// Copyright 2023 PingCAP, Inc.
//
// 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.

#pragma once

#include <IO/Buffer/WriteBuffer.h>
#include <common/ThreadPool.h>
#include <math.h>

#include <vector>


namespace DB
{
/** Writes data asynchronously using double buffering.
  */
class AsynchronousWriteBuffer : public WriteBuffer
{
private:
    WriteBuffer & out; /// The main buffer, responsible for writing data.
    std::vector<char> memory; /// A piece of memory for duplicating the buffer.
    legacy::ThreadPool pool; /// For asynchronous data writing.
    bool started; /// Has an asynchronous data write started?

    /// Swap the main and duplicate buffers.
    void swapBuffers()
    {
        buffer().swap(out.buffer());
        std::swap(position(), out.position());
    }

    void nextImpl() override
    {
        if (!offset())
            return;

        if (started)
            pool.wait();
        else
            started = true;

        swapBuffers();

        /// The data will be written in separate stream.
        pool.schedule([this] { thread(); });
    }

public:
    explicit AsynchronousWriteBuffer(WriteBuffer & out_)
        : WriteBuffer(nullptr, 0)
        , out(out_)
        , memory(out.buffer().size())
        , pool(1)
        , started(false)
    {
        /// Data is written to the duplicate buffer.
        set(memory.data(), memory.size());
    }

    ~AsynchronousWriteBuffer() override
    {
        try
        {
            if (started)
                pool.wait();

            swapBuffers();
            out.next();
        }
        catch (...)
        {
            tryLogCurrentException(__PRETTY_FUNCTION__);
        }
    }

    /// That is executed in a separate thread
    void thread() { out.next(); }
};

} // namespace DB