular non-windowed stream.
     *
     * <p>Arriving data is incrementally aggregated using the given reducer.
     *
     * @param reduceFunction The reduce function that is used for incremental aggregation.
     * @param function The window function.
     * @return The data stream that is the result of applying the window function to the window.
     */
    @PublicEvolving
    public <R> SingleOutputStreamOperator<R> reduce(
            ReduceFunction<T> reduceFunction, AllWindowFunction<T, R, W> function) {

        TypeInformation<T> inType = input.getType();
        TypeInformation<R> resultType = getAllWindowFunctionReturnType(function, inType);

        return reduce(reduceFunction, function, resultType);
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window for each key individually. The output of the window function is
     * interpreted as a regular non-windowed stream.
     *
     * <p>Arriving data is incrementally aggregated using the given reducer.
     *
     * @param reduceFunction The reduce function that is used for incremental aggregation.
     * @param function The window function.
     * @param resultType Type information for the result type of the window function
     * @return The data stream that is the result of applying the window function to the window.
     */
    @PublicEvolving
    public <R> SingleOutputStreamOperator<R> reduce(
            ReduceFunction<T> reduceFunction,
            AllWindowFunction<T, R, W> function,
            TypeInformation<R> resultType) {

        if (reduceFunction instanceof RichFunction) {
            throw new UnsupportedOperationException(
                    "ReduceFunction of reduce can not be a RichFunction.");
        }

        // clean the closures
        function = input.getExecutionEnvironment().clean(function);
        reduceFunction = input.getExecutionEnvironment().clean(reduceFunction);

        String callLocation = Utils.getCallLocationName();
        String udfName = "AllWindowedStream." + callLocation;

        String opName = windowAssigner.getClass().getSimpleName();
        String opDescription;
        KeySelector<T, Byte> keySel = input.getKeySelector();

        OneInputStreamOperator<T, R> operator;

        if (evictor != null) {
            @SuppressWarnings({"unchecked", "rawtypes"})
            TypeSerializer<StreamRecord<T>> streamRecordSerializer =
                    (TypeSerializer<StreamRecord<T>>)
                            new StreamElementSerializer(
                                    input.getType()
                                            .createSerializer(
                                                    getExecutionEnvironment().getConfig()));

            ListStateDescriptor<StreamRecord<T>> stateDesc =
                    new ListStateDescriptor<>("window-contents", streamRecordSerializer);

            opDescription =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + evictor
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new EvictingWindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            new InternalIterableAllWindowFunction<>(
                                    new ReduceApplyAllWindowFunction<>(reduceFunction, function)),
                            trigger,
                            evictor,
                            allowedLateness,
                            lateDataOutputTag);

        } else {
            ReducingStateDescriptor<T> stateDesc =
                    new ReducingStateDescriptor<>(
                            "window-contents",
                            reduceFunction,
                            input.getType()
                                    .createSerializer(getExecutionEnvironment().getConfig()));

            opDescription =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new WindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            new InternalSingleValueAllWindowFunction<>(function),
                            trigger,
                            allowedLateness,
                            lateDataOutputTag);
        }

        return input.transform(opName, resultType, operator)
                .setDescription(opDescription)
                .forceNonParallel();
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window for each key individually. The output of the window function is
     * interpreted as a regular non-windowed stream.
     *
     * <p>Arriving data is incrementally aggregated using the given reducer.
     *
     * @param reduceFunction The reduce function that is used for incremental aggregation.
     * @param function The process window function.
     * @return The data stream that is the result of applying the window function to the window.
     */
    @PublicEvolving
    public <R> SingleOutputStreamOperator<R> reduce(
            ReduceFunction<T> reduceFunction, ProcessAllWindowFunction<T, R, W> function) {

        TypeInformation<R> resultType =
                getProcessAllWindowFunctionReturnType(function, input.getType());

        return reduce(reduceFunction, function, resultType);
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window for each key individually. The output of the window function is
     * interpreted as a regular non-windowed stream.
     *
     * <p>Arriving data is incrementally aggregated using the given reducer.
     *
     * @param reduceFunction The reduce function that is used for incremental aggregation.
     * @param function The process window function.
     * @param resultType Type information for the result type of the window function
     * @return The data stream that is the result of applying the window function to the window.
     */
    @PublicEvolving
    public <R> SingleOutputStreamOperator<R> reduce(
            ReduceFunction<T> reduceFunction,
            ProcessAllWindowFunction<T, R, W> function,
            TypeInformation<R> resultType) {
        if (reduceFunction instanceof RichFunction) {
            throw new UnsupportedOperationException(
                    "ReduceFunction of reduce can not be a RichFunction.");
        }

