// 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.

// This file is adapted from

// https://github.com/ClickHouse/ClickHouse/blob/master/src/AggregateFunctions/AggregateFunctionExponentialMovingAverage.cpp

#pragma once

#include <cmath>

#include <memory>

#include "core/assert_cast.h"

#include "core/column/column_vector.h"

#include "core/data_type/data_type_number.h"

#include "core/types.h"

#include "exprs/aggregate/aggregate_function.h"

namespace doris {

class Arena;

class BufferReadable;

class BufferWritable;

class IColumn;

/**

 * Exponentially smoothed moving average over time.

 *

 * Each value corresponds to a timeunit index. The half_decay parameter is the

 * time lag at which exponential weights decay by one-half.

 *

 * State is a (value, time) pair representing the exponentially accumulated sum

 * at a reference time. To get the average, divide by sumWeights(half_decay).

 *

 * Formula:

 *   scale(dt, x) = 2^(-dt/x)

 *   sumWeights(x) = 1 / (1 - 2^(-1/x))

 *   add(v, t): merge current state with point (v, t)

 *   merge(a, b): move both to the later time, then sum values

 *   get():  value / sumWeights(half_decay)

 *

 * Usage: exponential_moving_average(half_decay, value, timeunit)

 *   - half_decay: constant double, the half-life period in timeunit units

 *   - value:      numeric column to average

 *   - timeunit:   numeric time index (not raw timestamp; use intDiv if needed)

 * Returns DOUBLE.

 */

struct ExponentialMovingAverageData {

    double value = 0.0;

    double time = 0.0;

    double half_decay = 0.0;

    static double scale(double time_passed, double hd) { return std::exp2(-time_passed / hd); }

    static double sum_weights(double hd) { return 1.0 / (1.0 - std::exp2(-1.0 / hd)); }

    void add(double new_value, double current_time, double hd) {

        half_decay = hd;

        ExponentialMovingAverageData other;

        other.value = new_value;

        other.time = current_time;

        merge_point(other, hd);

    }

    void merge_point(const ExponentialMovingAverageData& other, double hd) {

        if (time > other.time) {

            value = value + other.value * scale(time - other.time, hd);

        } else if (time < other.time) {

            value = other.value + value * scale(other.time - time, hd);

            time = other.time;

        } else {

            value = value + other.value;

        }

    }

    void merge(const ExponentialMovingAverageData& rhs) {

        double hd = half_decay != 0.0 ? half_decay : rhs.half_decay;

        if (hd == 0.0) {

            return;

        }

        half_decay = hd;

        merge_point(rhs, hd);

    }

    double get() const {

        if (half_decay == 0.0) {

            return 0.0;

        }

        return value / sum_weights(half_decay);

    }

    void write(BufferWritable& buf) const {

        buf.write_binary(value);

        buf.write_binary(time);

        buf.write_binary(half_decay);

    }

    void read(BufferReadable& buf) {

        buf.read_binary(value);

        buf.read_binary(time);

        buf.read_binary(half_decay);

    }

    void reset() {

        value = 0.0;

        time = 0.0;

        half_decay = 0.0;

    }

};

class AggregateFunctionExponentialMovingAverage final

        : public IAggregateFunctionDataHelper<ExponentialMovingAverageData,

                                              AggregateFunctionExponentialMovingAverage>,

          MultiExpression,

          NullableAggregateFunction {

public:

    AggregateFunctionExponentialMovingAverage(const DataTypes& argument_types_)

            : IAggregateFunctionDataHelper<ExponentialMovingAverageData,

                                           AggregateFunctionExponentialMovingAverage>(

                      argument_types_) {}

    String get_name() const override { return "exponential_moving_average"; }

    DataTypePtr get_return_type() const override { return std::make_shared<DataTypeFloat64>(); }

    void reset(AggregateDataPtr __restrict place) const override { this->data(place).reset(); }

    void add(AggregateDataPtr __restrict place, const IColumn** columns, ssize_t row_num,

             Arena&) const override {

        const double half_decay =

                assert_cast<const ColumnFloat64&, TypeCheckOnRelease::DISABLE>(*columns[0])

                        .get_data()[row_num];

        const double new_value =

                assert_cast<const ColumnFloat64&, TypeCheckOnRelease::DISABLE>(*columns[1])

                        .get_data()[row_num];

        const double current_time =

                assert_cast<const ColumnFloat64&, TypeCheckOnRelease::DISABLE>(*columns[2])

                        .get_data()[row_num];

        this->data(place).add(new_value, current_time, half_decay);

    }

    void merge(AggregateDataPtr __restrict place, ConstAggregateDataPtr rhs,

               Arena&) const override {

        this->data(place).merge(this->data(rhs));

    }

    void serialize(ConstAggregateDataPtr __restrict place, BufferWritable& buf) const override {

        this->data(place).write(buf);

    }

    void deserialize(AggregateDataPtr __restrict place, BufferReadable& buf,

                     Arena&) const override {

        this->data(place).read(buf);

    }

    void insert_result_into(ConstAggregateDataPtr __restrict place, IColumn& to) const override {

        assert_cast<ColumnFloat64&>(to).get_data().push_back(this->data(place).get());

    }

};

} // namespace doris