// 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 <condition_variable>
#include <exception>
#include <functional>
#include <mutex>
#include <queue>
#include <thread>
#include <vector>


namespace legacy
{
/** Very simple thread pool similar to boost::threadpool.
  * Advantages:
  * - catches exceptions and rethrows on wait.
  */

class ThreadPool
{
public:
    using Job = std::function<void()>;

    /// Size is constant, all threads are created immediately.
    /// Every threads will execute pre_worker firstly when they are created.
    explicit ThreadPool(
        size_t m_size,
        Job pre_worker = [] {});

    /// Add new job. Locks until free thread in pool become available or exception in one of threads was thrown.
    /// If an exception in some thread was thrown, method silently returns, and exception will be rethrown only on call to 'wait' function.
    void schedule(Job job);

    /// Wait for all currently active jobs to be done.
    /// You may call schedule and wait many times in arbitary order.
    /// If any thread was throw an exception, first exception will be rethrown from this method,
    ///  and exception will be cleared.
    void wait();

    /// Waits for all threads. Doesn't rethrow exceptions (use 'wait' method to rethrow exceptions).
    /// You should not destroy object while calling schedule or wait methods from another threads.
    ~ThreadPool();

    size_t size() const { return m_size; }

    /// Returns number of active jobs.
    size_t active() const;

private:
    mutable std::mutex mutex;
    std::condition_variable has_free_thread;
    std::condition_variable has_new_job_or_shutdown;

    const size_t m_size;
    size_t active_jobs = 0;
    bool shutdown = false;

    std::queue<Job> jobs;
    std::vector<std::thread> threads;
    std::exception_ptr first_exception;


    void worker();
    void finalize();
};
} // namespace legacy