        // clean the closures
        function = input.getExecutionEnvironment().clean(function);
        reduceFunction = input.getExecutionEnvironment().clean(reduceFunction);

        String callLocation = Utils.getCallLocationName();
        String udfName = "AllWindowedStream." + callLocation;

        String opName;
        KeySelector<T, Byte> keySel = input.getKeySelector();

        OneInputStreamOperator<T, R> operator;

        if (evictor != null) {
            @SuppressWarnings({"unchecked", "rawtypes"})
            TypeSerializer<StreamRecord<T>> streamRecordSerializer =
                    (TypeSerializer<StreamRecord<T>>)
                            new StreamElementSerializer(
                                    input.getType()
                                            .createSerializer(
                                                    getExecutionEnvironment().getConfig()));

            ListStateDescriptor<StreamRecord<T>> stateDesc =
                    new ListStateDescriptor<>("window-contents", streamRecordSerializer);

            opName =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + evictor
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new EvictingWindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            new InternalIterableProcessAllWindowFunction<>(
                                    new ReduceApplyProcessAllWindowFunction<>(
                                            reduceFunction, function)),
                            trigger,
                            evictor,
                            allowedLateness,
                            lateDataOutputTag);

        } else {
            ReducingStateDescriptor<T> stateDesc =
                    new ReducingStateDescriptor<>(
                            "window-contents",
                            reduceFunction,
                            input.getType()
                                    .createSerializer(getExecutionEnvironment().getConfig()));

            opName =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new WindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            new InternalSingleValueProcessAllWindowFunction<>(function),
                            trigger,
                            allowedLateness,
                            lateDataOutputTag);
        }

        return input.transform(opName, resultType, operator).forceNonParallel();
    }

    // ------------------------------------------------------------------------
    //  AggregateFunction
    // ------------------------------------------------------------------------

    /**
     * Applies the given {@code AggregateFunction} to each window. The AggregateFunction aggregates
     * all elements of a window into a single result element. The stream of these result elements
     * (one per window) is interpreted as a regular non-windowed stream.
     *
     * @param function The aggregation function.
     * @return The data stream that is the result of applying the fold function to the window.
     * @param <ACC> The type of the AggregateFunction's accumulator
     * @param <R> The type of the elements in the resulting stream, equal to the AggregateFunction's
     *     result type
     */
    @PublicEvolving
    public <ACC, R> SingleOutputStreamOperator<R> aggregate(AggregateFunction<T, ACC, R> function) {
        checkNotNull(function, "function");

        if (function instanceof RichFunction) {
            throw new UnsupportedOperationException(
                    "This aggregation function cannot be a RichFunction.");
        }

        TypeInformation<ACC> accumulatorType =
                TypeExtractor.getAggregateFunctionAccumulatorType(
                        function, input.getType(), null, false);

        TypeInformation<R> resultType =
                TypeExtractor.getAggregateFunctionReturnType(
                        function, input.getType(), null, false);

        return aggregate(function, accumulatorType, resultType);
    }

    /**
     * Applies the given {@code AggregateFunction} to each window. The AggregateFunction aggregates
     * all elements of a window into a single result element. The stream of these result elements
     * (one per window) is interpreted as a regular non-windowed stream.
     *
     * @param function The aggregation function.
     * @return The data stream that is the result of applying the aggregation function to the
     *     window.
     * @param <ACC> The type of the AggregateFunction's accumulator
     * @param <R> The type of the elements in the resulting stream, equal to the AggregateFunction's
     *     result type
     */
    @PublicEvolving
    public <ACC, R> SingleOutputStreamOperator<R> aggregate(
            AggregateFunction<T, ACC, R> function,
            TypeInformation<ACC> accumulatorType,
            TypeInformation<R> resultType) {

        checkNotNull(function, "function");
        checkNotNull(accumulatorType, "accumulatorType");
        checkNotNull(resultType, "resultType");

        if (function instanceof RichFunction) {
            throw new UnsupportedOperationException(
                    "This aggregation function cannot be a RichFunction.");
        }

        return aggregate(
                function, new PassThroughAllWindowFunction<W, R>(), accumulatorType, resultType);
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window for each key individually. The output of the window function is
     * interpreted as a regular non-windowed stream.
     *
     * <p>Arriving data is incrementally aggregated using the given aggregate function. This means
     * that the window function typically has only a single value to process when called.
     *
     * @param aggFunction The aggregate function that is used for incremental aggregation.
     * @param windowFunction The window function.
     * @return The data stream that is the result of applying the window function to the window.
     * @param <ACC> The type of the AggregateFunction's accumulator
     * @param <V> The type of AggregateFunction's result, and the WindowFunction's input
     * @param <R> The type of the elements in the resulting stream, equal to the WindowFunction's
     *     result type
     */
    @PublicEvolving
    public <ACC, V, R> SingleOutputStreamOperator<R> aggregate(
            AggregateFunction<T, ACC, V> aggFunction, AllWindowFunction<V, R, W> windowFunction) {

        checkNotNull(aggFunction, "aggFunction");
        checkNotNull(windowFunction, "windowFunction");

        TypeInformation<ACC> accumulatorType =
                TypeExtractor.getAggregateFunctionAccumulatorType(
                        aggFunction, input.getType(), null, false);

        TypeInformation<V> aggResultType =
                TypeExtractor.getAggregateFunctionReturnType(
                        aggFunction, input.getType(), null, false);

        TypeInformation<R> resultType =
                getAllWindowFunctionReturnType(windowFunction, aggResultType);

        return aggregate(aggFunction, windowFunction, accumulatorType, resultType);
    }

    private static <IN, OUT> TypeInformation<OUT> getAllWindowFunctionReturnType(
            AllWindowFunction<IN, OUT, ?> function, TypeInformation<IN> inType) {
        return TypeExtractor.getUnaryOperatorReturnType(
                function, AllWindowFunction.class, 0, 1, new int[] {2, 0}, inType, null, false);
    }

    private static <IN, OUT> TypeInformation<OUT> getProcessAllWindowFunctionReturnType(
            ProcessAllWindowFunction<IN, OUT, ?> function, TypeInformation<IN> inType) {
        return TypeExtractor.getUnaryOperatorReturnType(
                function,
                ProcessAllWindowFunction.class,
                0,
                1,
                TypeExtractor.NO_INDEX,
                inType,
                null,
                false);
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window for each key individually. The output of the window function is
     * interpreted as a regular non-windowed stream.
     *
     * <p>Arriving data is incrementally aggregated using the given aggregate function. This means
     * that the window function typically has only a single value to process when called.
     *
     * @param aggregateFunction The aggregation function that is used for incremental aggregation.
     * @param windowFunction The window function.
     * @param accumulatorType Type information for the internal accumulator type of the aggregation
     *     function
     * @param resultType Type information for the result type of the window function
     * @return The data stream that is the result of applying the window function to the window.
     * @param <ACC> The type of the AggregateFunction's accumulator
     * @param <V> The type of AggregateFunction's result, and the WindowFunction's input
     * @param <R> The type of the elements in the resulting stream, equal to the WindowFunction's
     *     result type
     */
    @PublicEvolving
    public <ACC, V, R> SingleOutputStreamOperator<R> aggregate(
            AggregateFunction<T, ACC, V> aggregateFunction,
            AllWindowFunction<V, R, W> windowFunction,
            TypeInformation<ACC> accumulatorType,
            TypeInformation<R> resultType) {

        checkNotNull(aggregateFunction, "aggregateFunction");
        checkNotNull(windowFunction, "windowFunction");
        checkNotNull(accumulatorType, "accumulatorType");
        checkNotNull(resultType, "resultType");

        if (aggregateFunction instanceof RichFunction) {
            throw new UnsupportedOperationException(
                    "This aggregate function cannot be a RichFunction.");
        }

        // clean the closures
        windowFunction = input.getExecutionEnvironment().clean(windowFunction);
        aggregateFunction = input.getExecutionEnvironment().clean(aggregateFunction);

        final String callLocation = Utils.getCallLocationName();
        final String udfName = "AllWindowedStream." + callLocation;

        final String opName;
        final KeySelector<T, Byte> keySel = input.getKeySelector();

        OneInputStreamOperator<T, R> operator;

        if (evictor != null) {
            @SuppressWarnings({"unchecked", "rawtypes"})
            TypeSerializer<StreamRecord<T>> streamRecordSerializer =
                    (TypeSerializer<StreamRecord<T>>)
                            new StreamElementSerializer(
                                    input.getType()
                                            .createSerializer(
                                                    getExecutionEnvironment().getConfig()));

            ListStateDescriptor<StreamRecord<T>> stateDesc =
                    new ListStateDescriptor<>("window-contents", streamRecordSerializer);

            opName =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + evictor
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new EvictingWindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            new InternalIterableAllWindowFunction<>(
                                    new AggregateApplyAllWindowFunction<>(
                                            aggregateFunction, windowFunction)),
                            trigger,
                            evictor,
                            allowedLateness,
                            lateDataOutputTag);

        } else {
            AggregatingStateDescriptor<T, ACC, V> stateDesc =
                    new AggregatingStateDescriptor<>(
                            "window-contents",
                            aggregateFunction,
                            accumulatorType.createSerializer(
                                    getExecutionEnvironment().getConfig()));

            opName =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new WindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            new InternalSingleValueAllWindowFunction<>(windowFunction),
                            trigger,
                            allowedLateness,
                            lateDataOutputTag);
        }

        return input.transform(opName, resultType, operator).forceNonParallel();
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window for each key individually. The output of the window function is
     * interpreted as a regular non-windowed stream.
     *
     * <p>Arriving data is incrementally aggregated using the given aggregate function. This means
     * that the window function typically has only a single value to process when called.
     *
     * @param aggFunction The aggregate function that is used for incremental aggregation.
     * @param windowFunction The process window function.
     * @return The data stream that is the result of applying the window function to the window.
     * @param <ACC> The type of the AggregateFunction's accumulator
     * @param <V> The type of AggregateFunction's result, and the WindowFunction's input
     * @param <R> The type of the elements in the resulting stream, equal to the WindowFunction's
     *     result type
     */
    @PublicEvolving
    public <ACC, V, R> SingleOutputStreamOperator<R> aggregate(
            AggregateFunction<T, ACC, V> aggFunction,
            ProcessAllWindowFunction<V, R, W> windowFunction) {

        checkNotNull(aggFunction, "aggFunction");
        checkNotNull(windowFunction, "windowFunction");

        TypeInformation<ACC> accumulatorType =
                TypeExtractor.getAggregateFunctionAccumulatorType(
                        aggFunction, input.getType(), null, false);

        TypeInformation<V> aggResultType =
                TypeExtractor.getAggregateFunctionReturnType(
                        aggFunction, input.getType(), null, false);

        TypeInformation<R> resultType =
                getProcessAllWindowFunctionReturnType(windowFunction, aggResultType);

        return aggregate(aggFunction, windowFunction, accumulatorType, aggResultType, resultType);
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window for each key individually. The output of the window function is
     * interpreted as a regular non-windowed stream.
     *
     * <p>Arriving data is incrementally aggregated using the given aggregate function. This means
     * that the window function typically has only a single value to process when called.
     *
     * @param aggregateFunction The aggregation function that is used for incremental aggregation.
     * @param windowFunction The process window function.
     * @param accumulatorType Type information for the internal accumulator type of the aggregation
     *     function
     * @param resultType Type information for the result type of the window function
     * @return The data stream that is the result of applying the window function to the window.
     * @param <ACC> The type of the AggregateFunction's accumulator
     * @param <V> The type of AggregateFunction's result, and the WindowFunction's input
     * @param <R> The type of the elements in the resulting stream, equal to the WindowFunction's
     *     result type
     */
    @PublicEvolving
    public <ACC, V, R> SingleOutputStreamOperator<R> aggregate(
            AggregateFunction<T, ACC, V> aggregateFunction,
            ProcessAllWindowFunction<V, R, W> windowFunction,
            TypeInformation<ACC> accumulatorType,
            TypeInformation<V> aggregateResultType,
            TypeInformation<R> resultType) {

        checkNotNull(aggregateFunction, "aggregateFunction");
        checkNotNull(windowFunction, "windowFunction");
        checkNotNull(accumulatorType, "accumulatorType");
        checkNotNull(aggregateResultType, "aggregateResultType");
        checkNotNull(resultType, "resultType");

        if (aggregateFunction instanceof RichFunction) {
            throw new UnsupportedOperationException(
                    "This aggregate function cannot be a RichFunction.");
        }

        // clean the closures
        windowFunction = input.getExecutionEnvironment().clean(windowFunction);
        aggregateFunction = input.getExecutionEnvironment().clean(aggregateFunction);

        final String callLocation = Utils.getCallLocationName();
        final String udfName = "AllWindowedStream." + callLocation;

        final String opName = windowAssigner.getClass().getSimpleName();
        final String opDescription;
        final KeySelector<T, Byte> keySel = input.getKeySelector();

        OneInputStreamOperator<T, R> operator;

        if (evictor != null) {
            @SuppressWarnings({"unchecked", "rawtypes"})
            TypeSerializer<StreamRecord<T>> streamRecordSerializer =
                    (TypeSerializer<StreamRecord<T>>)
                            new StreamElementSerializer(
                                    input.getType()
                                            .createSerializer(
                                                    getExecutionEnvironment().getConfig()));

            ListStateDescriptor<StreamRecord<T>> stateDesc =
                    new ListStateDescriptor<>("window-contents", streamRecordSerializer);

            opDescription =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + evictor
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new EvictingWindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            new InternalAggregateProcessAllWindowFunction<>(
                                    aggregateFunction, windowFunction),
                            trigger,
                            evictor,
                            allowedLateness,
                            lateDataOutputTag);

        } else {
            AggregatingStateDescriptor<T, ACC, V> stateDesc =
                    new AggregatingStateDescriptor<>(
                            "window-contents",
                            aggregateFunction,
                            accumulatorType.createSerializer(
                                    getExecutionEnvironment().getConfig()));

            opDescription =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new WindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            new InternalSingleValueProcessAllWindowFunction<>(windowFunction),
                            trigger,
                            allowedLateness,
                            lateDataOutputTag);
        }

        return input.transform(opName, resultType, operator)
                .setDescription(opDescription)
                .forceNonParallel();
    }

    // ------------------------------------------------------------------------
    //  Apply (Window Function)
    // ------------------------------------------------------------------------

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window. The output of the window function is interpreted as a regular
     * non-windowed stream.
     *
     * <p>Note that this function requires that all data in the windows is buffered until the window
     * is evaluated, as the function provides no means of incremental aggregation.
     *
     * @param function The window function.
     * @return The data stream that is the result of applying the window function to the window.
     */
    public <R> SingleOutputStreamOperator<R> apply(AllWindowFunction<T, R, W> function) {
        String callLocation = Utils.getCallLocationName();
        function = input.getExecutionEnvironment().clean(function);
        TypeInformation<R> resultType = getAllWindowFunctionReturnType(function, getInputType());
        return apply(new InternalIterableAllWindowFunction<>(function), resultType, callLocation);
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window. The output of the window function is interpreted as a regular
     * non-windowed stream.
     *
     * <p>Note that this function requires that all data in the windows is buffered until the window
     * is evaluated, as the function provides no means of incremental aggregation.
     *
     * @param function The window function.
     * @return The data stream that is the result of applying the window function to the window.
     */
    public <R> SingleOutputStreamOperator<R> apply(
            AllWindowFunction<T, R, W> function, TypeInformation<R> resultType) {
        String callLocation = Utils.getCallLocationName();
        function = input.getExecutionEnvironment().clean(function);
        return apply(new InternalIterableAllWindowFunction<>(function), resultType, callLocation);
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window. The output of the window function is interpreted as a regular
     * non-windowed stream.
     *
     * <p>Note that this function requires that all data in the windows is buffered until the window
     * is evaluated, as the function provides no means of incremental aggregation.
     *
     * @param function The process window function.
     * @return The data stream that is the result of applying the window function to the window.
     */
    @PublicEvolving
    public <R> SingleOutputStreamOperator<R> process(ProcessAllWindowFunction<T, R, W> function) {
        String callLocation = Utils.getCallLocationName();
        function = input.getExecutionEnvironment().clean(function);
        TypeInformation<R> resultType =
                getProcessAllWindowFunctionReturnType(function, getInputType());
        return apply(
                new InternalIterableProcessAllWindowFunction<>(function), resultType, callLocation);
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window. The output of the window function is interpreted as a regular
     * non-windowed stream.
     *
     * <p>Note that this function requires that all data in the windows is buffered until the window
     * is evaluated, as the function provides no means of incremental aggregation.
     *
     * @param function The process window function.
     * @return The data stream that is the result of applying the window function to the window.
     */
    @PublicEvolving
    public <R> SingleOutputStreamOperator<R> process(
            ProcessAllWindowFunction<T, R, W> function, TypeInformation<R> resultType) {
        String callLocation = Utils.getCallLocationName();
        function = input.getExecutionEnvironment().clean(function);
        return apply(
                new InternalIterableProcessAllWindowFunction<>(function), resultType, callLocation);
    }

    private <R> SingleOutputStreamOperator<R> apply(
            InternalWindowFunction<Iterable<T>, R, Byte, W> function,
            TypeInformation<R> resultType,
            String callLocation) {

        String udfName = "AllWindowedStream." + callLocation;

        String opName;
        KeySelector<T, Byte> keySel = input.getKeySelector();

        WindowOperator<Byte, T, Iterable<T>, R, W> operator;

        if (evictor != null) {
            @SuppressWarnings({"unchecked", "rawtypes"})
            TypeSerializer<StreamRecord<T>> streamRecordSerializer =
                    (TypeSerializer<StreamRecord<T>>)
                            new StreamElementSerializer(
                                    input.getType()
                                            .createSerializer(
                                                    getExecutionEnvironment().getConfig()));

            ListStateDescriptor<StreamRecord<T>> stateDesc =
                    new ListStateDescriptor<>("window-contents", streamRecordSerializer);

            opName =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + evictor
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new EvictingWindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            function,
                            trigger,
                            evictor,
                            allowedLateness,
                            lateDataOutputTag);

        } else {
            ListStateDescriptor<T> stateDesc =
                    new ListStateDescriptor<>(
                            "window-contents",
                            input.getType()
                                    .createSerializer(getExecutionEnvironment().getConfig()));

            opName =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new WindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            function,
                            trigger,
                            allowedLateness,
                            lateDataOutputTag);
        }

        return input.transform(opName, resultType, operator).forceNonParallel();
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window for each key individually. The output of the window function is
     * interpreted as a regular non-windowed stream.
     *
     * <p>Arriving data is incrementally aggregated using the given reducer.
     *
     * @param reduceFunction The reduce function that is used for incremental aggregation.
     * @param function The window function.
     * @return The data stream that is the result of applying the window function to the window.
     * @deprecated Use {@link #reduce(ReduceFunction, AllWindowFunction)} instead.
     */
    @Deprecated
    public <R> SingleOutputStreamOperator<R> apply(
            ReduceFunction<T> reduceFunction, AllWindowFunction<T, R, W> function) {
        TypeInformation<T> inType = input.getType();
        TypeInformation<R> resultType = getAllWindowFunctionReturnType(function, inType);

        return apply(reduceFunction, function, resultType);
    }

    /**
     * Applies the given window function to each window. The window function is called for each
     * evaluation of the window for each key individually. The output of the window function is
     * interpreted as a regular non-windowed stream.
     *
     * <p>Arriving data is incrementally aggregated using the given reducer.
     *
     * @param reduceFunction The reduce function that is used for incremental aggregation.
     * @param function The window function.
     * @param resultType Type information for the result type of the window function
     * @return The data stream that is the result of applying the window function to the window.
     * @deprecated Use {@link #reduce(ReduceFunction, AllWindowFunction, TypeInformation)} instead.
     */
    @Deprecated
    public <R> SingleOutputStreamOperator<R> apply(
            ReduceFunction<T> reduceFunction,
            AllWindowFunction<T, R, W> function,
            TypeInformation<R> resultType) {
        if (reduceFunction instanceof RichFunction) {
            throw new UnsupportedOperationException(
                    "ReduceFunction of apply can not be a RichFunction.");
        }

        // clean the closures
        function = input.getExecutionEnvironment().clean(function);
        reduceFunction = input.getExecutionEnvironment().clean(reduceFunction);

        String callLocation = Utils.getCallLocationName();
        String udfName = "AllWindowedStream." + callLocation;

        String opName;
        KeySelector<T, Byte> keySel = input.getKeySelector();

        OneInputStreamOperator<T, R> operator;

        if (evictor != null) {
            @SuppressWarnings({"unchecked", "rawtypes"})
            TypeSerializer<StreamRecord<T>> streamRecordSerializer =
                    (TypeSerializer<StreamRecord<T>>)
                            new StreamElementSerializer(
                                    input.getType()
                                            .createSerializer(
                                                    getExecutionEnvironment().getConfig()));

            ListStateDescriptor<StreamRecord<T>> stateDesc =
                    new ListStateDescriptor<>("window-contents", streamRecordSerializer);

            opName =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + evictor
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new EvictingWindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            new InternalIterableAllWindowFunction<>(
                                    new ReduceApplyAllWindowFunction<>(reduceFunction, function)),
                            trigger,
                            evictor,
                            allowedLateness,
                            lateDataOutputTag);

        } else {
            ReducingStateDescriptor<T> stateDesc =
                    new ReducingStateDescriptor<>(
                            "window-contents",
                            reduceFunction,
                            input.getType()
                                    .createSerializer(getExecutionEnvironment().getConfig()));

            opName =
                    "TriggerWindow("
                            + windowAssigner
                            + ", "
                            + stateDesc
                            + ", "
                            + trigger
                            + ", "
                            + udfName
                            + ")";

            operator =
                    new WindowOperator<>(
                            windowAssigner,
                            windowAssigner.getWindowSerializer(
                                    getExecutionEnvironment().getConfig()),
                            keySel,
                            input.getKeyType()
                                    .createSerializer(getExecutionEnvironment().getConfig()),
                            stateDesc,
                            new InternalSingleValueAllWindowFunction<>(function),
                            trigger,
                            allowedLateness,
                            lateDataOutputTag);
        }

        return input.transform(opName, resultType, operator).forceNonParallel();
    }

    // ------------------------------------------------------------------------
    //  Aggregations on the all windows
    // ------------------------------------------------------------------------

    /**
     * Applies an aggregation that sums every window of the data stream at the given position.
     *
     * @param positionToSum The position in the tuple/array to sum
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> sum(int positionToSum) {
        return aggregate(
                new SumAggregator<>(positionToSum, input.getType(), input.getExecutionConfig()));
    }

    /**
     * Applies an aggregation that sums every window of the pojo data stream at the given field for
     * every window.
     *
     * <p>A field expression is either the name of a public field or a getter method with
     * parentheses of the stream's underlying type. A dot can be used to drill down into objects, as
     * in {@code "field1.getInnerField2()" }.
     *
     * @param field The field to sum
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> sum(String field) {
        return aggregate(new SumAggregator<>(field, input.getType(), input.getExecutionConfig()));
    }

    /**
     * Applies an aggregation that gives the minimum value of every window of the data stream at the
     * given position.
     *
     * @param positionToMin The position to minimize
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> min(int positionToMin) {
        return aggregate(
                new ComparableAggregator<>(
                        positionToMin,
                        input.getType(),
                        AggregationFunction.AggregationType.MIN,
                        input.getExecutionConfig()));
    }

    /**
     * Applies an aggregation that gives the minimum value of the pojo data stream at the given
     * field expression for every window.
     *
     * <p>A field expression is either the name of a public field or a getter method with
     * parentheses of the {@link DataStream}S underlying type. A dot can be used to drill down into
     * objects, as in {@code "field1.getInnerField2()" }.
     *
     * @param field The field expression based on which the aggregation will be applied.
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> min(String field) {
        return aggregate(
                new ComparableAggregator<>(
                        field,
                        input.getType(),
                        AggregationFunction.AggregationType.MIN,
                        false,
                        input.getExecutionConfig()));
    }

    /**
     * Applies an aggregation that gives the minimum element of every window of the data stream by
     * the given position. If more elements have the same minimum value the operator returns the
     * first element by default.
     *
     * @param positionToMinBy The position to minimize by
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> minBy(int positionToMinBy) {
        return this.minBy(positionToMinBy, true);
    }

    /**
     * Applies an aggregation that gives the minimum element of every window of the data stream by
     * the given position. If more elements have the same minimum value the operator returns the
     * first element by default.
     *
     * @param positionToMinBy The position to minimize by
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> minBy(String positionToMinBy) {
        return this.minBy(positionToMinBy, true);
    }

    /**
     * Applies an aggregation that gives the minimum element of every window of the data stream by
     * the given position. If more elements have the same minimum value the operator returns either
     * the first or last one depending on the parameter setting.
     *
     * @param positionToMinBy The position to minimize
     * @param first If true, then the operator return the first element with the minimum value,
     *     otherwise returns the last
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> minBy(int positionToMinBy, boolean first) {
        return aggregate(
                new ComparableAggregator<>(
                        positionToMinBy,
                        input.getType(),
                        AggregationFunction.AggregationType.MINBY,
                        first,
                        input.getExecutionConfig()));
    }

    /**
     * Applies an aggregation that gives the minimum element of the pojo data stream by the given
     * field expression for every window. A field expression is either the name of a public field or
     * a getter method with parentheses of the {@link DataStream DataStreams} underlying type. A dot
     * can be used to drill down into objects, as in {@code "field1.getInnerField2()" }.
     *
     * @param field The field expression based on which the aggregation will be applied.
     * @param first If True then in case of field equality the first object will be returned
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> minBy(String field, boolean first) {
        return aggregate(
                new ComparableAggregator<>(
                        field,
                        input.getType(),
                        AggregationFunction.AggregationType.MINBY,
                        first,
                        input.getExecutionConfig()));
    }

    /**
     * Applies an aggregation that gives the maximum value of every window of the data stream at the
     * given position.
     *
     * @param positionToMax The position to maximize
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> max(int positionToMax) {
        return aggregate(
                new ComparableAggregator<>(
                        positionToMax,
                        input.getType(),
                        AggregationFunction.AggregationType.MAX,
                        input.getExecutionConfig()));
    }

    /**
     * Applies an aggregation that gives the maximum value of the pojo data stream at the given
     * field expression for every window. A field expression is either the name of a public field or
     * a getter method with parentheses of the {@link DataStream DataStreams} underlying type. A dot
     * can be used to drill down into objects, as in {@code "field1.getInnerField2()" }.
     *
     * @param field The field expression based on which the aggregation will be applied.
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> max(String field) {
        return aggregate(
                new ComparableAggregator<>(
                        field,
                        input.getType(),
                        AggregationFunction.AggregationType.MAX,
                        false,
                        input.getExecutionConfig()));
    }

    /**
     * Applies an aggregation that gives the maximum element of every window of the data stream by
     * the given position. If more elements have the same maximum value the operator returns the
     * first by default.
     *
     * @param positionToMaxBy The position to maximize by
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> maxBy(int positionToMaxBy) {
        return this.maxBy(positionToMaxBy, true);
    }

    /**
     * Applies an aggregation that gives the maximum element of every window of the data stream by
     * the given position. If more elements have the same maximum value the operator returns the
     * first by default.
     *
     * @param positionToMaxBy The position to maximize by
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> maxBy(String positionToMaxBy) {
        return this.maxBy(positionToMaxBy, true);
    }

    /**
     * Applies an aggregation that gives the maximum element of every window of the data stream by
     * the given position. If more elements have the same maximum value the operator returns either
     * the first or last one depending on the parameter setting.
     *
     * @param positionToMaxBy The position to maximize by
     * @param first If true, then the operator return the first element with the maximum value,
     *     otherwise returns the last
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> maxBy(int positionToMaxBy, boolean first) {
        return aggregate(
                new ComparableAggregator<>(
                        positionToMaxBy,
                        input.getType(),
                        AggregationFunction.AggregationType.MAXBY,
                        first,
                        input.getExecutionConfig()));
    }

    /**
     * Applies an aggregation that gives the maximum element of the pojo data stream by the given
     * field expression for every window. A field expression is either the name of a public field or
     * a getter method with parentheses of the {@link DataStream}S underlying type. A dot can be
     * used to drill down into objects, as in {@code "field1.getInnerField2()" }.
     *
     * @param field The field expression based on which the aggregation will be applied.
     * @param first If True then in case of field equality the first object will be returned
     * @return The transformed DataStream.
     */
    public SingleOutputStreamOperator<T> maxBy(String field, boolean first) {
        return aggregate(
                new ComparableAggregator<>(
                        field,
                        input.getType(),
                        AggregationFunction.AggregationType.MAXBY,
                        first,
                        input.getExecutionConfig()));
    }

    private SingleOutputStreamOperator<T> aggregate(AggregationFunction<T> aggregator) {
        return reduce(aggregator);
    }

    // ------------------------------------------------------------------------
    //  Utilities
    // ------------------------------------------------------------------------

    public StreamExecutionEnvironment getExecutionEnvironment() {
        return input.getExecutionEnvironment();
    }

    public TypeInformation<T> getInputType() {
        return input.getType();
    }
}
